Design guidance for roofs

The primary purpose of roof cladding is to maintain a weather-tight envelope for the effective life of the roof. This means both maintaining water-tightness and protecting the roof cladding material from undue and premature degradation. For these reasons, all roofs should be designed to avoid retention or ponding of water.

Roofs with a pitch greater than or equal to 4° can be easily designed and built to form a weather-tight envelope. Sheets must be fixed so as to overlap at sides and the ends to resist the entry of rainwater. The main cause of leakage comes from water which is forced in through laps by the force of the wind or pressure variations across laps. However, the appropriate use of sealants can easily eliminate this possibility.

Ideally, end laps should be eliminated by using sheets which cover the full distance from eaves to ridge. However, if end laps are unavoidable, they should be minimised in number and positioned as close as possible to the ridge. The minimum end lap should be 150 mm and any end lap details should be fully fixed across the full width of the sheet.

Very low pitched roofs

For very low pitched roofs, less than 4°, special considerations must be given. Such roofs should be constructed from an un-perforated roof cladding system, such as concealed fix or standing seam. This will avoid the possibility of deflection around fasteners which can, on very low pitches, give rise to ponding conditions at the weakest points of the roof. Since each roof cladding system is specific to its manufacturer, the recommendations of the manufacturer should be sought.

The design of very low pitch roofs must ensure that, under all foreseeable conditions of loading, and taking into account all probable tolerances on dimensions in manufacturing and site work, no ponding will occur. It is not recommended that profiled steel cladding be used for roofs of less than 1° pitch. Even if a roof is intended to have a pitch of 1°, it should be designed at a pitch of about 2.5° to ensure that, after all settlements, the slope is not less than 1°. For barrelroofs there is clearly an area of <1° pitch. This is considered acceptable provided the guidance above is followed. The design of the roof must ensure good air-movement over the relatively short low-pitch arc.