The document discusses national specifications that must be followed for erected roof structures using timber trusses in South Africa. There are strict guidelines regarding the design, fabrication, materials used, nail plates, joints, quality verification, storage, transportation and handling of the trusses. Non-compliance with these standards could result in roof failure and significant costs to repair or replace the roof. Following all applicable standards helps ensure a properly designed and manufactured roof structure that has eliminated the risk of failure due to defects.

1. INDUSTRYINSIGHT
Your erected roof
structure:
Will your bank or insurance company assume the risk?
THE DESIGN AND FABRICATION
MUST BE ACCORDING TO NATIONAL
SPECIFICATIONS – PART 1
The manufacture and erection of timber roof trusses in South Africa
is governed by SANS 10243 and if the fabricator is one of the select
few that are permitted to apply the SANS 1900 mark of quality, the
consumer can rest assured that the quality assurance of the
manufactured product is of the highest national standard. But what
are the consequences if it is not?
There are only four accredited system software suppliers in
South Africa that are permitted to supply software and nail
plates for the manufacture of monoplaner timber trusses in
South Africa. Site manufactured trusses, nail plates or bolted
and nailed trusses with hips and valleys, are not permitted
unless the contractor can prove that each and every truss and
connection detail has been designed by an engineer who is
generally recognised as having the necessary experience and
training to undertake rational designs in the field of roofs and
roofing.
7 EASY STEPS TO CONFIRM
COMPLIANCE:
ACCREDITATION:
Check the ITC-SA Web site to confirm that the fabricator is
not only a current member of the ITC-SA, but more
importantly, that the fabricator was certified competent to
manufacture a roof which falls in his category of competence?
There are four categories of roof structures and these can be
obtained from the ITC-SA website (www.itc-sa.org).
Both the fabricator and designer must be competent and
professionally certified, and it is always advisable to obtain
proof of this.
TIMBER:
Only structurally graded timber which is either graded to S5
or S7 may be used in a roof structure. The use of "XXX" or
‘black cross’ timber is strictly prohibited, even in battens or
purlins. Unscrupulous retailers have even described black
cross timber as ‘triple strength,’ which could not be further
from the truth. Structural timber must be marked along its
entire length at one meter intervals with its applicable grading
and the name of the supplying mill.
The moisture content of the timber may not exceed 17% when
fabrication commences, irrespective if the timber is treated or not.
The maximum difference in thickness of members in a joint
shall not exceed 1.0mm and the edges must be clean cut with a
sharp blade.
Chipping or breaking out of the edges will reduce the strength
of the joint. [See illustration A.]
Defects in the timber, such as dead knots, wane, or resin
infiltration, are not permitted in the plate contact area unless
a larger plate is used to compensate for loss of penetrating
nails.
NAIL PLATES:
The size of each nail plate in a joint is specially designed for that
joint. Some software systems specify exact positioning which
is not necessarily the centre of the joint and this must be
checked with regards to the software system involved. Equally
critical is the tolerance allowed in the designed position of the
plate which, according to SANS0163-2, is currently 5mm,
although some software systems will allow 10mm in either
direction. In either case, the margin for error is extremely
small. [See illustration B.]
JULY 2014
By Lyndsay Cotton, General Manager of LCP Roofing and Chairman of the ITC-SA.
Illustration A.
Illustration B.

2. JULY 2014
INDUSTRYINSIGHT
The direction of the teeth placement in a nail plate is specifically
designed for, and a square plate, for example, may only be
positioned in one direction. Joints and plating thereof are not
permitted in webs.
Flattening of teeth is when any given tooth in a nail plate has a
1/4 or more of its length visible within the tooth slot opening.
A tooth will also be considered flattened if the surface of the
wood has been raised. The number of ineffective teeth in a
plate due to flattening will be calculated as twice the number of
visibly flattened teeth. Even a small number of flattened teeth
will seriously compromise the joint. [See illustrations C & D.]
The gap between the nail plate and the timber is known as the
embedment gap and this is measured through the slot openings
of the nail plate. The tooth effectiveness table indicates the
significant reduction in teeth strength per metal plate as the
embedment gap increases. [See table below.]
Without nail plates the completed truss is just a number of
positioned loose timbers. Incorrectly sized, positioned and
embedded nail plates are, apart from defective bracing, the
single largest cause of roof failures.
The consumer cannot be expected to be able to check every
joint in his roof trusses, and yet, should there be a roof failure,
the consumer will need to prove that the roof was in full
compliance with all industry requirements.
This even if a loading certificate was provided by the fabricator.
A fraudulently issued certificate will naturally expose the
consumer financially and, in the worst case scenario, for
damages arising from a roof collapse. A bank may not pay out a
building progress claim or an insurer may repudiate a claim,
even if the failure is a number of years after erection.
Financiers and insurance houses are becoming more and more
aware of the importance of ensuring that their interests are
protected in this regard.
JOINTS:
All members in a joint must be tight fitting and according to
SANS1900, there may not be a gap greater than 2mm between
the members of a joint. Where the joint length is greater than
300mm, a gap of 5mm is allowed, but tapering to 0mm. This is
easily discernible on site and even a non-building professional
can determine compliance with this requirement. The
difference in thickness between members in the joints may not
exceed 1mm.
QUALITY VERIFICATION:
A consumer should require continuous verification of the
quality of the prefabricated timber roof structure, and should
not concentrate only on the finished product, but also direct
his attention to the fabricator’s quality management system.
It is therefore highly advisable that a consumer only transacts
directly with a fabricator that can prove his adherence to the
standards of SANS 1900 through his quality management
system. This quality management system is not to be confused
with the software systems accreditation or compliance to
SANS 9001.
Of the 213 fabricators in South Africa there are only 9 that are
certified to apply the SANS1900 quality mark to their finished
product. Where no information about a quality system exists,
the consumer can resort to a complex sampling procedure
according to the provisions of Annex B2.1 of SANS1900. This
criterion of compliance is very strict.
STORAGE:
Trusses, stored in the fabricator’s yard or on site, must be off
the ground on a reasonably level surface and in a manner that
distortion is prevented. Excessive exposure to inclement
weather must be avoided, but a few days of rainy weather will
not have any negative effect and the structure may be erected
and covered and, as long as the timber’s moisture content is
below 17%, it may be enclosed with ceilings.
TRANSPORTATION & HANDLING:
The consumer must insist that the trusses are transported in a
professional manner. Only purpose made and specially adapted
truss trailers should be utilised and where the trusses are
transported in an upright position.
A flatbed truck or trailer may be used but the truss must be
supported across its entire length and height and no portion
should hang off or protrude from the vehicle.
All parts of the truss should be supported whilst handling and
hoisting to prevent excessive bowing or twisting.
TOOTH
EMBEDMENT GAP (G)
TOOTH
EFFECTIVENESS (%)
0mm < G ≤ 0.80mm 100%
0.80mm < G ≤ 1.16mm 60%
1.16mm < G ≤ 2.40mm 40%
G ≥ 2.40mm 0%
Illustration C.
Illustration D.

3. INDUSTRYINSIGHT
THE CONSEQUENCES OF
NON-COMPLIANCE:
Simply said, the consequences of non-compliance of the
national standards will eventually result in a roof failure with
potentially disastrous results and repair costs far exceeding
the original roof costs by up to tenfold.
BENEFITS:
The benefits of insisting on the aforementioned standards will
ensure that the building owner has a professionally designed
and expertly manufactured roof structure and will furthermore
rest assured that the risk of failure due to design or fabrication
defects is eliminated.
In part two of this article we will investigate the dangers of
poor erection techniques and bracing. Even a perfectly
manufactured truss system will fail within the first 7 to 10
years if incorrectly erected and poorly braced.
For more information please do not hesitate to contact the
writer for clarification and expert advice.
FOR MORE INFORMATION, CONTACT:
Lyndsay Cotton
(c) 082 441 5026 | 0861 LCPROOF
(e) lyndsay@lcproofing.co.za
(w) www.lcproofing.co.za
JULY 2014