COP v3.0:roofing; end-laps

7.4 End Laps 

End laps should be avoided if possible when installing metal roof cladding as an incorrectly sealed end lap may entrap water and cause corrosion. When the sheets are too long to be transported or exceed the longest recommended length (see 7.3.2 Roof Cladding Expansion Provisions), the transverse or end lap joint can be avoided by using a waterfall step. (See Step Apron Details)

When long lengths outside the capacity of available transport are required, secret-fixed roof cladding can be supplied by using an onsite roll-forming machine.

Where end laps are unavoidable, a sealed joint should be made using sealant at both ends of the lap. The upper seal is critical as condensation entering the upper side of the lap from underneath can cause rapid corrosion. (See 14.12.1 Sealing End Laps.) Rivets are used to fix the sheets together and should not be fastened to the purlin. The sheets are fixed to the purlin using screw fixings.

The two lengths should be regarded as one length for expansion provisions.