What is apron flashing?

Apron flashing is lead flashing fitted at the foot of a rising wall or roof interruption to make the junction watertight. It is used where a roof slope meets a facade or upstand, or where a facade stands on a lower roof. In Dutch it is called voetlood (“foot lead”). In general it is a form of lead flashing; at the front of a chimney or dormer it is called an apron flashing.

Where is apron flashing found?

Between a facade and the flat roof of an extension below it.
Between a dormer and the roof slope, on the lower side.
At the front of a chimney that passes through the roof.
Between a roof and a facade below or above it.

Function

Apron flashing has two tasks:

Keeping out water from outside: rain running down the facade is led over the junction onto the roof.
Draining water from the cavity: water that penetrates the outer leaf and runs down inside the cavity has to get out somewhere. The flashing collects it and leads it outside through open perpend joints (weep holes). In practice this is the most important function.

Common mistakes

Too short: the lead does not reach far enough over the roof covering or tiles, so water runs behind it.
Not continued in the cavity: the lead must run up in the cavity and connect to the inner leaf, otherwise cavity water runs inwards.
No open perpends above the lead: the water in the cavity cannot escape.
Wrong overlap order: the upper part must always lap over the lower part, like roof tiles.
Lead sealed on instead of built in: sealant ages. Build the lead into a joint or use a cover flashing or clamping profile.

Tips

Use lead or an approved lead replacement and follow the supplier’s installation guidelines for thickness, overlap and fixing.
Apron flashing can be supplied made to measure, which saves a lot of fitting work in renovation.
A leak at an extension after wind-driven rain often points to a problem with the flashing or the cavity drainage.

Related terms

Lead replacement flashing
Roof lead flashing
Dormer
Chimney
Outer leaf
Perpend joint
Flat roof

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