COP v3.0:installation; double-lapping

14.8 Double Lapping 

Sheets should not be laid with a double lap, as this can cause accelerated corrosion in the lap area.  If the remaining area of a roof or wall section to be covered is less than a standard sheet width, the sheet should be cut lengthwise to fit the gap.

Similarly, when replacing a roof without removing existing flashings, for example apron or chimney flashings behind cladding, the area of the retained flashing should be reduced by cutting back to the minimum practically achievable.

If a roof sheet lap requires extra protection, for instance when over the apex of a barrel curved roof where pitch is less than the minimum stipulated for the product, laps should be sealed rather than double-lapped.  Lap tape is preferred over gunned sealant for this purpose, as it is less likely to become displaced while positioning the sheet.