Chimney Flashing

A chimney that rises through a sloping roof interrupts the covering on all four sides, and each side faces water differently. Chimney flashing is therefore not a single part but a system of pieces, each suited to one face, combined so that water is lapped outward at every point.

The four faces

• Front (downhill) face: an apron flashing laid over the roof covering and turned up the masonry.
• Sides: step flashing (North America) or soakers (UK and Ireland) interleaved with each course, turned up the side of the chimney.
• Back (uphill) face: a back pan or back gutter turned up the chimney and running under the covering above. On wider chimneys a cricket (saddle) is built here to divert water around the chimney.
• Counter-flashing: on all sides, a cap or cover flashing let into the mortar joints and turned down over the base flashings.

Crickets

The US IRC (section R903.2.2) requires a cricket or saddle on the ridge side of any chimney or penetration more than 30 in (762 mm) wide measured perpendicular to the slope. It may be covered in sheet metal or the same material as the roof.

Masonry issues

Brick and mortar absorb water, so even perfect flashings can be undermined by water soaking through the masonry above the roof. Chimney caps or crowns, and in some cases through-chimney flashing trays, address this. Mortar joints holding counter-flashings need to be maintained.

Common problems

• Counter-flashings set only with sealant rather than into a reglet.
• Missing or undersized crickets leading to debris and ponding behind the chimney.
• Corrosion where dissimilar metals meet.
• Old flashings reused on a new roof. Codes and good practice call for damaged or deteriorated flashings to be replaced when re-roofing.

UK practice

In Britain chimney flashings are typically formed in lead: a front apron, soakers with stepped cover flashings at the sides, and a back gutter at the top, with lead codes chosen for each piece.