What is Step Flashing?
Step flashing is a series of small L-shaped metal pieces installed one per shingle course wherever a sloped roof meets a vertical wall, such as the side of a dormer, the wall of a chimney, or the place where a single-story roof butts against a two-story sidewall. Each piece overlaps the one below in a stair-step pattern so water cascading off the roof is shed away from the wall at every layer. Step flashing is required by the International Residential Code on all asphalt shingle roofs at roof-to-wall intersections.
A residential roof meeting a vertical wall. The cyan stair-step column is the step flashing, with one L-shaped piece tucked under each shingle course and against the wall, so water cannot find a path behind the siding at any layer.
Where it goes and what it does
Step flashing lives at every junction where a sloped roof terminates against a vertical wall. The most common locations are the sides of dormers, the walls around a chimney, and the long sidewalls where a one-story roof attaches to a two-story house. Each piece of step flashing is a small L-shape with one leg laid flat on top of the roof underneath the shingle course and the other leg standing up against the wall behind the siding or counter-flashing.
The reason it has to be stair-stepped (rather than one continuous strip) is the way water moves down a wall during a storm. Rain that hits the wall above the roof runs down the wall, hits the top of each step-flashing piece, and is shed out onto the shingle below. With overlapping pieces at every shingle course, there is no continuous gap for water to follow, even in wind-driven rain.
Is step flashing required by code?
Yes, in Virginia and Maryland. Both states adopt the International Residential Code, which under section R905.2.8.3 requires base flashing at every roof-to-wall intersection on asphalt shingle roofs. Continuous (one-piece) flashing is permitted only in very limited situations; the standard residential practice required by every reputable inspector is overlapping step flashing tucked under each shingle course.
A new asphalt shingle roof installed without step flashing at a sidewall, dormer, or chimney is a code violation. It is also the single most common source of mystery interior leaks five to ten years after a budget re-roof.
Step flashing vs continuous (one-piece) flashing
Bargain contractors sometimes substitute a single long L-shaped strip (called continuous flashing) for step flashing at sidewalls because it is faster to install. It looks similar from the ground for the first few years. It fails earlier and more catastrophically. Here is why:
| Step Flashing | Continuous Flashing | |
|---|---|---|
| Shape | Many small L-shaped pieces, one per shingle course | One long L-shaped strip running the full sidewall |
| Water shedding | Sheds water onto the shingle at every course | Channels water down to a single exit point |
| Thermal movement | Each piece moves independently as the roof expands and contracts | One long piece develops stress, buckles, or pulls away |
| Repair | Damaged piece can be replaced individually | Whole strip must be removed and rebuilt |
| Required by IRC | Standard practice (R905.2.8.3) | Permitted only in very limited cases |
Bottom line: when a contractor quotes a roof replacement at a sidewall or dormer, the line item should read “step flashing” not “continuous” or “sidewall flashing.” If the quote doesn’t break this out, ask. The labor difference is small. The longevity difference is huge.
Does step flashing go behind the siding?
Yes. The vertical leg of each step flashing piece tucks up behind the siding or counter-flashing above. This is critical, because if the siding sits on top of the flashing, water running down the wall hits the front face of the siding, slides down, and lands on top of the exposed flashing, then runs onto the shingle. If the siding tucks behind the flashing (the wrong order), water runs behind the siding directly onto the roof deck.
On a siding replacement or new construction, the siding is installed AFTER the step flashing is in place, with the lap of the siding covering the vertical leg of the flashing. On an existing house where siding is already up, the proper repair is to remove the bottom course of siding, install the new step flashing, then reinstall the siding over it.
What DreamHome installs
DreamHome installs aluminum step flashing at every roof-to-wall junction on every roof replacement, sized correctly for the shingle exposure (typically 5 by 7 inches for standard architectural shingles). Each piece is overlapped to the next by half the piece’s height, fastened only on the top corner so it can move independently with thermal expansion, and tucked behind siding or counter-flashing where present. Step flashing is included in the base price on every DH quote at every roof-to-wall junction; it is never an upsell, and continuous one-piece flashing is never substituted to save labor.
Red flags on someone else’s roof
When you are getting roofing quotes, here is what to watch for at sidewalls, dormers, and chimneys:
- Continuous (one-piece) flashing at a sidewall. One long strip of metal running the full length of the wall instead of overlapping stair-step pieces. Faster to install, fails fast. Code violation in most jurisdictions.
- Caulk or sealant smeared along the wall-to-shingle joint. Sealant is a temporary fix, not a substitute for proper step flashing. If the only thing keeping water out is a bead of caulk, the underlying flashing is wrong or missing.
- Siding sitting on top of the flashing. The siding should cover the vertical leg of the step flashing, not sit on top. If you can see flashing exposed below the siding edge, the order is backwards and water can follow the wrong path.
- No step flashing at the chimney. Most chimney leaks are not the chimney; they are missing or failed step flashing where the chimney sidewall meets the roof.
- Old step flashing reused on a new roof. When the shingles come off, the step flashing comes off with them. Reusing 20-year-old aluminum to save $80 in materials is a corner-cut. Every new roof should get new step flashing.