What overlap is required for corrugated roofing sheets?

The required overlap is the profile-specific side lap and end lap that keeps water flowing over the roof rather than into the joint. There is no single measurement suitable for every corrugated sheet: the profile geometry, roof design, sheet orientation and site exposure all affect the detail. The manufacturer’s published cover width and lap arrangement should therefore be used when setting out the roof.

Side laps join adjacent sheets across the roof slope. The sheets should be aligned so that the corrugations interlock cleanly without forcing or flattening the profile. A side lap that is too small can allow wind-driven rain to track beneath the sheet, while an excessive lap reduces the effective cover width and may interfere with the intended fixing positions. Side laps should remain consistent along the roof and be fixed using the washers, fasteners and sealing method specified for the particular roofing system.

End laps are used where one sheet finishes and the next continues up the slope. The joint should be positioned over suitable support, normally a purlin or other member designed to receive the fixings. The roof pitch and the degree of exposure influence how much protection the joint needs. A shallow pitch or exposed location generally requires more careful detailing than a steep roof in a sheltered position, so an end-lap dimension should not be selected simply because it was used on another building.

Before fixing, check the sheet layout from the eaves to the ridge. Where the design uses end laps, joints should be arranged in accordance with the profile instructions and, where appropriate, staggered rather than concentrated in one line. This helps maintain a consistent load path and avoids creating a weak or visually uneven area. Allow for the specified eaves and ridge details as well as the laps when calculating sheet lengths; the nominal sheet length is not the same as the finished roof coverage.

Lap sealing must be compatible with the roofing material and the manufacturer’s fixing system. Apply any sealing strip, tape or sealant to clean, dry surfaces in the prescribed position, keeping it continuous around the joint where the detail requires it. Do not rely on sealant to compensate for misaligned sheets, an insufficient lap or fixings that are missing, loose or over-tightened. Fasteners should clamp the sheets securely without deforming the corrugations or damaging the sealing washers.

Good installation practice also includes:

• checking that the supporting steelwork is correctly spaced and level before sheets are laid;
• keeping laps facing in the intended direction for the roof layout and prevailing weather conditions;
• removing swarf and debris after drilling, as loose metal particles can promote surface staining or corrosion;
• using the specified flashings at verges, abutments, valleys, ridges and roof penetrations, because these areas require more than a standard sheet lap; and
• inspecting each joint before the next sheet or flashing prevents access for adjustment.

For a building with unusual exposure, a low roof pitch, long sheet runs or complex junctions, the overlap should be confirmed against the roofing system’s technical information and the building design rather than estimated on site. This is particularly important where the sheets form part of an agricultural or industrial building, as internal conditions, ventilation and condensation control may also affect the complete roof specification.