Apron and Headwall Flashing

An apron flashing is a single continuous flashing installed where a roof slope meets a vertical surface running horizontally across it. The two most common locations are a headwall, where a lower roof's upper edge abuts a taller wall (for example, a porch roof against the house wall), and the downhill face of a chimney. In North American usage the term headwall flashing is common; in the UK, "apron" is the standard term and the flashing is traditionally made of lead.

Shape and installation

An apron flashing has one leg lying over the top course of the roof covering and another leg turned up the wall. Because water runs straight down the slope onto it, it is installed over the roof covering, not under it. Its upper leg is then covered by the wall's water-resistive barrier and cladding, or by a counter-flashing let into masonry.

On a chimney, the apron is the first piece installed: it wraps across the front and around the lower corners, after which step flashing (or soakers) runs up the sides and a back gutter or cricket deals with the uphill face.

Details that matter

• The roof leg must be long enough to carry water well out onto the covering.
• Where the apron meets sidewall step flashing at an outside corner, the corner needs to be folded or soldered so it does not open.
• At the end of a headwall where the wall continues down past the roof, a kickout or diverter may be needed.
• On tile roofs in the UK, the lead apron is dressed down over the profile of the tiles below.

Materials

Lead, copper, aluminium, galvanised and coated steel are all used. Historically, the apron below a chimney was typically a strip of lead.