FEATURE
Truss ties need to be built into all the load-bearing walls at the locations where trusses will be installed .
1 . Load-bearing walls must be level and square . Failure to do this will complicate the roof design .
2 . Truss ties need to be built into all the load-bearing walls at the locations where trusses will be installed .
3 . In general , wire tie downs are used for tiled ( heavy ) roofs and metal strap tie downs are used for sheeted ( light weight ) roofs
4 . If any internal load-bearing walls have been considered in the truss design , these walls need to be built up to the same level / elevation as the external load-bearing walls . If these walls are not at the same level , this can cause stresses in the trusses that the truss was not designed for , resulting in a possible truss failure . Any internal walls that were not considered in the truss design , must not be built up tightly to the bottom chord of the truss , where this wall now gives support to the truss , as this could also cause stresses in the truss that the truss was not designed for .
5 . It is recommended that non-load-bearing walls be built after the roof has been loaded . This will allow for the trusses to settle before these walls are built .
6 . A gap must be provided between the top of the non-loadbearing wall and the bottom chord of the trusses .
There are several truss profiles available , and with the aid of computer truss software , trusses can be designed in many shapes .
The favoured roof covering material in South Africa is cement or clay tile or metal sheeting . Slate and metal tiled roof are other options .
ROOF COVERING COMPARISONS :
The table below provides a comparison between a clay or cement tile roof and a metal sheeted roof :
Max truss spacing ( mm )
Maximum batten / Purlin spacing ( mm )
Batten / Purlin size ( mm ) ( pine )
Batten / Purlin size ( mm ) ( eucalyptus )
Minimum roof slope generally ( degrees )
Tie Downs
Clay or Cement Tile ( battens )
760 – Pine 900 – Eucalyptus
Metal Sheeting ( purlins )
* 1200
Comments
* For Pine purlins , the truss spacing for metal sheeting can increase to 1500 , but the size of the ceiling brandering will need to increase .
* 400 * 1150 * In general , refer to the manufacturer ’ s specifications
36 X 36 48 X 73
36 X 36 48 X 48
* 17.5 * 5
Wire
Metal strapping
* dependent on the roofing product , refer to the manufacturer ’ s specifications
See SANS 10400 - K
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