Scott’s expert advice to avoid risky structural integrity
Scott’s tip – Trusses must be installed straight and vertical
Occasionally we hear of builders who do not fully understand the tolerance on “bow” or “plumb” in trusses. Here is a summary from the “Truss Installation Guide” from our website:
Trusses must be installed straight and vertical or the stability of the structure could be compromised. The tolerance for bow must not exceed the length of the bowed section divided by 200, or 50mm, whichever is the lesser.
So, if L = 3000mm, the maximum bow is 15mm
Similarly, no part of the truss should be out of plumb by more than the height of the section divided by 50, or 50mm, whichever is the lesser.
The recommended way to ensure the trusses are plumb is to erect the first truss correctly, straight and vertical, then temporarily brace it in position. Then space each successive truss correctly and fix it back to the first truss with temporary ties to top chord at a maximum spacing of 3000mm and to the bottom chord at a maximum spacing of 4000mm.
Temporary bracing for hip or dutch-hip end roof:
Temporary bracing for gable-end roof: