Bracing is one of the most overlooked aspects of roofing. With this in mind, SA Roofing toook an in-depth look at this important facet of the roof system – with the help of expert engineer, Errol Hobden of International Truss Systems.
In the real estate world the estate agents have a mantra, ‘Location, location, location.’ We have all certainly heard this phrase many times. You may wonder why the word location is repeated three times. The reason is to emphasize the importance of the location of the property when purchasing real estate. If you purchase a property in a good location, the property is most likely to increase in value over time, giving you a good return on your investment. Location is the number one rule in real estate, and it’s often the most overlooked rule.
In the roof truss industry, this phrase can be translated as, ‘Bracing, bracing, bracing.’ Bracing is one of the most important, integral parts of a roof structure, and it is also one of the most overlooked parts. Many contractors don’t realize that most roof failures are a result of inadequate bracing. A roof structure can be perfectly designed and manufactured, but if it is not correctly erected and braced, you will end up with an unsafe roof that will in all likelihood wind up costing you a considerable sum of money in the future.
SANS 10243:2004 is the current South African National Standard that covers the manufacture and erection of timber trusses. This code has been designed to act as a guide on the manufacture, erection and bracing of timber roof trusses.
SANS 10400-L: 2011 – The application of the National Building Regulations – Part L: Roofs. This code covers mainly site-made trusses with lapped members. This code has some ‘deemed to satisfy’ truss designs that one can use to construct nail and bolted trusses with lapped joints. The truss types are limited to a maximum truss span of 8m and a minimum pitch of 15 degrees. The roof type is limited to a gable-to-gable roof, meaning that the roof cannot contain any hips. The minimum timber size that can be used is also limited to 38mm x 114mm. The reader is advised to refer to the code for further limitations that are applicable to these deemed to satisfy truss types.
At present, both SANS 10243 and SANS 10400-L codes are in the process of being revised.
International Truss Systems (Pty) Ltd (ITS) and some of the other metal nail plate system suppliers, in conjunction with the Institute for Timber Construction South Africa (ITC-SA), have over the years developed numerous guidelines that explain the correct procedures in erecting and bracing a timber roof structure. All these details have been published by the ITC-SA and are contained in two publications: Roof Erector’s Handbook Volume 1 and Roof Erector’s Handbook Volume 2.
ITS as well as the other metal nail plate system suppliers also have their own bracing manuals that contain some of the details found in the ITC-SA Roof Erectors’ Handbooks, as well as details pertaining specifically to their own range of proprietary truss hardware. Erectors should be aware that only the proprietary hardware of the specific system whose engineering software was used to design the roof trusses may be used in the roof system. The reason for this is that the engineering software programmes use the tested design values of their specific, associated hardware in the design of the roof. Using hardware from an unknown source could relieve the metal nail plate system supplier from all responsibility for the roof. Roof Erector’s Handbook Volume 1 is designed as a pocket guide that can be carried in your shirt pocket for quick reference. It contains some of the basic dos and don’ts in roof erection and bracing.
Roof Erector’s Handbook Volume 2 is a more concise publication that contains all of the most common bracing requirements needed for the majority of the roof type structures found in South Africa.
Both Volumes 1 and 2 are deemed to satisfy bracing rules that will work for most roof type structures found in South Africa. In the case of very complicated or very large span roofs, it is advisable to have an accredited Timber Engineer or the System Engineer review the bracing requirements for the roof structure before any of the trusses are manufactured. All roof structures for public buildings are required to be reviewed and checked by an accredited Timber Engineer or the System Engineer.
The importance of bracing in a roof system cannot be emphasized enough. Bracing is a crucial, integral part of a roof system that is quite often not treated as such. Designers and erectors are often both at fault here.
Designers must pay more attention when preparing the truss erection drawings. Their drawings are to clearly communicate their intention of what is needed to correctly erect and brace the roof. All site erection drawing packages/job packages should contain a truss layout drawing, bracing details and the truss profile drawings. Each truss layout drawing must identify and show the positions of each truss, the position of each braced bay, and the position and type of each hanger or cleat needed in the roof. The bracing details can also be included on the truss layout drawing if possible; otherwise a separate drawing can be prepared for all the applicable bracing details for the roof. Quite often the truss profile drawings are excluded from the site erection drawing package. This is not good practice as it makes it harder for the erector to be able to clearly identify which webs need to be braced or the correct positioning of the bottom chord runners. Each truss profile drawing must clearly show the position of the required web and bottom chord runners.
Roof erectors should also pay more attention to the drawings contained in the site erection drawing package/job package. It is advisable for them to take some time and study the drawings to make sure they completely understand what is required in erecting and bracing the roof. If they have any questions they should contact the truss designer for explanation.
To read the full article in the September 2015 issue of SA Roofing click the link here.