How to Reflash a Chimney During a Reroof: Reuse, Replace and Sequence
A reroof is the one time the whole chimney is exposed. Decide what to keep, then build the flashing from the bottom up.
- Professional
- 4 to 8 hours
- Updated
The short answer
During 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.
A chimney is a single detail on the roof, but it carries more leak risk than the rest of the roof combined. A reroof gives a one-time opportunity: the old shingles are off, the deck is visible and the chimney flashing can be rebuilt as a proper system. Reusing tired flashing to save an hour is how a new roof ends up leaking around an old chimney.
The decision comes down to each part. The base flashing, meaning the apron, step flashing and back pan, is cheap and almost always replaced. The counter flashing is set into masonry and can sometimes be saved. The order of work matters as much as the parts. See chimney flashing parts explained for what each piece does, and how to tear off a shingle roof for the stripping stage.
Tools
- Angle grinder with a diamond masonry blade and a dust shroud
- Shop vacuum
- Flat bar and cold chisel
- Sheet metal brake
- Tin snips (straight cut and offset)
- Roofing hammer or nailer
- Caulking gun
- Respirator rated for fine dust
Materials
- New apron and step flashing, galvanized, prefinished steel or copper
- Back pan or cricket materials
- New counter flashing, if not reused
- Ice and water shield
- Lead or metal wedges
- Polyurethane or hybrid polymer masonry sealant
- Roofing nails, 1-1/4 inch
- Mortar for repointing
Safety first
- Work tied off. Tear-off around a chimney exposes the deck and the steepest part of the roof. WorkSafeBC requires fall protection at 3 metres (about 10 feet) and wherever a fall could cause serious injury.
- Grinding mortar and masonry releases respirable silica dust. Use a shrouded grinder with a vacuum and a respirator, and follow WorkSafeBC silica requirements.
- Loose brick at the top of a chimney can fall when the flashing is disturbed. Check the masonry before leaning on it, and keep people clear below.
Step by Step
Step 1: Inspect the old flashing and the masonry before tear-off
Look for rust, lifted edges, failed sealant, cracks at the corners and pieces that are too short. Check the brick and mortar for crumbling joints, cracked bricks and a failing crown. A chimney that needs masonry work should be repaired before flashing goes on. See how to inspect a chimney crown and cap.
Ask one more question before any metal goes on: is the chimney still used? A masonry chimney that no longer vents a fireplace, furnace or water heater is a leak risk with no benefit, and a reroof is the cheapest time to take it down below the roofline and deck over it. See how to remove an old chimney and roof over it. If it is kept, note its width across the slope now, because a chimney more than 750 mm (about 30 inches) wide needs a cricket under the BC Building Code, and many older chimneys never had one.
Step 2: Remove the old base flashing
Tear-off around the chimney is done by hand. Lift the old step flashing pieces out from under the shingles, pull the apron and back pan, and scrape away old cement and membrane. Do not pry against the brick. Check the deck around the chimney for soft spots, because the chimney is one of the most common places for hidden rot. See how to replace roof sheathing.
Step 3: Decide whether to reuse the counter flashing
Counter flashing set in a sound reglet can often be lifted and refitted, because it is not nailed to the roof. Check for rust, cracks, kinks and thin metal. Also check whether it will still lap the new base flashing by at least 4 inches, because a new thicker shingle or membrane layer can change the height.
If it is loose, rusted, surface-mounted with sealant only or set in a failing joint, replace it. A rebuilt reroof with old counter flashing is a weak link.
Tip: Lift the old counter flashing gently and number each piece so it goes back in the same place.
Step 4: Grind out and renew the reglets
Clean the groove to bare masonry. If a joint has crumbled, repoint it with mortar and let it cure before cutting a new reglet. Cut grooves at least 1 inch deep. See how to cut a reglet. Always use dust control and a respirator.
Choose joints so the counter flashing will lap the new base flashing by at least 4 inches.
Watch out: Cutting into failing mortar crumbles the groove. Repoint first and let it cure.
Step 5: Lay the membrane and field underlayment
Membrane goes up the chimney about 4 inches and onto the deck about 12 inches, with no gaps at the corners. It is the backup layer under all the metal. The field underlayment laps over the membrane. See how to install ice and water shield.
Step 6: Install the apron, step flashing and back pan in sequence
The apron goes on the downslope face over the shingles. Step flashing then goes up each side, one piece per shingle course, lapped over the apron's ends. The back pan or cricket goes last and laps over the top step flashing. See apron, step flashing, back pan and cricket.
Step 7: Set the counter flashing and seal
Install counter flashing last, after the shingles are laid around the chimney. Wedge it every 16 to 24 inches and seal it with polyurethane or hybrid masonry sealant. See how to install chimney counter flashing.
Finish with a hose test from above and a look in the attic.
Reuse or Replace: a Quick Guide
- Apron: replace. It is cheap and usually bent or rusted.
- Step flashing: replace. Old pieces are often too short or nailed through the wall leg.
- Back pan or cricket metal: replace. Corners and laps fail with age.
- Counter flashing: reuse only if sound, reglet-set, long enough and liftable. Otherwise replace.
- Membrane: always new.
Sequencing With the Roof
Do the chimney flashing as part of the roof, not after it. The base flashing has to be woven with the shingles, so the shingle courses and the flashing go up together. The counter flashing is the one piece that goes on last, because it covers everything. Plan the day so the chimney is not left open overnight, and see how to dry in a roof overnight if it cannot be finished.
What the BC Building Code Asks For at a Chimney
The code's rules for shingle roofs meeting masonry are short, and every reflash should meet them. They are minimums, and manufacturers' instructions or good practice often go further.
- Counter flashing: embedded at least 25 mm (1 inch) in the masonry, running at least 150 mm (6 inches) down the face, and lapping the base flashing by at least 100 mm (4 inches).
- Stepped pieces: the base step flashing and the stepped counter flashing both lap at least 75 mm (3 inches) at each step.
- Upslope side: where the roof rises from the back of the chimney, the flashing runs up the roof to a height equal to the flashing on the masonry, and at least 1.5 times the shingle exposure.
- Cricket: required where the upslope side is more than 750 mm (about 30 inches) wide, unless the flashing runs up the chimney at least one sixth of its width and not less than 150 mm.
- Crown: a sloped cap with a drip at least 25 mm out from the wall, and the flue liner finishing 50 to 100 mm above the cap.
Fraser Valley Notes
Chimneys here fail from the back first. Fir and cedar needles, moss and wet leaves pack into the space behind the chimney, hold water against the back pan for months, and rust it from the top down. When you strip the old flashing, look hardest at the upslope side and the deck under it. A back pan that was rusted through often means the sheathing behind the chimney has to be replaced too.
In Hope and up the canyon, heavy wet snow sits behind a wide chimney and melts slowly against the masonry. That is where a proper cricket earns its keep, even on chimneys close to the 750 mm threshold. Across the valley, the long wet season means brick and mortar that looked fine in August can be spalling by spring, so repoint any soft joints during the reroof rather than setting new counter flashing into them.
Common Mistakes
- Reusing old step flashing. It is cheap to replace and often rusted or short.
- Reusing counter flashing without checking the lap. New layers raise the roof surface, and the old piece may no longer cover the base flashing.
- Not checking the deck. Chimneys hide rotted sheathing that a new roof will cover.
- Cutting reglets in bad mortar. The groove crumbles. Repoint first.
- Flashing the chimney after the shingles. Base flashing must be woven with the shingle courses.
- Sealing instead of lapping. Sealant fills gaps but does not replace laps.
- Leaving old metal or cement under the new flashing. Lumps of old cement and bent scraps hold the new pieces off the deck and open gaps at the corners.
- Keeping a chimney nobody uses. An abandoned chimney still needs flashing, a crown and upkeep, and the reroof is the cheapest time to remove it.
When to Call a Professional
Chimney flashing during a reroof is the most detail-heavy part of the job, involving masonry cutting, metal work and ridge-side fall risk. If the chimney is tall, the masonry is crumbling or the roof is steep, hire a roofer. Masonry repair such as repointing and crown work is a mason's job.
If the chimney leaks after a reroof, see how to diagnose and fix a leaking chimney.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Roof Repair service or the towns we cover.
How to Reflash a Chimney During a Reroof: Questions
Should chimney flashing be replaced when you reroof?
Yes, in almost every case. The apron, step flashing and back pan are inexpensive compared with the roof, and old pieces are often rusted, too short or installed wrongly. A reroof exposes the whole chimney, so it is the best time to rebuild it as a system. Counter flashing in good condition can sometimes be reused.
Can you reuse chimney counter flashing?
Sometimes. If it is sound, set in a good reglet, can be lifted without damage and still laps the new base flashing by at least 4 inches, it can be refitted. If it is rusted, loose, surface-mounted with sealant only, or sits in crumbling mortar, replace it. Reusing tired counter flashing is a common cause of leaks after a reroof.
What order does chimney flashing go on?
Bottom to top. Membrane goes on first, then the apron on the downslope face, then step flashing woven into the shingles up each side, then the back pan or cricket on the upslope face, and finally the counter flashing set into the masonry over all of them. Each upper piece laps over the one below it.
Do you have to grind out the old reglet?
Usually you clean it out. Remove the old sealant and loose mortar with a grinder so the new metal and sealant bond to sound masonry. If the joint has crumbled, repoint it and let it cure before cutting a new groove at least 1 inch deep. Always use dust control and a respirator.
Should an unused chimney be removed during a reroof?
Often, yes. A masonry chimney that no longer vents any appliance still needs flashing, a sound crown and repointing, and it remains one of the most likely places for a roof to leak. During a reroof the shingles are already off, so taking the chimney down below the roofline, framing the opening and decking over it costs far less than doing it later. Confirm first that no furnace, water heater or fireplace vents into it, check permit requirements, and test older materials for asbestos.
Related Guides
All Chimneys guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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