THE LEGAL WAYS TO PROVIDE A TIMBER ROOF STRUCTURE

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

http://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.


Erection & Bracing

Prefabricated

Trusses


Causes of Failure in Timber Roofs


Poor Erection is the dominant

cause of Failure

This presentation shows

common problems and

their solutions


Stacking on Site

Don’t let the

grass grow


Stacking on SiteStack like this and

trusses can warp,

twist, deform ….


Stacking on Site

…and will never be

straight


Stack RightPack on level ground

Neat stacking enables profile

checks prior to erection


Handling


Handling


Handling


Lift Large Trusses in Braced

Sections

Poor practice, no

strongback, damage

likely

Solution -Pairs of

trusses, braced on

the ground


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.


Temporary Bracing

• Temporary Bracing is used to restrain the first

trusses erected until sufficient are in place for

the permanent bracing to be fixed.


Adequate Temporary Bracing

Unbraced rafters

buckle under own

weight


Hold Trusses Straight

& Vertical


Erect from a braced bay


Rafter Buckling

Without Diagonal Cross Bracing

Rafters Buckle together


Top Chords can buckle despite

frequent Purlins or Battens


Resistance of Compression

Members

Strengt

h

Strength increases greatly with more lateral restraint


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


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.


Rafter Bracing - connections

Top Chord Brace

Block

Top Chord Brace bolted

To shelf fitted between

Trusses & soundly connected


Cantilever Bracing

Cantilever Tie

Beams must be

soundly braced

with cross

braced runners


Design, Brace & Secure for Wind

Inadequately Braced Steel Structure


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

THE LEGAL WAYS TO PROVIDE A TIMBER ROOF STRUCTURE

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