 | Impact of joists on sloping roofs
When designing a sloping roof system consisting of joists perpendicular to the slope line (Figure 1), the building designer must be aware of certain aspects that may save time and money in terms of the overall building design. Since joists have reduced lateral strength, especially on sloping roofs, neglecting to consider this aspect at the design stage may result in additional expenses and increased time to install the roof structure.
Figure 1
The lateral slopes of any conventional roof system, comprised of girders, joists and steel deck, must be considered in the building design. The effect of loads on roofs with slopes of less than 2% and joist spans not exceeding 12 metres (40 feet) is negligible. This is generally the case for gravity loads and superimposed loads on a conventional roof. On an unconventional roof, such as one carrying the weight of a rooftop terrace, special design considerations must be taken, particularly when the joist spans exceed 12 metres, even if the slope is 2% or less.
Economical solutions
The following two solutions will eliminate the major added expense of a structural bridging and horizontal bracing system which, in most instances, is field-welded (Figure 2).
- One possible solution is to specify a steel deck fastener connection pattern which, with the help of the diaphragm, will transfer the lateral portion of the gravity load to the building’s vertical bracing system. The building designer must be aware that, during installation, the joists will deflect laterally due to self-weight. The extent of the deflection will be affected by the joist span and roof slope. Since this deflection generally exceeds L/500 (ref. CSA S16-09, Clause 29.3.4), the installer must take corrective action or use a special erection method prior to erecting the steel deck.
- Another solution is to design the roof with the joists installed in the direction of the slope and a diaphragm system provided by steel decking (Figure 3). This solution is especially economical when combined with a standing seam metal roof system. A standing seam metal roof alone does not provide sufficient lateral support to the joists, regardless of their direction. Therefore, this roof system would typically require the use of a structural bridging and horizontal bracing system.
In the latter solution, if the standing seam roof is covered over the joists and steel deck, then the decking will provide sufficient rigidity to the building through diaphragm action.
Although steel deck and fasteners represent an added expense, their costs are much less than those incurred to install a bridging and horizontal bracing system for the lateral stability of the joists and subsequently, the building.
Figure 2
Figure 3
Information
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- Telephone: 1-877-499-6049
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