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© 2011 Dodson Publications, Inc. Page 1 of 21 March 16, 2012 8:33 AM
Roof Drainage Issues
Designing Commercial Roof Systems In Compliance with the IBC Part 2:
Roof Drainage Issues
by John D. Shepherd, RRC, Shepherd Consulting Services
(Editor’s Note: John Shepherd is an RCI, Inc. Registered Roof Consultant
and Registered Roof Observer, a Building Science Certified Thermographer, a
Haag Certified Commercial and Residential Roof Inspector, and holds roofing
licenses in both Calif. and Ariz. He is also actively involved in RCI, Inc. and
the SoCal Chapter of RCI, and currently is a member of three committees
including the RCI Inc Advocacy Committee, the SoCal Chapter of RCI Hawaii
Winter Workshop Committee, and the ASTM D08 committee. You can reach
Shepherd at jds@shepherdconsulting.com)
After proper attachment and installation of the low-sloped roof assembly on
a commercial building, it is this consultant’s opinion that proper roof
drainage is the singular most important issue that impacts a commercial roof
system. Roof drainage issues appear to some degree or other on just about
every low-sloped commercial reroof project I have inspected in my 40 plus
years as a roofer, roofing contractor, and roofing consultant. Roof drainage
issues also crop up in new construction projects and those not resolved
during the original construction process will resurface during a reroof
project. That said, while the focus of this article is on drainage issues
related to reroofing projects, I have included a section on pre-construction
inspections for new construction roof projects that I hope will slightly reduce
the roofing contractor’s future liability on new construction projects.
With the exception of new construction projects, this article goes
forward assuming that the reroofing project does not include the
involvement of an architect, engineer, or register roof consultant. While I
highly recommend that all reroofing projects have specifications and details
© 2011 Dodson Publications, Inc. Page 2 of 21 March 16, 2012 8:33 AM
prepared for the specific project by one of these design professionals, I also
understand that this is not the reality of the world we all live in. In the vast
majority of cases, the roofing contractor has taken on the role as the
designer of record (refer to Part I of this series in the November/December
2011 issue of Western Roofing for a more detailed discussion on the
designer of record discussion).
Design related decisions made by the roofing contractor, in his/her
capacity as the prime contractor on the project, often include, but are not
limited to: the project’s overall scope of work, the type of materials to be
installed, which roofing material manufacturer to use, how existing
mechanical equipment and accessories will be addressed, addressing roof
drainage problems (ponding water conditions, missing overflow drains,
undersized crickets, etc.), how to deal with condensation and ventilation
issues, whether skylights are to be re-used or replaced, determining what to
do about areas with low base flashings, replacement of water damaged roof
sheathing, and support members, etc.
In this article I will discuss the typical roof drainage issues that a
contractor will run into on a reroofing project including ponding water
conditions, roof crickets and their impact on roof drainage, internal primary
and overflow roof drains, through-wall scupper primary and overflow drains,
raising and/or moving mechanical equipment, and penetrations in
waterways. This article will also include requirements and definitions for
roof drainage issues from the 2009 International Building Code (IBC) and
provide some common sense ideas on resolving the problems referenced
above.
2% Minimum Roof Slope Required
The 2009 International Building Code (IBC) and most roofing material
manufacturers required low-sloped roofs, often mischaracterized as flat roofs
© 2011 Dodson Publications, Inc. Page 3 of 21 March 16, 2012 8:33 AM
even by experienced roofing contractors, to have a minimum 2% slope.
Under IBC Section 1507 “Requirements for Roof Coverings”, the IBC
requires that the, “roofs shall have a design slope of a minimum of one-forth
unit vertical in 12 units horizontal (2%) for drainage.” In IBC Section 1507
2% is listed as the minimum roof slope requirement for seven different roof
systems including Standing Seam Metal Roof Panels (Section 1507.4), Built-
up Roofs (Section 1507.10), Modified Bitumen Roofing (Section 1507.11),
Thermoset Single-Ply Roofing (Section 1507.12), Thermoplastic Single-Ply
Roofing (Section 1507.13), Sprayed Polyurethane Foam Roofing (Section
1507.13), and Liquid-Applied Coatings/Roofs (Section 1507.14).
Based on these code requirements I will ask you each to consider two
questions: 1) Does the 2% requirement also apply to the roof deck? 2) Is
there any flexibility in the 2% requirement? The answer to #1 is yes and
no, the answer to #2 is yes. Let me explain.
Roof Slope Deck Requirement: When Section 1507 references steep-
sloped type roof system, it specifically references what the minimum deck
slope shall be. However for low-sloped roof systems the code reference is
specific that the roof shall have a design slope of a minimum of 2%. So how
this is achieved on a reroof project is up to the contractor. If you have
verified that the existing roof deck slope is a minimum of 2%, then the
contractor can hypothetically install the new roofing system directly to the
roof deck (this discussion excludes discussions on energy code requirements
and specific materials that may be required to address fire-ratings of a roof
system). However, if the roof deck does not have the required 2% slope
then this slope must be achieved via either modifications of the actual roof
deck or via addition of materials over the deck (tapered insulation, light-
weight insulating concrete, etc.). Alternatively, slope can be achieved by
using sprayed polyurethane foam, which as a roof system can address both
the slope deficiencies and provide a waterproof roof assembly.
© 2011 Dodson Publications, Inc. Page 4 of 21 March 16, 2012 8:33 AM
The methodology for addressing roof slope issues on projects without
the involvement of a design professional is really left up to the contractor,
who hopefully was prudent enough to address the issues in the pre-bid stage
and not after the roofing project is in progress. In the bid stage the
contractor will have the time to adequately evaluate the options and decide
whether they have the expertise to address the slope issues or recommend a
design professional to the building owner (see more on the above referenced
options later in this article). If decisions about slope modifications must be
made after the roofing project has started, the options available become
time dependent as the longer a roof is exposed the greater the risk of a
weather related catastrophe occurring.
When is a Slope of Less Than 2% Acceptable: Section 1510
“Reroofing” provides the one exception to the Section 1507 2% roof slope
rule. That exception, as listed under Section 1510.1 General, reads as
follows: “Reroofing shall not be required to meet the minimum design slope
requirement of one-quarter unit vertical in 12 units horizontal (2% slope) in
Section 1507 for roofs that provide positive roof drainage.” (It is important
to verify that the state or local jurisdiction the roof project is in has left this
code section intact when the code was adopted.)
So based on this code section, the contractor is not required to modify
slopes of less than 2%, as long as the roof provides positive drainage. This
code section now raises another question: What is “Positive Roof Drainage?”
Thankfully the IBC anticipates this question and provides us with a definition
for “positive roof drainage”.
Before moving on I want to recommend that you always consider
looking at either satellite images of the roof (via Google Earth or Bing) or
roof images taken via fixed wing aircraft (Pictometry, Eagleview, etc.) of any
building you are bidding on. Looking at such photographs prior to visiting
the site will provide a general understanding of the building layout, roof top
© 2011 Dodson Publications, Inc. Page 5 of 21 March 16, 2012 8:33 AM
objects, and whether there is any possible ponding water conditions (refer to
photographs #1 and #2).
Positive Roof Drainage & Ponding or Standing Water Conditions
One of the most commonly mischaracterized conditions I have seen in
low-sloped related defect litigation cases are those related to ponding or
standing water conditions. For the last 20 plus years the so-called “48 hours
rule” was most often referenced with regards to defining what is ponding or
standing water conditions on low-sloped roofs. However, for many of those
years we (roofing contractors and consultants) depended on what the NRCA
1 and roofing material manufacturer’s 2 had to say about ponding or
standing water conditions. The building code had references to positive
drainage however in the past it did not articulate what is positive drainage,
and what is not.
With the adoption of the IBC nationwide we in the Western states now
have a clear legal definition for what is Positive Roof Drainage, and therefore
a legal definition for what is ponding water on a low-sloped roof. IBC
Section 1502 definitions defines Positive Roof Drainage as follows: “The
drainage condition in which consideration has been made for all loading
deflections of the roof deck 3, and additional slope has been provided to
ensure drainage of the roof within 48 hours of precipitation.”
This is a pretty direct statement: if all water from a rainfall is
dissipated from the roof within 48 hours of the last rainfall, the 2% slope
rule is not applicable on a reroofing project. This same statement is
applicable to any so called ponding water areas on a roof. Let’s discuss the
ponding issue more in depth. First though some clarifications on my
experience in this area. As a construction consultant, the majority of my
business is working as an expert of construction defect litigation cases.
Generally I am an expert on roofing, waterproofing, and deck issues, though
© 2011 Dodson Publications, Inc. Page 6 of 21 March 16, 2012 8:33 AM
I also handle exterior wall issues as well (stucco, EFIS, siding, elastomeric
paint, windows and doors, etc). Though I occasionally work as a plaintiff
expert, I primarily work as a defense expert for either the developer or the
subcontractor (roofing, sheet metal, waterproofing, decks). It is my
experience as a roofing expert that has provided the basis for what I am
about to say.
In virtually every litigation case that I have worked on involving
commercial roofing (low-sloped roof systems) the allegation of ponding
water conditions is always on the defect list. Why, well it’s an easy target.
When the plaintiff’s expert sees staining and/or an accumulation of dirt and
debris in what appear to be low spots on a roof, typically in front of drains
and scuppers, or adjacent to cricket waterways and large mechanical
equipment, they automatically assume it’s a ponding water condition. In
most cases how I deal with this issue is I disagree with the allegation that
ponding water is actually present, until someone actually performs a 48-hour
ponding water test. At times when I am pretty certain that the water testing
will confirm there are no ponding conditions, I will actually push for the
water testing as part of any defense related testing.
Now sometimes this condition is actually present on a roof and it is
important to recognize it early in the litigation, or in the case of a reroofing
project, prior to starting the roof construction. From a contractor’s
perspective you want to know if there are issues you need to address related
to ponding that could void the manufacturer’s warranty. So it is important
to include some type of language in your proposal that includes the
possibility of additional work being conducted to correct ponding conditions.
It should include a reference to verify the adequacy of the roof slope and
roof drainage via a 48-hour water test prior to removing the roofing,
especially if your preliminary inspection leads you to suspect that ponding
water conditions do exist. One thing that doesn’t work is to put language in
© 2011 Dodson Publications, Inc. Page 7 of 21 March 16, 2012 8:33 AM
your contract that you are not responsible for ponding water conditions.
Regardless of what language you insert into your roofing contract, you are
the roofing professional and the party that should be familiar with the
building code requirements and the exclusions in the roofing material
manufactures warranty. The average building owner or property manager is
typically not aware of these issues until you inform them of these issues. So
if you suspect ponding water conditions then push for a water test. If your
client insists that the work be done and the ponding be ignored, you have to
make a decision to either walk away from the project (before signing the
contract) or risk ignoring the building codes and hope that nothing goes
wrong later that brings attention to the warranty exclusion.
The testing for ponding water is pretty simple. First check the weather
and determine whether rainfall is eminent. Is so let the rain water the roof
for you and then follow the below procedures. If no rain is predicted then
you will need to run water on the roof.
Start the testing by first sweeping up all loose debris from the
waterways and in front and around internal roof drains and through-wall
scuppers. Run the water over the roof until all little dips, birdbaths, and any
suspected ponding area is covered with water. Note the time the water is
turned off and then use spray paint (my preference) or chalk to mark the
outline of each area holding water (refer to photograph #3). Return to the
site 48-hours after you last turned off the water. If the roof is 100% dry,
there are no ponding water areas, then the roof has positive slope as defined
by the building code (refer to photograph #4). If there are small puddles of
water on site-specific areas of the roof then, using a different color of
marking paint, mark the perimeter of each remaining pond and then
photograph the condition. Minor ponding water conditions can often be
corrected with minor alterations to the roof slope. These include adding
© 2011 Dodson Publications, Inc. Page 8 of 21 March 16, 2012 8:33 AM
Henry’s Pond Patch, Siplast’s Paraslope, or small amounts of tapered
insulation to the location in question.
If the water test identifies significant ponding conditions you still need
to identify, mark and photograph the ponding conditions but now you need
to have a serious discussion with the building owner about the options for
addressing the ponding conditions.
Addressing Serious Ponding Conditions
There are a number of things that can be done to address serious
ponding water conditions, however the extent and location of the ponding
conditions is the key to the repair options. First, if the conditions are
extensive and widespread, it is likely that a complete resloping of the roof
may be required. Such resloping can be done with lumber and tapered 2x’s,
with light-weight insulating concrete, with tapered insulation and/or with
spray-polyurethane foam. With the exception of the wood framing option,
each of the choices can not only provide the positive slope needed, but also
provide additional R-value for the buildings roof via the insulating aspects of
the materials. The method and materials chosen for this task will depend on
the structural integrity of the roof, costs, the needs of the building owner,
and what the building is used for. If the building is primarily an office
building the long term benefits of energy savings should be calculated for
the building owner to assist in the methods used to address the inadequate
slope issues. If the building is in California, you may already need to add
insulation to the building to comply with the current energy codes so this
becomes a win-win situation.
With each of the choices above the roofing contractor should not try to
go-it alone. With tapered insulation you can get design assistance from the
tapered insulation manufacturers. While there is no direct cost for this
service the indirect cost could be high and the manufacturer’s design might
© 2011 Dodson Publications, Inc. Page 9 of 21 March 16, 2012 8:33 AM
include more insulation than say a design done by a third part design
professional.
With regards to modifying the roof slope with tapered 2x’s and
plywood or light-weight insulating concrete, you will need to have a
structural engineer determine if the building can handle the load. An
architect or engineer should design the new wood deck. The light-weight
insulation industry will provide design assistance for their product though,
like the tapered insulation you need to have the design reviewed by a third
party.
Addressing Moderate Ponding Conditions
The most common scenario related to a positive finding of ponding
water conditions is that the condition is not minor, but not so extensive that
the entire roof slope must be redone. In my experience I often found that
the problems can be resolved using a combination of one or more remedies.
This includes creating recessed sumps around ponding at roof drains and
through-wall scuppers, adding additional roof drains and/or through-wall
scuppers, adding tapered crickets on the upslope side of skylights and
mechanical equipment, rebuilding the existing crickets to have a steeper
slope and therefore more positive drainage, breaking up large crickets into
smaller crickets, and adding additional roof drains and/or scuppers. On
some commercial buildings tenant improvements or use changes in the
building create roof drainage problems by locating mechanical equipment in
the water-ways (refer to photographs #5 and 6). In these cases the only
option is to relocate the equipment that is blocking the direct flow of water
to roof drains. Again the choice of options used will depend on the specific
needs and issues on the building. And each option brings about another set
of questions to determine whether the option is actually viable.
© 2011 Dodson Publications, Inc. Page 10 of 21 March 16, 2012 8:33 AM
If the main area of ponding is along the waterways, and it is not
caused by mechanical equipment, it is likely that the slope of the cricket is
not double the roof slope. In this case the best solution to solve the
drainage problems may be increasing the slope of the large crickets, if there
is adequate room in the height of the parapet walls. However if the parapet
wall is too low for such an adjustment, then look at breaking up large
crickets to smaller crickets and adding roof drains or through-wall scupper
drains as an alternative option.
When it comes to roof penetrations and objects blocking the flow of
water to the drains, you need to consider all options. When evaluating the
relocation of mechanical equipment that has been set in the waterways first
look at relocating the unit and the platform. But also consider the option of
replacing a closed mechanical equipment platform with an opened frame
support system that allows water to flow to the roof drains. Individual roof
penetrations in waterways are often the source of future roof leaks and they
should be moved out of the waterway if at all possible.
Adding roof drains also provides its own complications. If you want to
add internal roof drains, can they be added with only minimal impact on the
interior of the building? For a warehouse type building adding the internal
drains is easily doable. But in an office building adding an internal drain
could cause a significant disruption to the building. Same goes for the
adding of through-wall scupper drains. Is the intended location directly over
a doorway or a window? Can downspouts be added? An important thing to
remember is that if you add roof drains or through-wall scuppers you will
also need to add overflow drains or scuppers.
It is important that your sales team, or at least one key person on
your staff, have a keen understanding of each option available, and how to
apply the various options to issues found at your specific project. Also keep
in mind that at some point you may cross a threshold where the cost of
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addressing each ponding area becomes more expensive than a whole roof
solution such as tapered insulation or even switching the roof system to an
SPF system.
Let’s move on to talk about existing roof drains and scuppers.
Verifying the Roof Has the Right Drain Size
As a consultant I always conduct what I call a preliminary evaluation of
the roof drain size for each specific roof area serviced by the roof drain to
determine if the roof drains are the correct size for the area of roof and
parapet walls being serviced by the roof drain. And I recommend that you
the contractor consider doing the same. I do these calculations primarily to
see if there are any red flags I need to caution my client about. However, I
caution you that unless you are an architect or an engineer licensed in the
state you are practicing, your calculations should used as an internal use
document only and should not be sent out to the client. If I find the drains
are undersized, I tell my client I am suspicious of the drain size and then
recommend that the drains be analyzed by a licensed engineer or architect.
The method of determining drain size for a given roof area is fairly
simple:
• Roof Drawing: Create a roof drawing that includes roof drains,
scuppers, crickets, ridge lines, etc. Identify the roof that sends water to
each drain and mark off the areas on the drawing. Also measure the
diameter of any internal roof drains and the area of any through-wall
scupper drains.
• Calculate the Total Roof Area: Calculate the total roof and wall area
service by each existing roof drain. Add 100% of the roof deck square
footage and 50% of the parapet wall square footage to determine the total
roof area serviced by each drain.
© 2011 Dodson Publications, Inc. Page 12 of 21 March 16, 2012 8:33 AM
• Determine Rainfall Intensity: For the Western USA use figure 1106.1
on page 87 of the 2009 International Plumbing Code. Look on the figure and
identify the geographical location of the building to be reroofed and
determine the maximum hourly rainfall in inches for your geographical area.
• Select the Table Needed for the Next Step: For internal drains use
Table 1106.2(1) and for through-wall scupper drains use figure 1106.2(2).
Both are on page 90 of the 2009 IPC.
• Determine the Roof Area the Drain Can Handle: Find the rainfall
intensity for your area (as obtained from figure 1106.1) along the top of the
table and the diameter or the size along the left hand column and cross
reference to determine the square foot area the buildings existing drains or
scuppers can handle.
• Compare Existing to Design Requirements: If the existing roof area
(and 50% of walls) is less than the design load for the specific drain then the
existing drain or scupper is acceptable. If the roof area exceeds the drain
design load then either the existing drains or through-wall scuppers must be
enlarger or additional drains or scuppers must be installed.
Requirement for Overflow Drains/Scuppers
As far back as I can remember the various building codes have always
required the installation of overflow drains and/or overflow scuppers in
conjunction with the installation of primary drain system of internal roof
drains and/or through-wall scupper drains. So I am still surprised by the
number of buildings I have encountered over the last five years that do not
have overflow drains or overflow scuppers (refer to photographs #5 and 6).
In each case I made recommendations for adding overflow drains or
scuppers. If you, the contractor, should come across a building without
overflow drains or scuppers you also should make recommendations for
adding the overflow drains or scuppers to your client.
© 2011 Dodson Publications, Inc. Page 13 of 21 March 16, 2012 8:33 AM
As with previous versions of the building code, 2009 IBC Section 1503
still required roof drainage systems to comply with the plumbing code.
Section 1503.4 read as follows: “Roof drainage. Design and installation of
roof drainage systems shall comply with Section 1503 and the International
Plumbing Code” (in California that would read “and the California Plumbing
Code). And though the wording had changed, the code made it relatively
simple for the contractor to determine if secondary roof drainage was
required on any building scheduled to have the roof replaced.
The 2009 IBC Section 1503.4.1 and IPC Section 1107.1 both read as
follows, “1503.4.1 Secondary drainage required. Secondary (emergency)
roof drains or scuppers shall be provided where the roof perimeter
construction extends above the roof in such a manner that water will be
entrapped if the primary drains allow buildup for any reason.” Translation, if
there are any parapet walls or any objects that would allow water to collect
on the roof if the roof drain should become blocked, the roof must have
overflow drains. This sounds straight forward enough. However the first
question I often hear from the roofing contractor, especially in construction
defect cases is, “This building was built X number of years ago under a
different code so why did I have to bring the roof and roof accessories into
compliance with the current code?”
The answer is, “because the building code says you have to.” Section
1510 Reroofing, includes the following statement: “1510.1 General.
Materials and methods of application used for recovering or replacing an
existing roof covering shall comply with the requirements of Chapter 15.”
So as there are no exceptions, all of the roof drainage requirements listed in
Chapter 15 would apply to a reroofing project. For clarity I think the
requirement bears repeating. “If any portion of the building could
accumulate water if the primary drain becomes blocked, and the building
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had no overflow drains or scuppers, then overflow drains or scuppers must
be added during the roof replacement project.”
The choice of using overflow drains or overflow scuppers is one of both
cost and access. Installing a through-wall overflow scupper could entail as
little as cutting a hole in the parapet wall and inserting a through-wall
scupper flashing. If concrete then little extra work is required. If wood-
framed and stucco covered then plaster and lath needs to be removed and
replaced, and the hole needs to be framed to allow attachment for new lath
around the area. Installing an internal overflow drain involves cutting
through the roof deck, adding extra structural support as needed around the
point where the drain penetrates the roof, and then running drainage pipe
down the interior of the building. This is an easy fix, though more costly
than a through-wall scupper in a concrete tilt-up wall, if the area below is a
warehouse. It is a more difficult task, though still doable, if the area below
was office space.
Placement & Size of Overflow Drains
Previous versions of the building code would reference internal drains
but would defer to the plumbing code. And though the wording has been
modified, IBC Section 1503.4 still directs the reader the IPC (or in the case
of California, the California Plumbing Code) for roof drain issues. A search of
the 2009 IPC finds two specific sections related to overflow drains including
IPC Sections 1107.2 and 1107.3.
Section 1107.2 is as follows: “Separate systems required. Secondary
roof drain systems shall have the endpoint of discharge separate from the
primary system. Discharge shall be above grade, in a location that would
normally be observed by the building occupants or maintenance personnel.”
Translation: the primary and overflow drain systems should not be using
the same drain pipe system to drain water from the roof, and the discharge
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point need to be highly visible so that if water is coming out of the overflow
drain discharge point someone will know the primary drains are blocked.
This section of the IPC may be the most overlooked section by any
roofing contractor, which is not good as it could also be the most costly issue
down the road if some existing condition does not comply with the code. I
have had a half dozen roof collapse cases over the years and each was
related to either the lack of an overflow drainage system, or as a result of a
lack of code compliance with the overflow drainage system that was in place
at the time the contractor installed the roof. Therefore it is my opinion that
on each commercial building that has overflow drains, the contractor has two
fundamental additional tasks. The first is to verify that that the overflow
drain system is separate from the primary drain system by both a visual
inspection from below the roof and by pouring water down the primary and
secondary drains and document the discharge points of both (alternatively,
the drains could be scoped with a camera by a plumber to provide a copy of
the film to keep in the contractors file). The second must-do task for the
contractor is to verify the location of all overflow drain discharge points and
then verify that it is not hidden behind landscaping or some other object. If
the discharge point is through a curb at a paved parking lot, the contractor
should also determine if the discharge point opening has not been partially
reduced in size by asphalt from a past paving of a parking lot.
Section 1107.3 of the IPC is as follows: “Sizing of secondary drains.
Secondary (emergency) roof drain systems shall be sized in accordance with
Section 1106 based on the rainfall rate for which the primary system is sized
in Tables 1106.2, 1106.3 and 1106.6.” To determine the correct size of
internal overflow drains, you use the same basic principal of drainage design
steps used to determine if the existing roof drains were the correct size. The
size of the overflow drains will be the same as the internal roof drains, if the
© 2011 Dodson Publications, Inc. Page 16 of 21 March 16, 2012 8:33 AM
existing internal drains size meets the requirements for the geographical
location and the roof area being services by the drain.
Placement & Size of Overflow Scuppers
In all previous versions of the building code it was easy to determine
the size and placement location of a through-wall overflow scupper. The
through-wall overflow scupper was to be three times the area of the primary
roof drain or scupper and was to be set 2” above the surface of the roof
drain or scupper. With the adoption of the 2009 IBC things related to
through-wall scuppers are no longer so simplistic. There are four sections of
the code to review for placement and sizing of scuppers including one
section in Chapter 15 of the 2009 IBC and three sections in Chapter 11 of
the IPC.
Starting out in the plumbing code the first reference to scupper
location is in Section 1106.5 which reads as follows: “Parapet wall scupper
location. Parapet wall roof drainage scupper and overflow scupper location
shall comply with the requirements of the International Building Code.” First
up in IBC Chapter 15 is Section 1503.4.1, which tells us when we need to
install secondary drainage on a roof (this was discussed in more detail
earlier in this article). Next up is Section 1503.4.2, which reads as follows:
“Scuppers. When scuppers are used for secondary (emergency overflow)
roof drainage, the quantity, size, location, and inlet elevation of the scuppers
shall be sized to prevent the depth of ponding water from exceeding that for
which the roof was designed as determined by Section 1503.4.1. Scuppers
shall not have an opening dimension of less than 4” (102mm). The flow
through the primary system shall not be considered when locating and sizing
scuppers.” Say what?
Let me start with the back half of this code section. The second and
third sentences are pretty clear: the scupper will have a minimum of a 4”
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opening dimension and the location and the size of the overflow scuppers
are not based on the primary drain system. Obviously this is a major
change from the past where the area of the overflow scupper was based on
three times the area of the roof drain. However the first sentence raises
some problems as it dictates that the size, quantity, and the location of the
overflow drains is based on what will not exceed the design load of the roof,
when it has X amount of water on it. At this point, unless you are an
engineer, we (consultants, architects, and contractors) must turn to an
engineer and say, “What size, where, and how many.”
The engineer will need to calculate the load capacity of the existing
roof deck (likely without the availability of original drawings) and then
determine what size the overflow scuppers must be, how many are required,
and where they should be installed. This requirement is supported by the
2009 IPC in section 1107.3, which reads similar to 1503.4.1: “Sizing of
secondary drains. Scuppers shall be sized to prevent the depth of ponding
water from exceeding that for which the roof was designed as determined by
Section 1101.7. Scuppers shall not have an opening dimension of less than
4” (102mm). The flow through the primary system shall not be considered
when sizing the secondary roof drain system.” Enough said on this part,
things get a bit more complicated with the above reference to IPC section
1101.7.
2009 IPC Section 1101.7 reads as follows: “Roof design. Roofs shall
be designed for the maximum possible depth of water that will pond there
on as determined by the relative levels of roof deck and overflow weirs,
scuppers, edges, or serviceable drains in combination with the deflected
structural elements. In determining the maximum possible depth of water,
all primary roof drainage means shall be assumed to be blocked.” This
section tells me that it is not about the structural load capacity of one roof
section when one drain is plugged. The IPC wants the engineer to determine
© 2011 Dodson Publications, Inc. Page 18 of 21 March 16, 2012 8:33 AM
the maximum possible depth of water on the roof when all drains are
plugged. Ouch.
Lets recap what we just covered. I now know that I, or you the
roofing contractor, can no longer just determine the size, location, or
number of through-wall overflow scuppers to install on a building without
overflow drains or scuppers, based on our years of experience and
knowledge of a past simplistic, yet easy to use, building and plumbing code.
Now an engineer needs to conduct a structural analysis of the roof in order
to allow for the installation of through-wall overflow scuppers. I think the
cost of installing internal overflow drains just might now be cheaper than
installing through-wall overflow scuppers.
Roof Crickets
Roof drainage crickets are located on every commercial building I can
ever remember being on. Crickets, made from either lumber, tapered
insulation, or concrete, are generally constructed between roof drains and
between a roof corner and a roof drain, with the overall number of crickets
being dependent on the number of roof drains. In order to push water
towards the roof drains, crickets typically have a slope that is twice or more
than the slope of the main roof area. And when there are ponding water
conditions on a roof I often find that the crickets are a contributing factor.
Either the slope on the cricket is less than double that of the roof field
and/or the crickets stop too far from the roof drains. Or in some cases the
crickets installed do not comply with the cricket drainage design that were
on the original construction drawings.
On a new construction project that does not have a consultant
involved or any third party inspections occurring, I want to encourage the
roofing contractor to review the construction drawing and determine if the
crickets built appear to be as designed. If not, I recommend you send
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documentation of such to the general contractor informing them of the issue
and the possible drainage problems that may arise in the future for which
you might get blamed for. If the building ends up in litigation in the future
and a roof consultant opines to ponding water conditions on the roof, your
letter could save your firm money in the future litigation.
On a reroofing project you are not usually going to have the luxury of
having the original plans to review so when you inspect the cricket areas of
the roof you need to look for the telltale signs of ponding water conditions.
As mentioned earlier in this article, the modification of roof crickets to
correct drainage problems is almost always doable, unless the parapet walls
are too low. However, my preference for addressing poor drainage issues at
wood framed crickets is to replace the large crickets with smaller crickets
and add more roof drains. With concrete crickets you generally have to find
a way to increase the slope of the crickets, which sometimes means adding
some height (if allowable) to the parapet walls. Regardless of the solution,
the one thing that will be required for cricket modifications is a structural
analysis of the roof area in question. So again we hit an area where even
when the roofing contractor is the prime contractor, there are things that
cannot be addressed without assistance.
Crickets on the upslope side of mechanical equipment and skylights
are an area that can be addressed by the roofing contractor. It is rare for
me to find a roof where these crickets were built large enough to drain off all
the water from the upslope side of the object. Even though the water is
often gone from the roof areas at each end of the cricket, I often find the
roof membrane slightly more deteriorated at the cricket ends than the roof
membrane in the field area of the roof. So on a reroof project, enlarge the
crickets when you can.
Closing
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Identifying drainage issues and then presenting the appropriate
solutions to correct the drainage deficiencies to the building owner prior to
the start of a roofing project can save everyone time and money in the long
run. The owner will have a roof that will not degrade early due to
inadequate roof drainage and will have a less chance of leaking. The
contractor will make more money on the project by being involved in the
extra work needed to correct the roof drainage deficiencies, and will have a
lower risk of being sued in the future if the deficiencies were ignored and the
roof fails prematurely or if a consultant comes along a few years later and
identifies the problems that should have been addressed.
Also consider where level of expertise your firm has in dealing with
roof drainage problems. Know your limitations, not just of what your firm
can do, but also what your insurance will and will not cover. Carefully
consider the level of design responsibility you want to take on. Be aware of
the overall scope of the drainage issues to be addressed and when in doubt
do the responsible thing and get a design professional involved.
What do you do about the building owner who wants the roof installed,
but doesn’t want to pay to correct any of the roof drainage issues that are
required by the building and/or plumbing code(s)? First off, when you
present the options available to address the problem conditions, also include
the specific section of the building or plumbing code that requires corrective
action be taken. Get the support of a design professional, the local building
official, and or the manufacturer’s representative as needed to convince the
owner of the need for addressing drainage issues. In the end, the prudent
roofing contractor should walk away and not get involved in the project if the
owner wants the reroof done without the needed corrections.
However, I understand that in this economy many of you may choose
to not walk away. If you find yourself in a situation where you don’t want to
walk away and the owner just wants the new roof installed, I suggest you at
© 2011 Dodson Publications, Inc. Page 21 of 21 March 16, 2012 8:33 AM
least list the drainage issues that need to be addressed, and get the owner
to sign a hold harmless agreement for these issues to limit your risk. You
may also want to choose a roof system that has no ponding water exclusion
and then try for a variance from the building department.
In the end the choice is yours. I only hope that I have provided you
with enough information to assist you in making the right choices.
If you want to learn more about roof drainage issues I suggest you
attend Roof Drainage – What You Don’t Know Can Hurt You! which is being
presented by Phil Dregger, FRCI, RRC, PE of DNG Group at the June 27th
RCI Region IV meeting in Las Vegas (as part of the 2012 WSRCA
Convention).
1 Though often mischaracterized as standards, the NRCA’s Roofing and
Waterproofing manual are not standards, though they are an excellent
reference source for learning about various roof decks, roofing systems, and
evaluating roofing-related issues. However, when it comes to specific roof
details, I find the lack of stucco related roof flashing details problematic and
therefore usually look to the WSRCA details, which do include stucco related
roof flashing details.
2 Today, most manufacturers have one or more products that are not
negatively impacted by ponding water conditions and therefore they allow
ponding water on some roofs, however the majority of each manufacturer’s
low-sloped roofing products still exclude damages from ponding water
conditions.
3 We will not get into the structural references in this statement in this
article.
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