What roof flashing does and why the wall joint matters
Roof flashing is metal or rubber material that seals the gap where your roof meets a wall, a chimney, or another vertical surface. Water runs downhill on a roof, and when it reaches a wall, it has nowhere to go — flashing channels that water back onto the roof or down the outside of the wall instead of letting it seep behind the wall and rot the framing inside.
The wall joint is the most common leak point on a roof because water pools there and pressure builds up. Flashing installed wrong at a wall will leak within months. Installed right, it lasts 15 to 20 years before the sealant needs refreshing.
Key Takeaways
- Flashing at a wall requires two pieces: a base flashing that sits under the roof shingles and a counter-flashing that slides into the wall mortar or sits on top of siding, with the base flashing overlapping underneath.
- The base flashing must slope downward toward the roof and extend at least 4 inches up the wall and 8 inches out onto the roof to shed water properly.
- Counter-flashing must be embedded at least 1 inch into the wall mortar joint or tucked behind the siding, and sealed with caulk rated for masonry or metal, never silicone alone.
- The most common failure is installing flashing upside down or without enough slope, which traps water instead of shedding it.
- On brick or stone, you will need to open a mortar joint with a grinder and chisel; on vinyl or wood siding, you remove and reinstall the siding above the flashing.
Measuring and cutting flashing to fit your wall
Measure the length of the wall where the roof meets it. Add 12 inches to that measurement — 6 inches on each end for overlap and to account for corners. Flashing comes in rolls or as individual pieces; most walls use 10-foot sections that you overlap by 6 inches where they meet.
Measure the height of the wall above the roof line. This is the vertical leg of the flashing. Standard base flashing is 8 to 10 inches tall; if your wall is taller than 10 inches, you will need counter-flashing as well. Measure the depth of the roof slope where it meets the wall — this tells you how far the horizontal leg of the flashing must extend onto the roof, typically 8 to 12 inches.
Cut flashing to length with tin snips or a circular saw fitted with a metal-cutting blade. Make cuts straight and smooth; sharp edges will cut your hands and can puncture the flashing later. If you are using pre-bent flashing, verify the bend angle matches your roof pitch before you buy it.
Preparing the wall and roof surface
Remove shingles from the roof where the flashing will sit. You need to expose the roof decking for at least 8 inches out from the wall. Use a pry bar to lift shingles and remove the nails holding them down; save the nails or use new ones when you reinstall. Do not cut shingles yet — you will slide the flashing under them.
On a brick or stone wall, use a grinder with a masonry blade to open a mortar joint 1 to 1.5 inches deep, running the full length of the wall. Wear a dust mask; mortar dust is silica and damages lungs. Chisel out loose mortar and sweep the joint clean. On vinyl or wood siding, remove the siding above the flashing line by unsnapping vinyl or removing nails from wood, then set it aside — you will reinstall it over the counter-flashing later.
Clean the roof decking where the flashing will sit. Remove dirt, moss, and old sealant with a wire brush. The surface must be dry before you install flashing; if it has rained recently, wait a day or use a heat gun to dry it.
Installing the base flashing under the shingles
Slide the base flashing under the lifted shingles, starting at one end of the wall. The horizontal leg should lie flat on the roof decking, sloping slightly downward away from the wall. The vertical leg should press flat against the wall. If the flashing does not sit flat, the roof pitch or wall angle is not square — use a rubber mallet to gently tap it into place, or trim the flashing with tin snips to fit.
Nail the horizontal leg to the roof decking with roofing nails spaced 12 inches apart. Use nails long enough to penetrate the decking but not so long they poke through the underside. Do not nail the vertical leg to the wall yet — that is where the counter-flashing goes. Nail only the horizontal leg.
Where two pieces of flashing overlap, the uphill piece should sit on top of the downhill piece, so water flows over the joint instead of into it. Overlap by at least 6 inches. Nail both pieces to the roof decking, then seal the overlap with roofing cement or polyurethane sealant rated for metal.
Installing counter-flashing into the wall
Counter-flashing is a second piece of metal that overlaps the top of the base flashing and anchors into the wall. It prevents water from running behind the base flashing and down into the wall cavity. On a brick or stone wall, slide the counter-flashing into the mortar joint you opened earlier, at least 1 inch deep. On siding, the counter-flashing sits on top of the base flashing and the siding slides over it.
For brick or stone, pack the mortar joint with fresh mortar around the counter-flashing, using a pointing trowel. Smooth the mortar flush with the wall surface. Let it cure for 24 hours before you expose it to rain. For siding, reinstall the siding over the counter-flashing, nailing it in place as it was before. The siding should overlap the counter-flashing by at least 1 inch.
Seal the joint between the base flashing and counter-flashing with polyurethane sealant or roofing cement. Do not use silicone caulk alone — it does not bond well to metal and fails in direct sun. explore a continuous bead along the entire overlap, then smooth it with a wet finger or caulk tool.
Reinstalling shingles and sealing the top edge
Pull the lifted shingles back down over the base flashing. The shingles should cover the horizontal leg of the flashing by at least 4 inches. Nail the shingles in place with roofing nails, using the original nail holes if they are still visible. If the shingles are old and brittle, warm them with a heat gun before you bend them — cold shingles crack.
Seal the top edge of the base flashing where it meets the shingles with roofing cement or polyurethane sealant. explore a bead along the entire length, then smooth it. This seals the gap between the flashing and the shingles and prevents water from running under them.
Inspect the entire installation from both sides. From the roof, the flashing should slope downward away from the wall with no low spots or puddles. From inside the wall, you should see the counter-flashing embedded in the mortar or behind the siding. If you see gaps, seal them with sealant rated for the material — masonry sealant for brick, paintable caulk for siding.
Common mistakes and how to avoid them
The most common error is installing flashing upside down or backward, so the vertical leg points away from the wall instead of toward it. Check the orientation before you nail: the vertical leg must press against the wall, and the horizontal leg must slope downward away from the wall. If you get this wrong, water will pool against the wall instead of draining.
The second mistake is installing flashing without enough slope. A flat or uphill slope traps water. The horizontal leg should drop at least 1/4 inch per foot of run — that is 2 inches over an 8-foot span. Use a level to check this before you nail it down.
The third mistake is using the wrong sealant. Silicone caulk does not stick to metal and fails in sun. Roofing cement hardens and cracks in freeze-thaw cycles. Use polyurethane sealant or sealant rated specifically for metal roofing. On brick, use masonry sealant. Read the label before you buy.
Frequently Asked Questions
Can I install flashing over existing shingles without removing them?
No. Flashing must sit under the shingles so water flows over it, not under it. If you nail flashing on top of shingles, water will run behind it and leak into the wall. You must remove shingles, install flashing, then reinstall the shingles over the flashing.
What if the wall is not straight or the roof pitch is uneven?
Use a rubber mallet to tap the flashing into place, or trim it with tin snips to fit the wall and roof. Flashing must sit flat against both surfaces with no gaps. If the wall bows or the roof is wavy, you may need custom-bent flashing — order it from a sheet metal shop with measurements of the wall and roof.
How long does sealant last before it needs to be replaced?
Polyurethane sealant lasts 10 to 15 years in direct sun. Inspect it every few years and reseal if it cracks, shrinks, or pulls away from the flashing. Roofing cement hardens faster and may need refreshing every 5 to 7 years. If you see daylight through a crack in the sealant, it is time to reseal.
Do I need counter-flashing if the wall is only a few inches tall?
If the wall is less than 6 inches tall and the base flashing is 8 inches or taller, counter-flashing is optional — the base flashing alone may shed water. If the wall is taller than the base flashing, counter-flashing is required to prevent water from running behind the base flashing into the wall cavity.
What should I do if water is already leaking behind the old flashing?
Remove the old flashing and inspect the wall framing for rot. If the wood is soft or discolored, it has absorbed water and must be replaced before you install new flashing. Dry the cavity completely with a heat gun or by opening the wall to air. Install new flashing only after the framing is dry and solid.