Steel Buildings in Europe

Part 8: Building Envelope 8 - 13 3.1 Weathertightness The primary function of the cladding system is to provide a weathertight building envelope, suitable for the intended use of the building. With this in mind, the cladding specifier must give careful consideration to the selection of the cladding components and the detailed design of the system. The location of the building, its orientation and the external climate should all be considered when specifying the cladding. The satisfactory performance of the system also depends on the correct assembly of the components in the factory and/or on site. In general, roofs are at greater risk of leakage than walls, and this risk increases as the roof pitch decreases. This is an important factor in the design of modern non-domestic buildings, since many have low pitch or flat roofs in order to minimise the volume of empty roof space. Not all types of roof cladding are suitable for use on low pitch roofs. Specifiers must, therefore, pay careful attention to the minimum pitch recommended by the manufacturers, together with the published guidance on detailing and installation. Trapezoidal metal roof sheets with through fix fasteners are generally suitable for slopes of 4º (7%) or steeper. This 4º limit is critical to the performance of the cladding and should take into account deflections in the supporting steelwork and localised cladding deformations that may lead to ponding. Where the primary steelwork is precambered to off-set the deflections due to permanent actions, great care must be taken to ensure that excessive precamber does not result in local high points, as these could also cause ponding. For shallower pitches, down to 1,5º (1,5%), a secret fix system with no exposed through fasteners, special side laps and preferably no end laps should be used. Secret fix systems may also be used on steeper roofs where increased reliability is desired. For low pitch roofs, ponding is a potential problem that must be considered at the design stage in order to avoid the deleterious effects of prolonged soaking and the increased loading due to the weight of the water. Where ponding occurs on rooflights, there is also the additional problem of the water leaving dirt deposits as it evaporates. Side and end laps in profiled sheeting are weak points in the building envelope, where the wind and rain could potentially penetrate the cladding. The design and construction of the laps is therefore critical to the weathertightness of the cladding system. End laps typically consist of two continuous butyl sealant strips, which are compressed to form a weathertight seal by the clamping action of the fasteners. The pitch of fasteners required to achieve a proper seal will depend on the profile geometry, but one fastener per trough is common. A typical side lap between trapezoidal sheets is formed by overlapping the profiles with a strip of butyl sealant positioned on the weather side of the fastener to provide a weather-resistant seal. The side laps should be stitched at 500 mm centres or closer using steel stitcher fasteners. Further information on side and end lap details is given in MCRMA Technical Paper No. 6 Profiled metal roofing design guide [4] and Technical Paper No. 16 Guidance for the effective sealing of end lap details in metal roofing constructions [5] . Reference can also be made to ECCS-TC7 Publication 41 Good practice in steel cladding and roofing [6] .