How to Step Flash the Sides of a Chimney on a Shingle Roof
One piece per shingle course up each side, one nail in the roof leg, and the wall legs covered by counter flashing.
- Advanced
- 2 to 4 hours
- Updated
The short answer
To 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.
The sides of a chimney are the long walls of the flashing job. Water runs down the side of the chimney and meets the roof surface, and the flashing has to catch that water and deliver it onto the shingles, one course at a time. The method is the same as step flashing along a sidewall: small L-shaped pieces woven into the shingles, each lapped over the one below.
What makes a chimney different is the masonry. Brick, block and stone do not take nails and do not move with the roof, and the wall legs have to be covered by counter flashing set into the mortar rather than by siding. This guide covers the roof side. The masonry cover is in how to install chimney counter flashing.
Tools
- Roofing hammer or coil nailer
- Tin snips (straight cut)
- Hand seamer
- Hook blade utility knife
- Tape measure
- Chalk line
- Cut resistant gloves
Materials
- Pre-bent step flashing, galvanized or prefinished steel, at least 4 by 4 inches and 10 inches long for laminated shingles
- Ice and water shield
- Roofing nails, 1-1/4 inch
- Shingles
- Polyurethane or hybrid polymer sealant, where the detail calls for it
Safety first
- Work tied off. You are working at the ridge end of the roof, close to the chimney with your back to the eave. WorkSafeBC requires fall protection at 3 metres (about 10 feet) and wherever a fall could cause serious injury.
- Keep away from the chimney top. Loose masonry and bricks fall without warning, and a chimney can shed mortar when tapped.
- Cut sheet metal edges are razor sharp. Wear cut resistant gloves, and file or hem edges.
Step by Step
Step 1: Turn the membrane up the masonry
Cut strips long enough for each side, fold them lengthwise and press them into the corner between the roof and the chimney. The membrane is a backup to the metal and is what stops water reaching the deck if a piece of flashing lets go. Overlap at the corners with the membrane placed around the base of the chimney so there is no gap.
Lap the membrane over or under the apron's end according to the shed: higher pieces lap over lower ones.
Step 2: Size the step flashing to the shingle exposure
The BC Building Code calls for step flashing at masonry to be stepped with at least a 75 mm (3 inch) head lap, in both the base and the counter flashing. For laminated shingles with about 5-5/8 inches of exposure, that means 10 inch long pieces. For a chimney on a steep roof or exposed to driven rain, use larger pieces, such as 6 by 6 inches. The roof leg should be wide enough to sit under the next shingle and past the wall, and the wall leg at least 4 inches tall. See how to install step flashing for the general sizing.
Use galvanized or prefinished steel, or copper. Bare aluminum corrodes in contact with wet mortar.
Step 3: Lap the first piece over the apron
The first piece sits on top of the apron's folded tab, so water running down the side of the chimney passes from the step flashing onto the apron and then onto the shingles. Place it over the lowest shingle course at the chimney, with its wall leg tight against the brick. See how to install chimney apron flashing.
Step 4: Lay a shingle course to the chimney
Trim the shingle end so it does not touch the chimney. Shingles pressed tight against the brick wick water and buckle as the roof moves. A small gap lets water drain down the flashing. Some manufacturers call for the end of the shingle to be bedded in cement, so read the instructions on the wrapper.
Step 5: Nail once in the roof leg, never the wall leg
One nail, high on the roof leg, where the next shingle and the next piece of flashing will cover it. The roof and chimney move independently, and a fastened wall leg tears. Keep nails away from the bend where water flows.
Never nail into the brick. Nailing into mortar cracks it and puts a hole in the path of water.
Watch out: A nail through the wall leg is the most common hidden chimney leak and sits where no one looks.
Step 6: Repeat course by course up each side
Work up the roof one course at a time so each piece is woven in, never all installed together afterward. Check every few courses that the pieces lap properly and that none have slipped down. Each side should end at the upslope corner with a last piece that extends around or past the corner so the back pan or cricket can lap over it.
Step 7: Finish with the back pan or cricket and the counter flashing
The last step flashing piece on each side sits under the ends of the back pan or cricket, which laps over it. See how to install a chimney back pan and how to build a chimney cricket.
Finally, the counter flashing covers the tops of the wall legs, set into a reglet in the mortar joint. See how to cut a reglet.
Flashing vs Masonry Movement
A masonry chimney sits on its own footing and does not move as the wood roof framing dries and shifts, while the roof deck moves with the seasons. If the flashing is fixed to both, it tears. Step flashing works because its roof leg is nailed to the deck and its wall leg is free against the masonry, and the counter flashing slides over the wall leg. That is why the wall leg is never nailed.
Membrane Turned Up the Masonry
The membrane should lap about 4 inches up the chimney, and the counter flashing should reach below its top edge so the metal covers it. If the membrane sticks above the counter flashing, sunlight will degrade it. Trim it to sit below the reglet.
Common Mistakes
- Reusing old step flashing. Old pieces are often rusted at the bend or were never properly lapped.
- Nailing the wall leg. It locks the roof to the chimney and tears when they move apart.
- Pieces too short for the shingle exposure. A gap opens between pieces and water runs through.
- Shingles jammed against the masonry. It wicks water and buckles the course.
- No membrane up the chimney. The membrane is the backup if the flashing leaks.
- Using one long strip instead of steps. A long strip has no lap to fall back on.
- Bare aluminum against mortar. It corrodes within a few seasons.
When to Call a Professional
Step flashing a chimney means working near the ridge, handling tall masonry and cutting into mortar for counter flashing. If the chimney is tall, the roof is steep or the mortar is crumbling, hire a roofer. The whole assembly should be built as one system, because one missing piece lets the rest fail.
If water is already staining a ceiling near the chimney, have the entire flashing checked, not only the side you can see.
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 Step Flash a Chimney: Questions
Do you use step flashing on a chimney?
Yes. On shingle roofs the sides of a chimney are flashed with step flashing, one piece per shingle course, woven into the shingles. The front has an apron, the back has a back pan or cricket, and counter flashing set into the masonry covers all the wall legs. Each piece laps over the one below it.
Where do you nail chimney step flashing?
One nail per piece, high in the roof leg, into the roof deck, where the next shingle and flashing piece cover it. Never nail the wall leg or into the masonry, because the chimney and the roof move independently. A nail through the wall leg tears the flashing and is one of the most common hidden chimney leaks.
How big should chimney step flashing be?
At least 4 inches on the roof leg and 4 inches up the wall, and at least 3 inches longer than the shingle exposure, since the BC Building Code calls for a 75 mm head lap. With about 5-5/8 inches of exposure, 10 inch long pieces do the job. Exposed or steep roofs benefit from larger pieces. Confirm the dimensions against the shingle manufacturer's instructions.
Can you reuse step flashing on a chimney?
It is possible if the pieces are in perfect condition and the right size, but it is rare. Old step flashing is often rusted at the bend, too short for modern shingle exposure or nailed through the wall leg. Because step flashing is cheap compared with a leak, most roofers replace it during a reroof.
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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