Review of Gutter Systems
and Fixing Methods
This document contains the findings from two earlier reports:
1. ADDENDUM to Review of Gutter Systems and Fixing Methods: 29 May 2008
2. Review of Gutter Systems and Fixing Methods: 10 April 2008
These documents should be read in conjunction.
Chief Investigator: Dr Shaun Manning (PhD, B.Engineering)
24 June 2008
ADDENDUM Review of Gutter Systems and Fixing Methods pdf Dr S. Manning 10 April 08
DATE: 29 May 2008
2
ADDENDUM to
Review of Gutter Systems
and
Fixing Methods Chief Investigator: Dr Shaun A. Manning (PhD, B.Engineering)
Newcastle Innovation Limited –
Industry Development Centre
University Drive
Callaghan NSW 2308
Australia
Date prepared: 10 April 2008
Addendum prepared: 29 May 2008
ADDENDUM Review of Gutter Systems and Fixing Methods pdf Dr S. Manning 10 April 08
DATE: 29 May 2008
3
Executive Summary
After circulation of the report entitled - Review of Gutter Systems and Fixing Methods - dated 10 April the Author was alerted to two Planning Circulars:
1. P&D Circular Number 1/2008 ( 7 March 2008) published by the Committee on
Uniformity and Plumbing and Drainage Regulations in NSW (CUDPR) entitled:
“Requirements for the installation of eaves gutters”
2. Planning Circular no. BS 08-001 (10 April 2008) published by the NSW
Department of Planning entitled – “High Front Gutters”.
P&D Circular Number 1/2008 reminds plumbers and roofing plumbers that “the
requirements for Roof Drainage Systems – Design and Installation, which are contained
in AS/NZS 3500.3 (2003) Section 3 and 4 …must be complied with.”
One of the important requirements set out in AS/NZS 3500.3 (2003) is stated in Clause
3.5.1:
“Eaves gutter systems, including downpipes, shall be designed and installed in
accordance with Clause 3.2 so that water will not flow back into the building”
The P&D Circular also directs the reader to examples of overflow provisions set out in
AS/NZS 3500.3 (2003) Appendix G. As explained in the original report - Review of Gutter Systems and Fixing Methods - dated 10 April - examples given in Appendix
G show how compliance with overflow provisions can be met with all examples
involving full-length (continuous) overflow measures. Non-continuous overflows are
not recognised in the Standard because non-continuous overflows such as slotted
gutters may not be adequate, as the BCA 2007 advises that:
“in heavy downpours a slotted gutter may be inadequate.”
Planning Circular no. BS 08-001 was published to “raise awareness and remind
practitioners of the regulatory provisions applying to the design and installation of
gutters”. This circular was released because “issues have been raised in relation to
water penetration in certain houses that have been fitted with high front gutters”
ADDENDUM Review of Gutter Systems and Fixing Methods pdf Dr S. Manning 10 April 08
DATE: 29 May 2008
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This Planning Circular directs the target reader (all building practitioners and certifying
authorities) to requirements set out in the BCA 2007 Vol.2 part 3.5.2. Notably, the reader
is reminded of the BCA 2007 Compliance and Performance Requirements (Part 3.5.2)
and Deemed to Satisfy Provisions with direct reference to AS/NZS 3500.3 (or AS/NZS
3500.5 2000). These were the provisions assessed for high front gutters in the original
report - Review of Gutter Systems and Fixing Methods date 10 April-. It was
concluded in the original report that:
“results indicate that during heavy rainfall events or if a blockage occurs, slotted gutters
cannot be relied upon to prevent water overflowing the gutter. In the case of high fronted
gutters, without appropriate continuous overflow provisions, the water would be directed
back into the building in breach of the BCA 2007”.
Concerns with current gutter design and installation practices have been raised
concurrently in the two circulars and in the original report. Each if these publications
were issued to remind manufacturers, installers and authorising bodies of their
obligations to adhere to the legislative requirements stated in the BCA 2007 and in
AS/NZS3500.3. The risks to property resulting from the use of high front gutters,
particularly when attached to fascia with spring clips, can only be addressed when
designers, manufactures and installers discontinue current practices and begin
conforming to the legislative requirements of the BCA 2007.
Review of Gutter Systems and
Fixing Methods Chief Investigator: Dr Shaun A. Manning (PhD, B.Engineering)
Newcastle Innovation Limited –
Industry Development Centre
University Drive
Callaghan NSW 2308
Australia
Date: 10 April 2008
Review of Gutter Systems and Fixing Methods pdf Dr S. Manning 10 April 08 5
Review of Gutter Systems and Fixing Methods Executive Summary Current eaves gutter systems were assessed against requirements for overflow
measures and fixing methods as set out in the Building Code of Australia (BCA) 2007
and The NSW Code of Practice for Plumbing and Drainage, 3rd Edition 2006. Provisions
given in Australian/New Zealand Standards referred to in these Codes were also
reviewed. Two additional reports were drawn on for insight, one that reviewed the
efficacy of installing slots into the front face of high fronted gutters, and another that
reviewed the corrosion potential of zinc coated spring clips (which are commonly used to
attach high fronted gutters to metal fascia). The major findings can be summarised as:
(a) Gutter systems must be designed with appropriate overflow measures to ensure
water does no pool against, damage or enter a building when a gutter overflows,
whether by excessive rainfall or from a blockage.
(b) Examples given in the Standards of possible designs for overflow measures all
involve full length (continuous) overflow.
(c) Slotted gutters are not recognized in the Australia Standards due to reservations in
their ability to adequately divert overflowing water. This inadequacy has also been
confirmed by fluid mechanics modeling
(d) An assessment of common methods of fixing high fronted gutters to fascia revealed
that gutters attached with spring clips are unable to direct overflowing water away from
the building when a blockage occurs. This method of gutter attachment does cannot
comply with BCA requirements for providing overflow measures when required.
(e) Departure from the requirements established in the BCA 2007 (which includes
compliance with relevant Standards) is not permitted without appropriate verifications.
(f) The practice of electroplating spring clips does not meet the Standard’s requirement
and is unlikely to ensure satisfactory service for the design lifetime of the drainage
installation.
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Background Guttering
Eaves gutters come in many proprietary designs. Recent trends have leaned toward the
manufacture and supply of “high front” gutters. High front gutters are popular as they are
seen to improve the street appeal of the gutter systems. High front gutters are available
in many styles and are manufactured in slotted and unslotted forms.
Fascia
Fascia is available in timber and pre-painted metal. Metal fascia has become
increasingly popular in domestic installations due to its ease of installation, neat
appearance and because it needs no priming or painting. These considerations have led
to metal fascia becoming more common than timber fascia in modern domestic
dwellings.
Fixing methods for guttering to fascia
Manufacturer’s supply accessories and generally recommend fixing methods of
attaching guttering to fascia. The accessories available for timber fascia are external or
internal brackets to suit each gutter type. The accessories for metal fascia are spring
clips used in conjunction with over-straps. The design of the spring clip requires the clips
to be attached over the top of the fascia, when the guttering is installed, it is held tight
and high on the fascia. These fixing methods are common to all manufacturers.
Overflow Measure
“Measure to divert water from flowing back into a building from a blockage anywhere
along or at the outlet of an eaves gutter” AS/NZS 3500.3.2003.
Average recurrence interval (ARI)
“The expected or average interval between events of a rainfall intensity of a given
magnitude being exceeded.” AS/NZS 3500.5.2000.
References
AS/NZS 3500.3.2003 Storm water drainage
AS/NZS 3500.5.2000 Domestic installations
AS/NZS 2179.1.1994 Specification for rainwater goods, accessories and fasteners
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BCA 2007 requirements –Overflow Measures The BCA Part 3.5.2 describes the required design methods for the design and
installation of eaves gutters. A number of quotes taken directly from the BCA give insight
into design requirements and the intent of the Code.
Part 3.5.2.0
“Performance Requirement P2.2.1 is satisfied for gutters and downpipes if they are
designed and constructed in accordance with AS/NZS 3500.3. – Stormwater drainage,
or AS/NZS 3500.5 – Domestic installations, Part 5 – Stormwater drainage.”
By invoking this requirement, the design and construction of eaves gutters must comply
with the specified standards. However, if an alternative solution is proposed that does
not follow the design requirements of Part 3.5.2 then it still must comply with the
performance requirement P2.2.1 and the relevant performance requirements set out in
accordance with P1.0.10.
PART 3.5.2.3
“The size of guttering must –
(a) be in accordance with Table 3.5.2.2; and
(b) be suitable to remove rainwater falling at the appropriate rainfall intensity listed in
Table 3.5.2.1 as follows-
(i) for eaves gutters – 20 year average recurrence interval ..”(ARI)
This part of the BCA describes how to design the size of guttering to cope with the
expected 1 in 20 year high rainfall event. Further guidance is given in AS/NZS
3500.3:2000 which states:
“Note: For Australia Table 3.1 should be used in conjunction with the BCA, which
has requirements to prevent rain and stormwater from roof drainage from entering
certain buildings” and “The ARI’s for eaves gutters given in Table 3.1 assume the
provision of appropriate overflow measures.”
As described in the Explanatory information in the BCA 2007, this design method does
not protect the guttering from overflowing if rainfall intensity is sufficient. The explanatory
information states:
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“Stormwater drainage systems specified in the Housing Provisions are not designed to
remove all the water during exceptionally heavy rain, especially in tropical areas.
Accordingly, it is necessary to design and install the system so that when
overflowing occurs any water is directed away in a manner which ensures it does
not pond against, or enter into, the building.”
This part of the BCA 2007 clearly indicates that specified design methods for gutter
systems are not intended to prevent overflowing during extreme weather. Furthermore,
when overflowing does occur, the water must be directed away from the building to
prevent damage to the dwelling.
The explanatory notes go on to state:
“This (directing water away from the building) may be achieved by using slotted gutters,
oversized gutters and downpipes, locating the gutter so that it is below the top of the
fascia or the installation of rainwater heads with overflow slots”………………”There are
many options available to designers using the requirements of the Housing Provisions.
The designer will need to choose an overflow system that will cope with the expected
rain intensity, ie in heavy downpours a slotted gutter may be inadequate.”
Two comments can be drawn from this quotation:
1. the design of the spring clip prevents high fronted gutters from “locating the gutter so
that it is below the top of the fascia”
2. In this part, the BCA 2007 acknowledges a common practice of installing slots in to
the front face of high front gutters but casts doubt on this design’s effectiveness of
meeting the requirement that “ when overflowing occurs any water is directed away
in a manner which ensures it does not pond against, or enter into, the building.”
See Performance of gutter slots (Appendix A)
The BCA 2007 relies on further detail to be provided in the referenced Australian/New
Zealand Standards. However, before reviewing the Standards, it is important to note that
the BCA at clause AO.5 ‘Meeting the Performance Requirements’, requires that
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compliance with the Performance Requirements can only be achieved by complying with
the Deemed-to-Satisfy provisions or formulating an Alternative Solution which complies
with the Performance Requirements or is shown to be at least equivalent to the Deemed-
to-Satisfy provisions or a combination of both aspects.
The BCA states that Assessment Methods must be followed in developing any
Alternative Solution to determine that a Building Solution complies with the Performance
Requirements.
These include:
1. Evidence to support that the use of a material, form of construction or design
meets a Performance Requirement or a Deemed-to-Satisfy Provision; and/or
2. Verification Methods such as the Verification Methods in the BCA or others that
the local government accepts; and/or
3. Comparison with the Deemed-to-Satisfy Provisions; and/or
4. Expert Judgment.
In order to comply with all sections of the BCA it is necessary to adhere to the following
method when determining whether an Alternative Solution meets all the relevant BCA
Performance Requirements by:
1. Identifying the relevant Deemed-to-Satisfy provisions of each Section or Part that
is to be the subject of the Alternative Solution; and
2. Identifying the Performance Requirements from the same Sections or Parts that
are relevant to the identified Deemed-to-Satisfy provisions; and
3. Identifying Performance Requirements from other Sections and Parts that are
relevant to any aspects of the Alternative Solution proposed or that are the
application of the Deemed-to-Satisfy provisions that are subject of the Alternative
Solution
Part 1.1.2 of the BCA 2007 states that:
“Where a Deemed-to-Satisfy Provision adopts a Standard, rule, specification or provision
included in any document issued by Standards Australia or other body, that adoption
does not include a provision –
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(e) Permitting departure from the code, rule, specification or provision at the sole
discretion of the manufacturer or purchaser, or by any arrangement or agreement
between the manufacturer and purchaser.”
Alternative solutions to those specifically mentioned in the Housing Provisions outlines in
the BCA 2007 are permitted, but only if the following is satisfied (Part 3.3):
“Alternative solutions may we used provided they comply with the Performance
requirements listed in Part 2 (for further explanation see part 1.0).”
Part 1.0.8 states:
(a) “An Alternative Solution must be assessed according to one or more of
the Assessment Methods.
(b) An Alternative Solution will only comply with the BCA if the Assessment
Methods used to determine compliance with the Performance Requirements
have been satisfied.
(c) The Performance Requirements relevant to an Alternative Solution must
be determined in accordance with 1.0.10.”
Part 1.0.9 states:
“The following Assessment Methods, or any combination of them, can be
used to determine that a Building Solution complies with the Performance
Requirements:
(a) Evidence to support the use of the material, form of construction
or design meets a Performance requirement or a Deemed to
Satisfy Provision as described in 1.2.2.
(b) Verification methods such as:
(i) the Verification Methods in the BCA; or
(ii) such other Verification Methods as the appropriate
authority accepts for determining compliance with the
performance Requirements
(c) Comparison with the Deemed-to-Satisfy Provisions.
(d) Expert Judgment.”
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The BCA 2007 clearly indicates that deviation from performance requirements must be
demonstrated to satisfy the verification rules outlined in Part 1.0.9.
Summary As established in the BCA 2007, when gutters overflow the water must be directed away
from the building to prevent damage to the building. Part 1.1.2 reiterates the need for
compliance with AS/NZS 3500.3:2003 and AS/NZS 3500.5:2000 in order to satisfy the
performance standards required by BCA 2007. Departure from the requirements
established in the BCA 2007 (which includes compliance with relevant Standards) is not
permitted without appropriate verifications.
The NSW Code of Practice for Plumbing and Drainage, 3rd Edition 2006 The code was prepared by the Committee on Uniformity of Plumbing and Drainage in
New South Wales (CUPDR) to provide uniform administrative and technical
requirements for the installation of plumbing and drainage in NSW.
This Code replaces the New South Wales Code of Practice; Plumbing and Drainage,
Edition No 2 July 1999.
This Code adopts the National Plumbing and Drainage Code AS/NZS 3500:2003 and
amendments and Part 5:2000, with New South Wales variations and additional
provisions as set out in this Code.
Requirements of AS/NZS 3500.5:2000 (DOMESTIC INSTALLATIONS)
The gutter and downpipe design methods given in BCA 2007 are also included in
AS/NZS 3500.5:2000, with additional detail. In Clause 5.5.1 the Standard states:
“Roof drainage systems are designed in respect to the risk of monetary loss, property
damage (including contents of buildings) and injury to persons due to overtopping. A
frequent cause of such overtopping is inadequate inspection and cleaning and not the
intensity of rainfall.”
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Clause 5.5.1 -
“Roof Drainage systems shall comply with the following:
(C) Be designed in accordance with-
“(i) eaves gutter systems (eaves gutters and associated downpipes with
appropriate over flow devices) method of Clause 5.5.4.”
In this section, the Standard has acknowledged the potential risks of overtopping.
Furthermore, it is noted that even during modest rainfall events, conditions exist that can
produce overtopping. Examples of such conditions are given as:
Clause 5.5.1-
“Notes-2 The design methods do not include an allowance for the following:
(b) Blockages of roof drainage systems, e.g. by snow, hail and debris”
Clause 5.5.4.1-
” Eaves gutters shall be designed so that they will not overflow with flows resulting from
a storm burst with an average recurrence interval (ARI) of not less than 20 years in
Australia” and “Appropriate measures shall be taken, if necessary, to ensure that
overflowing water will not flow into the building but will fall to the ground”
Clause 5.5.4.2-
“Method for design of eaves gutters and downpipes with appropriate overflow measure
shall be as follows:
“STEP 1 Determine 5min/20 year ARI”
Note that the ARIs for eaves gutters given in Table 3.1 assume the provision of
appropriate overflow measures - AS/NZS 3500.3.2003.
STEP 3 Select eaves gutter and the gutter slope
Note that Clause 5.5.4.4 states:
“The selection of eaves profiles shall suit the type of eaves construction. Where
stormwater from overflowing eaves gutters can flow over the eaves lining and back into
the building, overflow measures shall be required.”
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STEP 9 Select an overflow measure if required-
Note that Clause 5.5.4.3(f) explains Step 9 as:
“Blockages may occur anywhere along an eaves gutter causing overtopping which
would not be prevented by an overflow device at the outlet of an eaves gutter. An
overflow method may be selected to suit building design or gutter shape. See Figure 5.3
for examples of overflow measures for eaves gutters for use where overflows may cause
significant damage. “
Figure 5.3 provides examples of overflow measures suitable to a variety of gutter styles.
All of the overflow measures suggested in Figure 5.3 provide for full length (continuous)
overflow in order to provide sufficient overflow capacity to remove water away from the
building during heavy storms or blockages.
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Summary AS/NZS 3500.5:2000 The standard, consistent with the BCA 2007, requires that” the
roof drainage system is designed so that any overflow during heavy rain periods is
prevented from flowing back into the building”. The design guidelines above incorporate
appropriate overflow measures as part of the design criteria. The method of overflow
required is dependant on the gutter type selected.
Requirements of AS/NZS 3500.3:2003 (Stormwater Drainage) This standard further qualifies the requirements for overflow measures in the design and
installation of gutters. Clause 3.2 states:
“The general method assumes regular inspection and cleaning (see Clause 4.6) and is
applicable to-
(a) eaves gutters and associated vertical downpipes with appropriate
overflow measures (see Clause 3.5)
NOTES: 1 The general method does not include allowance for any of the following:
(b) Blockages of roof drainage systems, e.g., by snow, hail and debris.”
Clause 3.3.1
“Roof drainage systems are designed in respect to potential monetary loss, property
damage (including contents of buildings) and injury to persons due to overtopping.
NOTES:. 1. A frequent cause of such overtopping is inadequate inspection and cleaning
(see Clause 4.6) and not to the intensity of rainfall.”
Clause 3.3.4
“The ARI (Average Recurrence Interval) shall be given in Table 3.1
NOTE: For Australia Table 3.1 should be used in conjunction with the BCA, which has
requirements to prevent rain and stormwater from roof drainage from entering certain
buildings.”
Clause 3.5.1
–“ Eaves gutter systems, including downpipes, shall be designed and installed in
accordance with Clause 3.2 so that water will not flow back into the building”
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Cause 3.5.2 –
“ The design procedure shall follow the general method for design of eaves gutters
systems flow chart, in Figure 3.4 (l) Select over flow measures in accordance with
Clause 3.5.
NOTE: An example of the application of the design procedure is given in Appendix H.”
Clause 3.5.3 –“NOTE: Examples of overflow measures for eaves gutters are given in
Appendix G.”
Appendix G provides examples of overflow measures suitable to a variety of gutter
styles. All of the overflow measures suggested in Appendix G provide for full length
(continuous) overflow in order to provide sufficient overflow capacity to remove water
away from the building during heavy storms or blockages. The examples in Appendix G
are consistent with the examples given in AS/NZS 3500.5:2000 Figure 5.3.
Summary AS/NZS 3500.32003, consistent with AS/NZS 3500.5:2000, provides further guidance on
how to comply with the BCA performance requirements. Examples given in Appendix G
on how compliance with overflow provisions can be met all involve full length
(continuous) overflow measures.
OBSERVATIONS
The design methods for gutters and downpipes given in the BCA 2007 and the
Standards are designed to prevent water flowing back into a building and causing
damage to the building. The design method assumes that the gutters are regularly
cleaned to prevent blockages. In this way, the gutters are designed to collect and convey
rainwater except in exceptional rainfall events. However, the Standards recognize that
blockages do occur, and exceptional rainfall events also occur. The BCA 2007 states
that stormwater drainage systems are not designed to remove all of the water during
exceptionally heavy rain. The BCA states that:
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“it is necessary to design and install the system so that when overflowing occurs any
water is directed away in a manner which ensures it does not pond against, or enter into,
the building.“
AS/NZS 3500.5:2000 states the “Where stormwater from overflowing eaves gutters can
flow over the eaves lining and back into the building, overflow measures shall be
required.”
The design methods outlined in the Standards include the provision of overflow
measures to ensure that in extreme rainfall events and when blockages occur, the intent
of the BCA 2007 is satisfied.
Further clarification is found in AS/NZS 3500.5.2000 Figure 5.3 and AS/NZS
3500.3:2003 Appendix G which provide examples of acceptable overflow measures. All
of these measures provide full-length (continuous) overflows. This is to ensure that water
can escape at a sufficiently high rate and to ensure anywhere overtopping occurs, water
will not enter the building.
It is noted in Appendix G that:
“Blockages can and do occur anywhere along an eaves gutter causing overtopping that
would be affected by an overflow device located at the outlet of an eaves gutter
e.g.rainhead (see Figure 3.7(a)). The overflow devices given in Paragraph G2 are
located along an eaves gutter so that any overtopping is unlikely to cause monetary loss
of property damage including damage to contents of buildings. The ARIs for eaves
gutters given in Table 3.1 assume the provision of appropriate overflow measures.”
As stated in Appendix G, it is assumed that the general design method based on ARIs
(average recurrence intervals) includes appropriate overflow measures to ensure that
when blockages occur, the water is directed away from the building.
It is clear from the assessment of the BCA 2007 and the referenced AS/NZS Standards
that overflow protection is needed to protect buildings from water damage in all cases
where there is a risk of property damage. This is confirmed as the Standards and the
BCA design methods for gutters use the ARI values including allowance for appropriate
overflow measures.
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Summary of BCA 2007 Requirements for Overflow Measures The preceding assessment has identified a number of requirements for eaves-gutters
which can be summarised as:
1. Gutter systems must be designed and installed using appropriate overflow
measures to ensure water cannot pond against or flow into a building.
2. Overflow measures designed in accordance with AS/NZS 3500.5.2000 and
AS/NZS 3500.3:2003 involve continuous overflows, since blockages can occur
at any point on the gutter. In all examples given in the Standards, overflowing
water is directed away from the building.
Additional requirements for gutter fixing components
3. Roof drainage system components made from aluminium alloys,
aluminium/zinc alloy-coated steel, copper, copper alloys, zinc-coated steel and
zinc shall comply with AS/NZS 2179.1.
4. 3500.5 Clause 5.3.1 “The selection and use of materials and products shall
ensure satisfactory service for the design lifetime of the Stormwater drainage
installation”.
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Assessment of common eaves-gutter designs for compliance with BCA
2007
In order to assess compliance of an eaves-gutter system with the BCA 2007
requirements, the following tests can be applied:
1. Does the system have an overflow measure designed such that whenever
overflowing of a gutter occurs for any reason, is all the water directed away
from the building in a manner compliant with AS/NZS 3500.5.2000 Figure 5.3
and AS/NZS 3500.3:2003 Appendix G.
2. Are roof drainage components designed in accordance with ASA/NZS
2179.1:1004 and be compatible, and ensure satisfactory service for the design
lifetime of the drainage installation
There are many gutter designs on the market, but the most common design are high
fronted quad style gutters also referred to as D gutters which are installed by either
external brackets, internal brackets or spring clips with over-straps. These systems
will be assessed in turn.
1. Overflow measures:
High Fronted Gutters attached to fascia with external or internal brackets Due to the front of the gutter being higher than the back, overflowing water will be
directed into the building unless an appropriate overflow measure is adopted. With
reference to AS/NZS 3500.5.2000 Figure 5.3 or AS/NZS 3500.3:2003 Appendix G,
the three methods for high fronted gutters attached with brackets are to:
(a) Pack the brackets away from the fascia to provide 10 mm continuous weir
at the back off the gutter.
(b) Attach the brackets low on the fascia so that the front of the gutter is a
minimum 20mm freeboard (figure 5.3 domestic installations) lower than the
top of the fascia at all points. Or calculate the amount of freeboard
required to comply with details in Appendix G.
(c) Provide continuous flashing to the rear of the gutter with the top edge of
the flashing not less than the required freeboard in accordance with the
standards (20mm domestic installations).
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High Fronted Gutters attached to metal fascia with spring clips and over-straps As stated, due to the front of the gutter being higher than the back, overflowing water
will be directed toward the building unless an overflow measure is adopted. Unlike
brackets, spring clips are designed to hold the gutter tightly against and at the top of
the fascia. This fixing method cannot be installed with any of the three continuous
overflow measures listed above for high front gutter. With spring clips the option of:
(a) Packing the gutter to provide a 10 mm weir gap is impossible as the spring
clip design requires the gutter to be held tight against the fascia.
(b) Installing the gutter lower so that the front bead of the gutter will be below
the top of the fascia is impossible. The spring clip is fixed over the top
edge of the fascia, the short length of the spring clip does not allow for
fixing the gutter with the required freeboard (20 mm domestic installations)
and fall.
(c) Providing a continuous flashing to the rear of the gutter is impossible as
the spring clip is attached over the top of the fascia making it impossible
for flashing to be installed.
Without providing an appropriate overflow measure, high front gutters attached with
spring clips cannot satisfy the requirements of the BCA 2007.
High front gutters with slots Gutter manufactures recognize the need for overflows and offer the option of slots in
the front face of high front gutters to act as the overflow measure to remove excess
water. The BCA 2007 makes note of this method but advises that:
“in heavy downpours a slotted gutter may be inadequate.”
The Standards do not recognize slotted gutters as a suitable overflow method. However,
tests were conducted to assess the effectiveness of slotted gutter systems. Newcastle
Innovation conducted an assessment on a number of slotted gutter profiles obtained
from NSW manufacturers. The assessment involved applying hydrodynamic principles of
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fluid flow using the Bernoulli equation. Results from this report are given in Table 1, with
the full report attached.
Table 1 Measured properties of tested gutter
Gutter Brand Slot Length (mm)
Slot Width (mm)
Gap between neighbour slots (mm)
Slot Area (mm2) per hole
ACE 17 1.5 120 25.5
STRATCO 22 2 80 44
LYSAGHTS 45 3 110 135
STRAMIT 50 2 90 100
Table 2 Flowing rates (L/s) for various gutter brands
Gutter Brand Flowing Rate (L/s)
For slots every 1 metre gutter
Flowing Rate (L/s) For 10 mm gap
every 1 metre gutter
Flow through slots compared to flow over required 10 mm gap
every 1 metre gutter
ACE 0.078 6.264 1.24%
STRATCO 0.181 6.264 2.89%
LYSAGHTS 0.366 6.264 5.83%
STRAMIT 0.300 6.264 4.79%
It can be seen from Table 2 that the calculated flowrates of overflow water through the
slots in the front face of proprietary slotted gutter systems is between 1.24% to 5.83% of
the flowrate capable of being removed by other recommended overflow measures. The
assessment was made assuming a 20 mm head of water above the slots. The
recommended system used in the comparison in Table 1 is shown in Figure 1. Note that
if the 10 mm gap were not provided, the water would overflow the fascia toward the
building.
Review of Gutter Systems and Fixing Methods pdf Dr S. Manning 10 April 08 21
Figure 1: Example of 10 mm gap to provide continuous over flow as described in AS3500
The relatively poor overflow capacity of slotted gutter systems (all tested gutters were
high front) explains the comments in the BCA 2007 ie “in heavy downpours a slotted
gutter may be inadequate.” The results given in Table 2 confirm that slotted gutters are
unlikely to satisfy the BCA requirement that “when overflowing occurs any water is
directed away in a manner which ensures it does not pond against, or enter into,
the building.”
Standards Australia does not recognize slotted gutters as an acceptable overflow
measure. These results indicate that during heavy rainfall events or if a blockage occurs,
slotted gutters cannot be relied upon to prevent water overflowing the gutter. In the case
of high fronted gutters, without appropriate continuous overflow provisions, the water
would be directed back into the building in breach of the BCA 2007.
Fixing accessories: Compliance AS/NZS 2179.1.1994 The Standards require that roof drainage components are designed in accordance
with ASA/NZS 2179.1:1004 and be compatible, and ensure satisfactory service for
the design lifetime of the drainage installation.
The common practice of using aluminium/zinc alloy-coated steel plain or pre-painted
steel shall comply with AS1397 with a minimum Class AZ150. Zinc-coated steel shall
Review of Gutter Systems and Fixing Methods pdf Dr S. Manning 10 April 08 22
comply with AS1397 with a minimum coating of Class Z275. Brackets must be
manufactured to these specifications in order to be compliant with AS/NZS
2179.1:1994.
Internal and External Brackets
External and internal brackets have been used for many years with success the
method of manufacture appear to satisfy the appropriate standards. Brackets are
manufactured using aluminium/zinc alloy-coated steel or zinc-coated steel.
Spring Clip with Overstrap
Spring clips with overstraps are a relative recent method of fixing gutters to fascia,
and has grown in popularity with the increasing use of metal fascia.
The common practice of using aluminium/zinc alloy-coated steel for overstraps
complies with AS1397 with a minimum Class AZ150 and satisfies AS/NZS
2179.1:1994.
Spring clips are manufactured using spring steel with zinc electroplating. An
assessment was made on whether spring clips satisfy requirements set out in
AS/NZS 2179.1:1944. The common practice of zinc electroplating spring clips does
not comply with AS1397 which requires a minimum coating of Class Z275. In this
way, the manufacturing process for spring clips does not comply with AS/NZS
2179.1.1994 (see Evaluation of Gutter Fixtures).
Zinc electroplating is not recognized in AS/NZS 2179.1.1994 as a suitable coating for
rainwater goods and accessories. Therefore this method of coating spring clips fails to
meet the requirements of the standards and the Codes.
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SUMMARY
The use of the spring clip as a fixing method does not meet the requirements of the
following Standards: AS/NZS 3500.5.2000, AS/NZS 3500.3.2003, AS/NZS 2179.1.1994
and AS 1397. The failure to meet the requirements of the Standards prevents the spring
clip fixing method from complying with the NSW Code of Practice for Plumbing and
Drainage, 3rd Edition 2006. The spring clip fixing method does not meet the Performance
Requirements or comply with the Deemed-to-Satisfy Provisions of the BCA 2007. The
practice of electroplating the spring clip does not meet the Standards requirement and is
unlikely to ensure satisfactory service for the design lifetime of the drainage installation
(see Evaluation of Gutter Fixtures). These factors demonstrate that the spring clip is
not fit for purpose and therefore should not be used as a method of installing gutters..
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APPENDIX A
PERFORMANCE OF GUTTER SLOTS COMPARED TO AS/NZS REQUIREMENTS
Chief Investigator: Dr Shaun A. Manning (PhD, B.Engineering)
Newcastle Innovation Limited –
Industry Development Centre
University Drive
Callaghan NSW 2308
Australia
Date: 10 April 2008
Client: Jason Higgins
Jason Higgins Plumbing
55 Cumberland St Cessnock
NSW 2325
Date: 3 March 2008
Review of Gutter Systems and Fixing Methods pdf Dr S. Manning 10 April 08 25
Performance of Gutter slots compared to AS/NZS Requirements & NSW Plumbing and Drainage Code of Practice & Building Code of Australia Four gutter profiles, containing slots in the front face, were assessed for ability to
discharge rainwater. The gutters under test were manufactured by:
1. Ace Gutters
2. Stratco
3. Lysaghts
4. Stramit
Performance was based on the ability of the gutter systems to discharge rain
water through the provided slots, compared to the ability of the gutter to
discharge rain water via a 10 mm gap at the rear of the gutter. Examples of gap
requirements for continuous overflows are given in AS/NZS 3500.3 2003 and
AS/NZS 3500.5 2000 (Figure 1).
The basis of calculation assumed a water level in the gutter 20 mm above the
slots, reflecting conditions where the fascia extends 20 mm higher than the back
of the gutter. Such water depths in the gutter could arise if the downpipes were
blocked (leaf debris, hail etc.).
The calculation procedure involved application of the Bernoulli fluid mechanics
equation. A correction factor of 0.67 was applied to the slot calculations to allow
for flow effects at the edges of the slots. A factor of 1 was applied for the 10 mm
gap calculations. Slots were treated as being rectangular due to the relatively
large length to height ratios. All predicted rainfall discharge calculations are
reported as litres/second (L/s).
Data used in the calculations and final results are given in Table 1. It can be seen
from Table 1 that the flowrates through the slots in each gutter system are
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significantly lower than (well less than 10%) the predicted flowrate of water flow
over a 10 mm gap as required by the Standards. For the conditions simulated in
this study, the slots in the front face of the gutters are shown to be ineffective in
discharging rainfall overflow when compared to the 10 mm gap at the rear of the
gutter specified in the Standards.
Table 3 Measured properties of tested gutter
Gutter Brand Slot Length (mm)
Slot Width (mm)
Gap between neighbour slots (mm)
Slot Area (mm2) per hole
ACE 17 1.5 120 25.5
STRATCO 22 2 80 44
LYSAGHTS 45 3 110 135
STRAMIT 50 2 90 100
Table 4 Flowing rates (L/s) for various gutter brands
Gutter Brand Flowing Rate (L/s)
For slots every 1 metre gutter
Flowing Rate (L/s) For 10 mm gap
every 1 metre gutter
Flow through slots compared to flow over required 10 mm gap
every 1 metre gutter
ACE 0.078 6.264 1.24%
STRATCO 0.181 6.264 2.89%
LYSAGHTS 0.366 6.264 5.83%
STRAMIT 0.300 6.264 4.79%
Review of Gutter Systems and Fixing Methods pdf Dr S. Manning 10 April 08 27
Figure 1: Example of 10 mm gap to provide continuous over flow as
described in AS3500 Required Design Method for Eaves-Gutters
The 2007 and associated Standards indicate intentions and mandate design and
installation procedures for gutters to ensure rainfall does not enter and damage
buildings. It is stated in Part 3.5.2 of the BCA 2007:
“when overflowing occurs any water is directed away in a manner
which ensures it does not pond against, or enter into, the
building.”
The BCA 2007 gives no detail on how to comply with this intent and relies on
guidance given in AS/NZS3500.3:2003 and AS/NZS3500.5:2000.
The design procedure for Eaves-Gutter systems is given in AS/NZS 3500.3:2003
Clause 3.5.3 (Figure 4) where the gutter system is designed to prevent
overtopping based on an expected 1:20 year rainfall event. That is, assuming a
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gutter is free of blockages it should not overtop more frequently than once each
20 years. Recalling that the BCA 2007 states that whenever overtopping occurs,
the system should direct water away from the building, the design method in
Figure 4 requires overflow measures to be added to in all cases where
overtopping occurs because:
AS/NZS3500.5:2000 Clause 5.5.4.1 states:
“Appropriate measures shall be taken, if necessary, to ensure that
overflowing water will not flow into the building but will fall to the
ground”
By following the design flow chart given in AS/NZS 3500.3:2003 Clause 3.5.3
(Figure 4) the designer is required to:
“Select over flow measures in accordance with clause 3.5”
Clause 3.5.1 states:
“Eaves gutter systems, including downpipes, shall be designed
and installed in accordance with Clause 3.2 so that water will not
flow back into the building.
Clause 3.2 states:
“The general method assumes regular inspection and cleaning
(see Clause 4.6) and is applicable to-
eaves gutters and associated vertical downpipes with appropriate overflow
measures (see Clause 3.5)”
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