the legal ways to provide a timber roof structure
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THE LEGAL WAYS TO PROVIDE A TIMBER ROOF
STRUCTURE
“DEEMED TO SATISFY” RULE AS PER THE NBR’s
NAILED & BOLTED (Site Made) ROOF TRUSSES
Use Table 4 – Part L of NBR
(a) Sizes
(b) Grades of Timber
specified for various spans and roof coverings:
Local Authority Inspects
“RATIONAL DESIGN” RULE
2.1 NAILED & BOLTED ROOF TRUSSES
If: • Span greater than 8 metres
• Hips and Valleys
• Not a Howe Configuration
• Pitch < 15° > 35° (refer table 4)
Then: • Owner appoints Designer to take responsibility
• Designer submits Completion Certificate to Local Authority
2.2 PREFABRICATED ROOF TRUSSES (NAIL PLATED)
• Owner appoints Designer to take responsibility
• Designer submits Completion Certificate to Local Authority
What a finished roof should
NOT look like!
Site application plates & ungraded timber
What a roof structure should
NOT look like!
What a roof structure
SHOULD look like!
Enjoy the peace of mind of a rationally designed
and safely erected and inspected roof structure
with Professional Indemnity Cover, to ensure the
structural stability of your lifetime investment,
together with the personal well-being of having a
secure roof over your head.
The ramifications of using roof trusses of inferior
quality, poor erection and inadequate inspection
procedures, are far-reaching.
Make it your policy to use SAFE timber roof
structures by ensuring that the trusses are
manufactured, erected and inspected by ITC-SA
Certificated Members and Certificated Inspectors.
from design to certification
Importance of A19 Inspection
The implications of using roof trusses of inferior quality, along
with poor erection and inadequate inspection procedures, are
far-reaching.
Make it your policy to use SAFE timber roof structures by ensuring
that the trusses you use are manufactured, erected and inspected by
ITC-SA Certificated Members.
COMPLETION CERTIFICATE REQUIRED
Issued by Registered Prof Engineer or Technologist
Authorities
Mandatory under section 14(2A) of the Act
NHBRC
Requires designer/engineer to provide Completion
certificate for a rationally designed roof (Part 3, section 4
of manual part 3)
Banking Institutions
Required by banking institutions
Resale of dwelling
(Appointment of person responsible for design, inspection,
completion)
“Where in terms of these regulations, a rational design for:-
The structural system of the building is to be submitted to the
LA, the owner of any building shall, appoint and retain any
person who is a professional engineer or other approved
competent person to undertake responsibility for each such
design, and also for inspection, during construction, of such
structural system”
INFORMATION ESSENTIAL FOR ROOF
INSPECTION
1. ROOF LAYOUT PLANS:
►The loading, truss centres and batten / purlin centres
►Position of trusses, beams and joints and braced bays
►The type of bracing and runners
►Erection details for hips and / or valleys
►Details of all cleats, hurricane clips and hangers
►Joint details
►Member sizes and grades
►Span, pitch, overhangs
2. BRACING DETAILS:
►ITC-SA Standard Details
►Special Bracing Details:
-These must be provided where non-standard bracing is required
The 7 Important Rules!
1. MAKE USE OF THE ITC-SA
ACCREDITED & AND REGULATED
VALUE CHAIN
Verify current certificate
www.itc-sa.org
2. Ensuring that Quality Material
is used in the manufacture of
the roof trusses.
3. Ensuring that imported
structural material is
acceptable by means of
accredited certification by
either SABS/SATAS
4. Ensuring that rational design
requirements are met
5. Ensuring that nail plates are fully
embedded in the timber, and
placed as per the rational design
prescriptions.
6. Ensuring that the Erector receives
comprehensive information in regards
to the design, placing and bracing of
the roof structure.
7. Ensuring that all roofs receive a
ROOF LOADING & ENGINEER CERTIFICATE
Volume 1
which is a pocket size edition, is
geared specifically towards the roof
erector, professional or amateur,
erecting roof trusses over a domestic
dwelling up to 9 metre span.
Roof Erectors Handbook
Volume 2
The Volume 2 Roof Erectors Handbook is a
comprehensive manual, which
Illustrates every standard aspect of erecting
and bracing pre-fabricated
timber roof trusses for almost every category
of timber roof
configuration and load.
Training DVD
Clear and concise pictures and graphic
illustrations indicate the rights and wrongs of
bracing installation and structural connections.
from design to certification
Erection & Bracing
Prefabricated
Trusses
from design to certification
Causes of Failure in Timber Roofs
from design to certification
Poor Erection is the dominant
cause of Failure
This presentation shows
common problems and
their solutions
from design to certification
Stacking on Site
Don’t let the
grass grow
from design to certification
Stacking on SiteStack like this and
trusses can warp,
twist, deform ….
from design to certification
Stacking on Site
…and will never be
straight
from design to certification
Stack RightPack on level ground
Neat stacking enables profile
checks prior to erection
from design to certification
Handling
from design to certification
Handling
from design to certification
Handling
from design to certification
Lift Large Trusses in Braced
Sections
Poor practice, no
strongback, damage
likely
Solution -Pairs of
trusses, braced on
the ground
from design to certification
Main Elements of Bracing
The bracing must:
• Keep trusses vertical
• Prevent rafter buckling
• Prevent buckling of long webs
• Prevent tie beam instability
• Stabilise walls where required.
Battens, Purlins
Runners & Brandering
Must be triangulated
To provide restraint.
from design to certification
Temporary Bracing
• Temporary Bracing is used to restrain the first
trusses erected until sufficient are in place for
the permanent bracing to be fixed.
from design to certification
Adequate Temporary Bracing
Unbraced rafters
buckle under own
weight
from design to certification
Hold Trusses Straight
& Vertical
from design to certification
Erect from a braced bay
from design to certification
Rafter Buckling
Without Diagonal Cross Bracing
Rafters Buckle together
from design to certification
Top Chords can buckle despite
frequent Purlins or Battens
from design to certification
Resistance of Compression
Members
Strengt
h
Strength increases greatly with more lateral restraint
from design to certification
Top Chord Bracing with Tile Battens
Continuous bracing member
With splice as required
Nailed to underside of rafters
At approximately 45 degrees
Connection to wall
from design to certification
Diagonal Braces
Diagonal braces should be fixed
at an angle of about 45 deg to the
line of the rafter. This angle can
be changed to between 35 and 55
deg to avoid obstructions such as
internal members and chimneys
or to enable fixing to the wall
plate.
Braces should run from eaves to
ridge and be nailed to each
trussed rafter as detailed by
designer.
from design to certification
Rafter Bracing - connections
Top Chord Brace
Block
Top Chord Brace bolted
To shelf fitted between
Trusses & soundly connected
from design to certification
Cantilever Bracing
Cantilever Tie
Beams must be
soundly braced
with cross
braced runners
from design to certification
Design, Brace & Secure for Wind
Inadequately Braced Steel Structure
from design to certification
Rafter / Purlin connections
Poor connections allow
roof sheets to work loose
High winds & progressive
failure will then remove
whole roof sections
Rafter / Purlin Connections
Hurricane Clips offer the best
connection between rafter and
purlin
Single Hurricane clips may be
used with Serrated nails.
Ridge , Barge and eaves
areas require double
Hurricane clips
Hold Down the Trusses
Nail securely
as specified
On construction details
Steel Strap built into
masonry or cast into
reinforced concrete.
Connections
Remedy on Site?
Any modification required on site must be specified
by the designer
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