Chimneys: Flashing, Crickets, Crowns and Leaks
10 step by step guides
A chimney is a masonry or framed tower poking through the roof, and it is one of the hardest things on the roof to make watertight. It interrupts the slope on all four sides, it moves differently from the roof around it, and the masonry itself soaks up water. Chimney flashing is built in layers, each one lapped over the one below, and a missing layer is usually where the leak is.
Begin with chimney flashing parts explained to see how the apron, step flashing, counter flashing, back pan and cricket fit together. Then work through the installs in the order they go on: apron flashing, step flashing up the sides, a back pan or a cricket on the upslope side, and finally the counter flashing set into the masonry.
Not every chimney leak is a flashing leak. A cracked crown, a missing cap or porous brick can all look like flashing failures from inside the house. See how to diagnose and fix a leaking chimney and how to inspect a chimney crown and cap before tearing anything apart. If a chimney is no longer needed, read how to remove an old chimney and roof over it first, because permits and appliance venting are involved.
Chimneys Guides
- Guide 1Chimney Flashing Parts ExplainedChimney flashing has four metal parts over a self-adhered membrane. The apron covers the downslope face and laps over the shingles. Step flashing runs up each side, woven into the shingle courses. The back pan or cricket on the upslope side divides water around the chimney. Counter flashing is set into the masonry and laps over the others from above. Every layer laps over the one below so water always sheds outward.IntermediateReading guide, 15 minutes
- Guide 2How to Install Chimney Apron (Base) FlashingApron flashing is one bent piece of metal on the downslope face of a chimney. Cut it about 8 inches longer than the chimney width, bend it so the roof leg is at least 4 inches and the wall leg 4 to 6 inches, fold the ends, and lay it over the shingle courses. Nail the roof leg only, keep nails out of the wall leg, and seal the corner folds. Counter flashing then covers its top edge.Advanced1 to 2 hours
- Guide 3How to Step Flash a ChimneyTo step flash a chimney, turn ice and water shield up the masonry, then install one L-shaped piece of flashing per shingle course along each side. Start with a piece lapped over the apron end, nail each piece once in the roof leg only, lap each by at least 3 inches, and end with the back pan or cricket lapped over the top piece. Counter flashing covers the wall legs.Advanced2 to 4 hours
- Guide 4How to Install Chimney Counter FlashingChimney counter flashing is a separate piece of metal that laps over the wall legs of the apron, step flashing and back pan. On masonry, cut a reglet at least 1 inch deep into a mortar joint, set the bent top edge into it, wedge it every 16 to 24 inches and seal it. Lap the base flashing by at least 4 inches. On siding-clad chimneys, surface mount it under the cladding.Professional3 to 6 hours
- Guide 5How to Build a Chimney Cricket (Saddle)A chimney cricket, or saddle, is a small peaked roof framed on the upslope side of a chimney to divert water around it. The BC Building Code requires one where the upslope side of the chimney is more than 750 mm (about 30 inches) wide. Frame two triangular planes from a ridge, sheath them, cover with ice and water shield and metal, and lap the metal over the step flashing and under the roof above.Professional4 to 8 hours
- Guide 6How to Install a Chimney Back PanA back pan is a small sloped metal saddle on the upslope face of a chimney up to 750 mm (about 30 inches) wide. Fold the corners, slope the pan to each side, turn it up the chimney 4 to 6 inches, and run the upper flange under the shingles and underlayment far enough up the slope to be at least as high as the wall leg, never less than 1.5 times the shingle exposure. Lap its ends over the top step flashing, then counter flash.Advanced1 to 3 hours
- Guide 7How to Reflash a Chimney During a ReroofDuring a reroof, tear off the old base flashing, inspect the masonry and deck, and replace the apron, step flashing and back pan or cricket. Counter flashing in good condition and properly set can sometimes be lifted and reused, otherwise grind out the old reglet, repoint if needed and set new. Install in order: membrane, apron, step flashing with the shingles, back pan or cricket, then counter flashing.Professional4 to 8 hours
- Guide 8How to Diagnose and Fix a Leaking ChimneyTo diagnose a leaking chimney, check the attic, then look at the flashing, the masonry, the crown and the cap. Water test from the bottom up in sections, running a hose for about 10 minutes on each: the flashing first, then the brick faces, then the crown and cap. Leaks in every rain suggest flashing. Leaks only in wind-driven rain suggest brick or mortar. Winter dampness with no rain suggests condensation.Advanced2 to 6 hours
- Guide 9How to Inspect a Chimney Crown and CapA chimney crown is the sloped slab of mortar or concrete on top of the masonry. Check that it overhangs the brick by about 2 inches with a drip edge, slopes away from the flue, has no cracks wider than a hairline, and that the flue liner stands above it. A cap over the flue with a screen keeps out rain and animals. Crown repair and rebuilding is a mason's job.Intermediate30 to 60 minutes
- Guide 10How to Remove an Old Chimney and Roof Over ItTo remove an unused chimney and roof over it, first confirm that no appliance vents through it, check permit requirements and test for hazardous materials. Take the masonry down below the roof deck, frame the opening with headers and trimmers sized to the rafters, sheath it, lay ice and water shield and underlayment, and shingle over. A chimney still in use must never be removed or capped.Professional1 to 3 days
Also on this site
Chimneys: Common Questions
What is chimney flashing made of?
Chimney flashing is most often galvanized or prefinished steel, aluminum, copper or lead-coated copper, cut and bent into an apron, step pieces, a back pan or cricket covering, and counter flashing. Steel and copper are the usual choices against masonry because aluminum can corrode in contact with wet mortar. Sealant is used only at the joints, never in place of lapped metal.
Does a chimney need a cricket?
A cricket, also called a saddle, is a small peaked roof built behind a chimney to split the water and steer it around. The BC Building Code requires one where the upslope side of the chimney is more than 750 mm (about 30 inches) wide, unless taller flashing is used as the code allows. Many roofers build one on narrower chimneys too in snow country or under trees.
Why does my chimney leak when it rains?
The cause is usually one of four things: failed or missing flashing at the base or sides, a cracked crown or missing cap letting water in from the top, porous or cracked brick and mortar, or a flue condensing in cold weather. Leaks that show only in wind-driven rain often point to the masonry. Leaks in every rain point to flashing.
Can chimney flashing be repaired or must it be replaced?
Small problems such as a lifted counter flashing edge or failed sealant can be repaired. Rusted, undersized or incorrectly lapped flashing should be replaced, and a reroof is the best time to do it because everything is exposed. Counter flashing set in good masonry can sometimes be reused if it is sound and can be refitted over the new step flashing.
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