Corrugated roofing sheets on steel purlins: what must the design specify?

Use the current spacing data for the precise sheet profile. The distance from one purlin to the next determines the distance the sheets must span. The span of each purlin runs between supports in the primary frame. Mark both dimensions on the drawing so that a single reference to “spacing” does not lead the contractor or owner to use the wrong value.

Check steel sections for load capacity, deflection and stability. Establish where lateral restraint comes from. Do not assume the corrugated sheets act as a structural diaphragm, particularly where their fixings allow movement. If the original design relied on another cladding material, review the assumptions when changing the covering. Similar sheet dimensions do not establish equivalent structural behaviour.

Specify the screw for the substrate, its thickness range and the design loads. Length alone is insufficient. Match the head, seal and hole detail to the fibre-cement system and select corrosion resistance for the environment. List the fixing type and installation method. A substitute can alter capacity or clamping pressure. If the steel is thicker or thinner than specified, review the fastener selection before changing driver settings.

Check that purlins are aligned and lie in the intended roof plane before laying sheets. A twisted section or a high spot can stress the sheet as screws are tightened. Correct the supports rather than using brittle sheets to pull the steelwork into line. Check lap locations and end supports. An extended overhang cannot compensate for a missing purlin. Once the roof is covered, correcting one error may require several rows to be removed.

Wind can cause uplift, while snow distribution depends on roof shape and neighbouring parts of the building. Equipment can change conditions locally. Check the sheets and their fixings for the relevant load cases. Specify the fixing pattern by zone. A catalogue span for a stated load does not establish a universal screw count or a spacing that applies on every site.

Support roof equipment from the structure. The covering must shed rain around a purpose-designed fixing or penetration. Heavy units cannot simply bear on sheets between purlins. Allow for vibration, thermal movement and maintenance. For photovoltaics, verify the load path through the full assembly. Locate brackets before selecting sheet lengths, as a bracket can conflict with a lap or leave too little room to seal the opening.

Document the installed purlins, fasteners and sheets, including changes approved on site. Inspect supports, joints and the alignment of the rows together. Photograph connections, identify any substitute fasteners and record their assessment. Mark safe access routes so maintenance staff can reach the roof without walking directly on the sheets. These records help keep later repairs and new installations compatible with the existing system.

Agree a coordinated layout for sheets, purlins and fixings before installation. Have changes assessed against the actual loads and the system documentation.