Chimney Flashing Installation

A masonry chimney passing through a roof needs a multi-part flashing because the chimney and roof frame move independently and water arrives from several directions. The standard assembly has four parts: a front apron, step (base) flashings along the sides, a back pan or cricket at the upslope side, and counterflashing let into the masonry and lapping over everything else.

Components

• Apron flashing. Laid over the shingles on the downslope face and turned up the chimney.
• Step flashing. One piece per course along each side, woven into the shingles. Canadian guidance specifies pieces 75 mm (3 in) longer than the shingle exposure, with 100 mm (4 in) legs on roof and chimney.
• Back flashing or cricket. On the upslope side, a pan or a small ridged structure diverts water around the chimney.
• Counterflashing. Metal let at least 25 mm (1 in) into the mortar joints, extending at least 150 mm (6 in) down the masonry and lapping the base flashing by at least 50 mm (2 in).

Cricket (saddle)

A cricket prevents water and snow from piling up behind a wide chimney. Canadian guidance recommends one where the chimney is 600 mm (24 in) or more wide. It is framed from wood sheathing fastened to the deck, built to the same pitch as the main roof and covered in sealed sheet metal, with its lower edge about 100 mm (4 in) wide to form a water trough.

Sequence

1. Install underlayment, turning it up the chimney sides.
2. Fit the apron at the front after shingling up to the chimney.
3. Weave step flashings into each course up both sides.
4. Build and flash the cricket or back pan at the top.
5. Cut a reglet into the mortar joints and fit counterflashing, sealing the joint.

Common problems

Sealant smeared over a joint instead of a proper counterflashing, missing crickets, and step flashings nailed into the masonry are frequent causes of chimney leaks. In the UK and Europe, lead is the traditional material, using lead soakers, stepped cover flashings and a back gutter.