Fibre cement roofing on an industrial building: what each layer does

The corrugations do not make the whole building airtight. Joints, eaves and penetrations all need detailing. Rain runs over the outside of the roof, while air can move through the assembly. The layers used to manage these two paths often differ.

For a heated building, design the roof to meet the thermal requirements. Asbestos-free fibre cement roofing can be used above an insulated build-up. Determine insulation thickness and position separately, with due allowance for supports, ventilation and internal layers. Filling gaps between structural members may leave thermal bridges and discontinuities, especially at the walls. If the building’s use may change, assess the future conditions before enclosing the roof.

Water vapour can diffuse through layers, and leaking joints can carry moist air from the interior into colder parts of the roof. Continue the air control layer through the roof-to-wall junction. Fibre cement has limited moisture-buffering capacity. Where a process produces significant amounts of vapour, consider extraction and general ventilation. To investigate dripping, trace the moisture source and inspect the operating conditions of the whole roof.

The underside may include an internal lining, an air space or a support for insulation. Choose each element for its task. The term asbestos-free fibre cement describes a material, and other boards made of it may have different intended uses. Specify what the inner lining must do so that substitutions can be evaluated. Similar-looking linings can have different fixing requirements, moisture resistance and compatibility with insulation.

Purlins and their connections must carry the design loads. Determine the spacing from the covering properties and site conditions. Count sheets as a structural diaphragm only if calculations and system documentation support that use. In a steel building, check how thin-walled sections are restrained laterally. When changing the covering, review the original structural assumptions. Matching fixing holes alone do not demonstrate that the supporting system is suitable.

At eaves, walls and service penetrations, the flashing sheds water while other layers provide insulation and airtightness. Detail the junction without gaps in the insulation or obstructions in the ventilation path. Surface sealant cannot restore missing insulation. Draw the water path, air path and load transfer so the roofer, cladding installer and other trades know which part of the junction they must complete.

Inspect and photograph critical joints before they are covered. Check the sheets against the installation instructions and establish acceptance criteria for the remaining layers. An even exterior can conceal a fault in the insulation or lining. Record the actual build-up, materials and installation sequence in the as-built file. These records help distinguish rain leakage, condensation and a thermal bridge, each of which needs a different investigation and remedy.

Assign a function to each layer in the specification. Use those functions to select the sheets, fasteners and details for the conditions in the building.