Ensuring Sound timber roof structures

by Ofentse Sefolo
Ensuring Sound timber roof structures

A roof is an essential part of a building, not only to enclose the building and protect the interior and occupants from the elements, but also to complete the design in an aesthetic way. When it comes to residential homes or other one- or two-storey developments, the roof is often one of the largest, heaviest and most costly structures in the design.
And whether tiles or metal sheeting are chosen as roof covering, the structure should be properly designed to carry the weight of the completed roof and any potential add-ons, as well as ensure maximum longevity. For a sound structure, a lot of planning, design and know-how need to be invested in a roof’s design and construction.

Sadly, the Institute for Timber Construction South Africa (ITC-SA) indicates that poorly constructed roofs have become a real problem in residential South Africa since many building owners and contractors opt for price over quality when it comes to workmanship and materials.

Handmade vs prefabricated roof trusses
In fact, internal investigation and findings by ITC-SA have confirmed that 90% of hand- or site-made trusses do not comply with the relevant building regulation and SANS material and design specifications. This is especially prevalent in the residential market and can lead to costly, disastrous and sometimes even life-threatening situations.

Prefabricated roof trusses are cut by advanced, specially designed machinery and are therefore far more accurate than handmade ones. When covered by a rational design, they also use less timber and are more likely to comply with National Building Regulations (NBR) requirements if they are designed by a qualified engineer.

In all industry-related tests, the ITC-SA has found hand- or site-made timber trusses to be on average up to 20% more expensive than prefabricated timber roof trusses.

What’s more, prefabricated roof trusses come with guarantees in the form of a manufacturing warranty and an engineer’s design certificate, which the owner may call upon to have the roof structure repaired. It is important to note that the so-called “engineer’s certificate” often referred to on quotations, refers to the certificate which certifies that the timber structure has been erected according to the design intent.

In addition to this, building owners should insist that the fabricator proves that the roof structure has been designed according to a rational design. This is because prefabricated roof trusses can only be approved when a rational design is available, confirming that the material and truss design meet and/or exceed the SANS requirements.

A timber house. Courtesy of Logo Homes

The 4 keys of roof truss construction
The ITC-SA identifies four elements in roof truss construction that are vital for a sound end-result:
1.    Structural timber that complies with the design intent must be used (SATAS- or SABS-marked): Structural timber needs to be marked with red ink on the face of the timber at 1m intervals. If it does not bear these structural markings, it will be condemned. One should beware of unmarked timber, as this is not structurally graded timber.

2.    The designer must create an accurate cutting bill: The cutting bill will dictate the exact lengths and angles at which the timber must be cut for proper assembly.

3.    ITC-SA accredited System Suppliers must be used: The metal connector plates used must be supplied by the specific engineering system that is being used. In addition, these plates must be the correct size, and positioned in the right locations as per the standard methods and tolerances.

4.    All connections and bracing details must be in accordance with the design intent: Everything that holds the structure together, such as the number of nails, bolts, washers, brackets and cleats, must be in accordance with the engineering design. All necessary bracing accessories must be stipulated on the design plans.

Legal standards
Timber roof truss designs are assessed in accordance with Part L of the National Building Regulations (SANS 10400). The legal mandate for the National Building Regulations is the National Building Regulations and Building Standards Act (Act 103 of 1977) (NBR).

In addition, as per the Construction Regulation 2014, Government Gazette No 37305, dated 7 February 2014, Structures 11, (2), an owner of a structure must ensure that:
a)    Inspections of the structure are carried out periodically by competent persons to render the structure safe for continued use.
b)    The inspections contemplated in Paragraph (a) are carried out at least once every six months for the first two years and thereafter yearly.
c)    The structure is maintained in such a manner that it remains safe for continued use.
d)    The records of inspections and maintenance are kept and made available on request to an inspector.

Although this points out the responsibilities of building owners, this does not free the designer, the truss plant and ultimately the system supplier of their moral responsibility in the matter.

Quality workmanship
A good start is to make use of reputable fabricators and qualified structural engineers with experience in roof construction to design and sign off the roof.

When it comes to timber roof trusses, all ITC-SA approved fabricators are audited on an annual basis, and are assessed according to quality, competence and compliance with the South African National Standards, SANS 10243. Should the audit be successful, an annual certificate of competence is issued as confirmation.

A comprehensive list of all the ITC-SA approved timber roof truss manufacturers across the country, noting the engineering systems each fabricator uses, is available on the ITC-SA website (www.itc-sa.co.za) and categorises each truss manufacturer in accordance with complexity ratings.

Full thanks and acknowledgement are given to ITC-SA for the information given to write this article.

Interesting facts:
•    90% of hand- or site-made trusses do not comply with the relevant building regulation and SANS material and design specifications.
•    Hand- or site-made timber trusses are on average up to 20% more expensive than premanufactured timber roof trusses.

Caption Main Image: A roof truss installation at the Vergelegen Shopping Centre.
Courtesy of Rooftek Trusses



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