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Temporary Roofs

The Story

The winter months have seen a distinct upturn in the number of temporary roofs we have been asked to design at 48.3: since October 2011 we have completed 19 temporary roof design projects and currently have 7 on-going.

There are a few contributing factors to this in our eyes. Firstly, the obvious: the winter months see harsh weather conditions that can significantly affect work progress and impact tight programmes. The temporary roof with associated encapsulation provides effective weather protection and significantly mitigates the risk of disruption from adverse weather.

Secondly, financial and work pressures within the industry have forced down the cost of scaffolding in general, making the temporary roof a more attractive option to contractors with good temporary works planning. As main contractors are under their own budget constraints the relative cost of a temporary roof in comparison with work delays and/or missing programme deadlines is low. The result is that the temporary roof option becomes attractive in the eyes of the Quantity Surveyor as well as the Construction Manager.

Finally, there are now three or four good temporary roof systems on the market, all of which offer good weather protection combined with a safe (SG4:10 compliant) method of erection. As scaffolding companies have invested in these products they have also become very skilled in their use and erection. The result is that companies can predict with confidence cost and erection times and also provide a track record of successful projects.

The design of a temporary roof throws up a series of distinct challenges:

Structural Considerations

The first consideration for the scaffold design engineer is the required span and shape of the roof. Different temporary roof systems and beam sizes offer varying capacities, ranging from 2 m to 40 m spans (dependent upon many other factors, more on which to follow). The choice of temporary roof system may be dictated by the equipment that a particular scaffolding contractor has available. The choice of whether the temporary roof will be mono pitch or duo-pitch also bears on the maximum span, with mono-pitch roofs spanning further. Again, this choice may not be left to the scaffold design engineer as the form and/or surroundings of the existing structure may necessitate one of these two (or another more complex e.g multi-pitch) temporary roof shape.

Various other factors must also be taken into account when considering the structural capacity of the temporary roof. The choice of support detail at each end of a duo-pitch roof (pinned/roller supports) has a considerable influence on both the roof beams and the support scaffold (in particular the loads at the top ties). For larger spans, a tension bar or cable is often employed to increase the capacity of the temporary roof under snow loading conditions. The quantity and extent of lacing tubes, bracing frames and plan bracing is generally dependent upon a couple of factors. Overall rigidity of the roof and how wind loads are distributed and mitigated throughout the structure is one factor, another being the restraint requirement of the beam chords and how their buckling length is minimised.

Part 2 will follow next week and include 'Buildability' and 'Environmental' Considerations.