Graphical Construction Glossary >> Roofs and roofing. >> Roof Coverings >> Roofing Lap

Roofing Lap

The amount by which roofing tiles, slates, shingles, flashings, etc overlap each other to achieve waterproofing.

Side-lap

Refers to the overlap in a sideways or horizontal direction.

End-lap

Refers to the amount of overlap in a direction following the roof slope.

Above is a sketch showing the end lap on slates. Basic to any material like this is that we lay the bottom row first and overlap each row after by a certain amount.

The concept of lapping one part of roof sheeting is the most basic principal of roof cladding or sheeting. Very few roofs do not use lapping in one way or another.

The amount of overlap varies from material to material and the amount varies also depending on the pitch of the roof.

Material manufacturers provide data on how much overlap to use in certain circumstances. If in doubt use more lap than recommended.

Here is a photo of an old tired roof, but thanks to the use of overlap in it's various parts it is still watertight after 60 years.

After the carpenters have finished the roof framing including the fascias and bargeboards then the roofer starts.

1. There is an old rolled barge flashing which is like a letter "P" lying on it's back. Theses would be fixed first.
2. The sarking is laid from the bottom up. It end-laps the fascia to carry condensation and other moisture that gets though the roofing to the outside. The sarking side-laps the barge flashing.
3. Each succeeding layer of sarking end-laps the lower one until at the ridge the last one overlaps both sides of the roof.
4. The bottom row of tiles (imitation, asbestos cement in this case) end-laps the fascia to suit the gutter.
5. Each layer of tiles has end-lap over the one before and also has half a tile side-lap.
6. At the level of the chimney lead flashing end-laps and side-laps the tiles so far and then flashing is laid higher than the chimney so that the next rows of tiles end-lap the lead.
7. Both sides of the roof are tiled to the ridge and then the ridge capping overlaps them.

Roofing lap in it's many forms is essential. Do not be tempted into the easy fix with silicone sealants. Get the lap right and they are not needed.

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