CROSS Safety Report
Low pitched metal sheet roofs
This report is over 2 years old
A reporter is concerned about low pitch metal sheet roofing attached to trussed rafters.
Key Learning Outcomes
For civil and structural design engineers:
The cladding design and installation should be given the same degree of attention as the primary structure during both design and construction to improve safety, reliability and longevity
Consider appointing a single entity (or Chartered Engineer) to have overall control of the design of the cladding and stability system
Connections can often be the weak link in structures and attention to detail is required
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A reporter is concerned about low pitch metal sheet roofing attached to trussed rafters. They say that the weight of such cladding is about 20% of that of concrete interlocking tiles so that there may be severe wind uplifts. The low pitch and building size result in large areas of higher wind pressures. BS 6399: Part 2 Table 10 gives Cpe as high as -2.6 compared with a typical uplift Cpe = -0.6.
Roofing contractors are, according to the reporter, using self drilling and tapping screws with a minimum diameter of 5.5mm into the top surface of the trussed rafters. The edge distance from BS 5268: Part 2 for this screw should be 27.5mm, requiring at least 55mm thick timber for the trussed rafters. Since trussed rafters are usually made from either 35mm or 47mm thick timber such screws are likely to split the timber even if placed accurately on the trussed rafter centreline. BS 5258: Part 2 also requires a check on loss of section of the timber for screws not less than 5mm diameter.
Since trussed rafters are usually made from either 35mm or 47mm thick timber such screws are likely to split the timber even if placed accurately on the trussed rafter centreline
In the reporter’s experience the trussed rafter design software used does not include for zones of enhanced wind pressures. This, in their opinion, is of no importance for tiled or slated roofs of traditional pitches but gives grossly wrong fixing forces for light weight low pitch roofs. It would be very rare for a trussed rafter designer to be aware that designing to support metal sheet cladding requires professional structural engineering input.
Enhanced wind uplift
Taking into account these enhanced wind uplifts the reactions from the trussed rafters can be as high as 5kN compared with the strength of a standard truss clip of about 2kN. Standard strapping at industry standard spacings of 2m is clearly appreciably less strong than the truss clips. Indeed safe anchorage may require substantial heights of wall to provide dead weight.
The high wind uplifts can result in stress reversal in the trussed rafters. This is seldom bad enough to require larger timbers or nail plates, but it does require a check that the bracing is adequate. The reporter is usually involved because the trussed rafter designer realises that the external wall heads need lateral restraint but frequently has to fight hard to ensure that the trussed rafters are correctly braced and safely secured to the wall plates. The problems need sorting out during the design period, not as the brickwork approaches the wall plate level.
Expert Panel Comments
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This is an example of insufficient attention being paid to the overall stability of a system and the problem of co-ordination between designers, suppliers and builders. Codes require one person to have overall charge of structural stability and this should be someone who is competent to do so. No failures were mentioned but with the threat of increasing wind loading in the future such issues have to be taken seriously.
Again, the emphasis is on the importance of choosing the correct fixings at an early stage. The guidance for designers which accompanies the revised CDM 2007 Regulations, clearly indicates the responsibilities of all those in the supply chain beginning with the scheme designer, and the need for adequate co-operation and co-ordination.
When there is no designer in the conventional sense the builder or contractor takes on the design responsibilities. When purchasing such components, either the purchaser should stipulate the required capability for the truss manufacturer to match, or the trussed rafter manufacturer should stipulate limitations of use. Further advice can be obtained from the Trussed Rafter Association.
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