Base flashing: the lower layer that carries water onto the roof
Base flashing is the piece that runs up the wall and under the shingles. When it fails, water goes straight into the roof edge.
Base flashing is the lower layer of metal that runs up a wall, chimney or curb and laps under the shingles. Its job is to carry water down the wall and out onto the roof surface instead of behind it.
Where base flashing sits
You find it wherever a roof meets something vertical. The bottom of the metal slides under the shingles. The top runs up the wall, where a second piece of metal covers it. That upper piece is the cap flashing.
Together the two form a shingle-style overlap. Water runs down, never up, and each layer sheds onto the one below it.
Base flashing and step flashing
On a sloped roof against a wall, base flashing is often made of many small bent pieces, one per shingle course. That version is called step flashing. On a chimney front, the base piece may be one long strip. All of it is base flashing, because all of it is the lower layer.
For the staircase version, see step flashing around a chimney.
How base flashing fails
It sits where water collects, so it takes more punishment than any other metal on the roof. The usual problems are slow ones.
- Rust, especially at the bottom edge where it stays wet.
- Pieces that have lifted or slipped out from under the shingles.
- Nail heads that have backed out and left holes.
- Tar or sealant smeared over a joint that should be metal.
A thick line of tar along the bottom of a wall is the sign to take seriously. It often means the metal underneath has already given up.
What to check from the ground
Use binoculars from the yard. Look along the line where the roof meets the wall. Rust streaks, a wavy edge or a dark bead of tar all deserve a photo.
Inside, check the ceiling and the attic framing along the same wall with a flashlight, staying on the joists. A stain that shows up near the wall after heavy rain points at the flashing line before it points at the shingles.
Do you need to replace it?
Not every old piece of base flashing has failed. Metal that is sound, lapped properly and free of rust can stay. Metal that is rotted, loose or buried under sealant usually should not. Ask the roofer which pieces are bad and ask to see them.
Why we do not show the repair
Replacing base flashing means lifting shingles and working along the roof edge beside a wall. That is high, awkward work, and we do not publish the steps. Take your photos, note the stained room and have a roofer explain what needs to change. To compare the two layers, see what is roof flashing.
Frequently asked questions
What is base flashing?
It is the lower layer of flashing that runs up a wall or chimney and laps under the shingles, so water is carried down onto the roof surface instead of behind it.
What is the difference between base flashing and cap flashing?
Base flashing is the lower layer lapped under the shingles. Cap flashing is the upper layer that covers the top of it. They are installed as a pair.
Is step flashing the same as base flashing?
Step flashing is a kind of base flashing. It uses many small pieces, one for each shingle course, along a wall that meets a sloped roof.
How do I know if base flashing has failed?
From the ground, look for rust, a wavy edge, lifted pieces or a thick line of tar along the roof and wall joint. Stains inside near that wall after heavy rain are another clue.
Can I replace base flashing myself?
We do not publish steps for it. It means lifting shingles and working at the roof edge. Document what you see from the ground and have a roofer do it.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- NRCA. National Roofing Contractors Association
- US OSHA. 1926.501 — Duty to have fall protection (OSHA)