Roof Trusses & Elements
Origins of the Truss
Trussed rafters are defined as triangulated timber frames. Ordinarily, trusses will be located at a centre (distance from the outer face to the inner face) of 600mm, this may be altered by a designer or structural engineer but must never be altered by the installing Trades without seeking permission first.
In 1952, in Pompano Beach Florida, after experimenting with plywood gusset plates and varying concoctions and combinations of glue, staples, nails and screws, the metal plate connected engineered wood truss was invented and patented. The inventor, A. Carroll Sanford, founder of Sanford Industries, marked the beginning of the truss industry (Freimans, 2010). The idea of trussed rafters arrived in this country from the States during the 1960’s and quickly became the standard method of constructing roof structures in domestic construction. (Margetson, 2010-13)
Twist and Shout
Often the ‘hand lifting’ of trusses will loosen gusset plates (small grey section) which are factory fitted and pressed into the timber sections. This causes twists which a truss cannot withstand, even if the plates are loosened they will lose their resistance to force causing future issues such as sag and deflection. If a truss is damaged, please seek the advice from the manufacturer on replacement or additional repairs that may be suitably carried out on site, however the yellow area gives an example of the sizing difference between a site fitted plate and the factory fitted gusset, the site fit would need to be sequence screwed through the truss with up to 20 screws per side of the truss at specified centres, this can often be very difficult if already in situ when the damage is identified so caution should be taken when lifting and installing all trusses.
When identifying what bracing requirements are needed the key element is firstly to identify the overall span of the trusses, this is taken from supporting walls of the truss clips.If the span is greater than 8m then chevron bracing is required in addition to the longitudinal and diagonal bracing which will be identified on your roofing layout plan.
Publications such as British Standards and Trussed Rafter Association gives guidance on bracing, however it is normally depicted on plan views designed by the manufacturer or structural engineer. When originally sourced from America the truss was fitted with Plywood to stop the racking effect or twisting of the trusses. However, this is not a material commonly used in the UK for construction and so roofs fitted around the 60s-70s had very little in the way of restraint or bracing, this has resulted in a twisted or sunken effect due to loading placed on an incorrectly fitted roof.
If in doubt about what bracing is required it is best to seek the advice of your appointed AHCI Surveyor, and additional bracing is better than too little, you cannot over brace a roof structure such as the Fink truss seen above. However, planning should be sought as too movements in and around the roof space as later trades may remove bracing as rough sawn timbers are often used making it seem a temporary fit whilst spreading the trusses. This should be communicated to all workers within the roof voids prior to removal or cutting of any timbers. If this does occur additional bracing should be fitted immediately to avoid future complications
Lock and Load
When assessing the design of a truss attention to the tiles and coverings should be given as often concrete interlocking tiles will be assumed for a loading self-weight however if this is not the case then a lighter tile will be able to be supported by the truss however the issue of uplift may occur where the bracing and strapping of the trusses is also reliant on the weight of the tiles. Additional wall straps may be required to the gables and wall plates if this does become an issue. Alternatively, an overloaded roof is the most likely issue to occur, this may not be down to a tile weight but rather additional features being added to the roof causing disproportionate loading. For example, a new sky light or dormer as previously mentioned when cutting into a truss or its bracings specifications should be sought from a designer or structural engineer to ensure the roofs triangulation is maintained and evenly spread, this too can be the case when fitting stairs into a trussed roof, the stairs should run at the same direction of the trusses and of the dormer windows to ensure loadings and bracings are maintained.
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