How to Flash a Roof to a Wall: Vinyl, Fibre Cement, Stucco and Masonry
The roof side is always the same. What changes is how each cladding covers the flashing and where the wall's water barrier goes.
- Advanced
- 2 to 6 hours per wall
- Updated
The short answer
Roof to wall flashing is the same on every wall, but the cladding changes how it is covered. On vinyl and fibre cement, the wall leg goes behind the siding with the barrier lapped over it and a clearance of about 1/2 to 2 inches above the roof. On stucco, a receiver or weep detail is needed. On brick and stone, counter flashing goes into a reglet over the base flashing.
The flashing pieces at a wall, step flashing on a sidewall and apron flashing at a headwall, are installed the same way on every roof. What varies is the wall. Siding, stucco and masonry each stand differently above the metal, and each has its own way of shedding the water that runs down it.
The governing idea is the lapping rule: the wall's water resistive barrier (building paper or housewrap) must lie over the top of the flashing's wall leg, and the cladding must stay clear of the roof. If either is backwards, water runs behind the metal. See the lapping rule.
This guide covers what has to come off, what gets cut and what clearance to leave for each major cladding. Always follow the cladding manufacturer's installation instructions where they differ from the common values given here.
Tools
- Flat bar for lifting siding
- Zip tool for vinyl siding
- Hook blade utility knife
- Tin snips (straight cut)
- Hand seamer
- Grinder with diamond blade and dust shroud for masonry
- Caulk gun
Materials
- Step flashing, apron flashing or counter flashing as the wall requires
- Ice and water shield
- Housewrap or building paper and compatible flashing tape
- Two-piece receiver or stucco weep screed where needed
- Sealant rated for the cladding and metal
- Paint for cut fibre cement edges
Safety first
- Work tied off. WorkSafeBC requires fall protection at 3 metres (about 10 feet) and wherever a fall could cause serious injury. Walls rise above roofs, so ladder set-ups matter as well.
- Cutting fibre cement and masonry produces silica dust. Use a shroud and vacuum, eye protection and a respirator rated for silica.
- Old stucco and some older siding products can contain hazardous materials. If the house is older, have a suspect material tested before cutting or grinding.
Step by Step
Step 1: Identify the cladding and the barrier behind it
Look at the wall and, if you can, lift a piece of cladding. Behind siding there should be building paper or housewrap over sheathing. Behind stucco there should be two layers of paper or a drainage plane. Behind brick there is an air space and a mortar-free cavity with its own flashing. If the barrier is missing or torn, it has to be repaired as part of the flashing work.
Step 2: Vinyl siding: lift the courses and keep the J-channel off the roof
Unlock each course with a zip tool from the bottom up so you do not crack cold vinyl. Vinyl is brittle in winter and needs to be handled carefully below about 5 degrees C. Install the base flashing with the wall leg against the sheathing, lap the barrier over it, and reinstall. The lowest course should clear the roof by about 1/2 inch to 1 inch in most vinyl instructions, but follow the vinyl manufacturer's published figure.
Vinyl siding expands and contracts with temperature, so nail in the centre of slots and never drive nails tight. A J-channel at the roof line is not used to catch water. It is only trim, and the wall flashing behind it does the real work.
Step 3: Fibre cement: keep it off the roof and seal cut edges
Fibre cement takes up moisture through its cut edges and bottom edge. Manufacturers commonly ask for a clearance of 1 to 2 inches above roofs, more than vinyl. Check the product instructions. Seal or paint any cut edges before installation, and use a kickout at the lower end of the wall. See kickout flashing.
Cut outdoors with a dust-reducing cutter or shear and follow silica precautions. Never use a standard circular saw without dust collection.
Step 4: Stucco: use a receiver or a weep detail
Stucco is rigid and does not move with the roof. Bridging from stucco to shingle, or running stucco down over the flashing, cracks and lets water behind it. The common detail is to stop the stucco about 2 inches above the roof with a weep screed or a receiver channel, so water drains onto the flashing and out.
Stucco walls usually have a layered barrier. The top layer must lap over the flashing's wall leg and any receiver top. Ask a stucco contractor to work with you on the order, since cutting stucco or patching it afterwards is a separate trade.
Step 5: Brick and stone: counter flashing in a reglet
Masonry cannot take nails, so the wall leg of step flashing is covered by a separate counter flashing set into a slot cut in the mortar. See counter flashing and how to cut a reglet. Brick veneer has its own drainage cavity, so keep flashing and weep holes at the base of the brick clear and do not seal them shut.
Step 6: Lap the barrier over the flashing on every wall type
Tuck the lower edge of the barrier over the flashing, tape the seam with tape the barrier manufacturer approves for the product, and never lap it behind the metal. Water that gets behind the cladding should run down the barrier, over the flashing and out. See roof flashing types for where each piece sits.
Step 7: Check the clearance before you close the wall
A gap that is too small lets cladding wick water and trap moss and needles. Too large and wind-driven rain can get up behind it. Commonly this is between about 1/2 inch and 2 inches depending on the product. Write the figure from the instructions on the wall and check it with a tape.
Step 8: Finish with sealant only where the cladding detail allows
Never seal the bottom edge of siding tight to the flashing. Water that gets behind the cladding has to drain out. See roofing sealants for where a bead helps and where it hurts.
Quick Reference by Cladding
- Vinyl: lift courses, keep the J-channel and lowest course roughly 1/2 to 1 inch off the roof, nail in slot centres.
- Fibre cement: commonly 1 to 2 inches clearance, seal cut edges, use a kickout.
- Stucco: stop above the roof with a weep screed or receiver, never bridge to the shingles.
- Brick and stone: counter flashing in a reglet, keep weep holes open.
- Wood and cedar siding: similar to vinyl but cut and prime cut ends, and leave about 1 to 2 inches of clearance.
Fraser Valley Notes
Most Fraser Valley houses are wood-framed and clad in vinyl, fibre cement or stucco, and all see many months of wind-driven rain. The barrier lap and the cladding clearance carry more of the load than the flashing does in these conditions. On exposed lots, keep to the larger clearance figures and check the joint each fall.
Common Mistakes
- Lapping the wall barrier behind the flashing, which sends water behind the metal.
- Siding or trim touching the roof, wicking water and holding moss against the wall.
- Running stucco down onto the roof so a rigid joint cracks at the first movement.
- Sealing weep holes or the bottom of the cladding so water behind the wall cannot drain.
- Skipping the kickout at the lower end of the wall, regardless of cladding.
- Not sealing cut fibre cement edges, which soak up water and swell.
When to Call a Professional
Stucco and masonry walls need trades who can cut and patch them. If the stucco has to be opened or the wall has hairline cracking near the roof, have a roofer and a stucco or masonry contractor coordinate the repair.
If the barrier behind the cladding is missing or rotten, or a wall is more than one storey above the roof, the job goes beyond flashing alone. A professional can assess what is behind the wall.
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 Flash a Roof to a Wall by Cladding Type: Questions
How do you flash a roof to a vinyl siding wall?
Unlock the lowest vinyl courses, install step flashing or apron flashing with the wall leg against the sheathing, lap the housewrap or building paper over the top of the wall leg, then relock the siding. Leave the vinyl clearance the manufacturer specifies above the roof, often about 1/2 to 1 inch, and nail in the centre of the slots.
How much clearance should siding have above a roof?
Commonly between about 1/2 inch and 2 inches, depending on the cladding. Vinyl often needs about 1/2 to 1 inch, fibre cement commonly 1 to 2 inches, and wood siding about 1 to 2 inches. The exact figure is in the cladding manufacturer's installation instructions. The gap stops water wicking into the siding and lets debris drain.
How do you flash a roof against a stucco wall?
Do not run stucco onto the roof. Install step or apron flashing with the wall leg behind the stucco's paper layers, then stop the stucco above the roof with a weep screed or receiver so water can drain. The building paper must lap over the flashing. Stucco and flashing work is often shared between a roofer and a stucco contractor.
Does step flashing go behind siding?
Yes. The wall leg of step flashing goes up behind the siding and on top of the wall sheathing, under the wall's water resistive barrier, and the barrier laps over the top of the metal. The siding then covers both. Never nail through the wall leg, and leave a gap between the siding bottom and the roof surface.
What goes behind brick where it meets a roof?
Brick is flashed with base flashing at the roof and counter flashing set into a cut mortar joint, called a reglet, above it. Brick veneer also has a drainage cavity and weep holes at its base, which must stay open. Counter flashing overlaps the base flashing by at least 4 inches and is sealed at the top of the reglet.
Related Guides
All Flashing Details 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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