Roof Trim · Metal building trim guide
Roof to Wall Flashing for Metal Lean-Tos and Add-Ons
Roof-to-wall flashing seals the joint where a metal roof runs into a taller wall, like a lean-to against a shop.
Also called: endwall flashing, headwall flashing, sidewall flashing, lean-to flashing, wall flashing
Roof to wall flashing seals the joint where a roof runs into a taller wall. The classic case is a metal lean-to built against the side of a shop or barn. That joint is one of the most leak-prone spots on any building, because roof water and wall water both end up there.
This guide covers the two types, endwall and sidewall flashing, how to measure them with a real lean-to example, how many pieces to order, and how to install them so the water stays on the outside. It is written for contractors and DIY builders around Houston.
What Is Roof to Wall Flashing?
It is a bent piece of sheet metal with one leg that lies on the roof panels and one leg that runs up the wall. It bridges the gap so water running down the wall and across the roof stays on top of the metal.
There are two kinds. Endwall flashing, also called headwall flashing, goes where the top (high end) of the roof slope dies into a wall. Sidewall flashing goes along the slope where the side edge of the roof runs beside a wall. Both keep water, wind, birds and bugs out, and both give the joint a clean finished look.
Where Roof to Wall Flashing Goes on a Metal Building
The most common spot is a lean-to attached to the side of a metal building, house or shop. The high side of the lean-to roof meets the main building's wall, so it needs endwall flashing along that whole length.
Sidewall flashing shows up when a roof runs beside a wall, for example a lean-to tucked into the corner of an L-shaped building, a porch roof beside a taller wall, or a lower roof between two buildings. If the high side of a single-slope roof stands free with no wall, use high-side eave trim instead.
Wall Flashing Sizes, Gauge, Profile and Colors
This flashing is typically 26 gauge to match R-panel and PBR, in Galvalume or painted colors. Endwall flashing is bent to your roof pitch, so tell us the pitch. Sidewall flashing is bent to fit the panel's edge rib and the wall.
Leg sizes vary. A taller wall leg gives more room to tuck under the siding. Pieces are typically 10 ft long; ask us. Because every wall is a little different, this is one of the most common custom-bent trims. A photo of the joint with a tape measure helps.
How to Measure Roof to Wall Flashing and How Many Pieces to Order
Formula: measure the length of each joint where roof meets wall. Endwall = the length of the wall the high side touches. Sidewall = the slope length (roof run x pitch factor; 1:12 = 1.00, 2:12 = 1.01, 3:12 = 1.03, 4:12 = 1.05). Add 10% and round UP to whole pieces for each run.
Worked example: a 20 x 24 single-slope lean-to built against a shop wall, 2:12 pitch, roof panels running 20 ft from the shop wall out to the low eave. The joint along the shop wall is 24 ft. 24 x 1.10 = 26.4 ft = 3 pieces of endwall flashing. Outside closures under it: 24 / 3 ft panel coverage = 8, plus a spare or two.
If the same lean-to also sits against a wall on one end, add sidewall flashing: 20 ft x 1.01 = 20.2 ft. Two full pieces cover about 19.75 ft after the lap, so order 3 pieces for that run.
How to Install Roof to Wall Flashing
The golden rule is that every piece must shed water onto the piece below it. The wall leg should go behind the wall covering or under a counterflashing. A bead of caulk along the top edge is a backup, not the main seal.
- Finish and fasten the roof panels first. For sidewall flashing, plan the panels so a full rib runs along the wall.
- For endwall flashing, lay butyl tape across the panels at the high end, set the outside foam closures, and add tape on top.
- On a metal-sided wall, loosen the bottom screws of the wall panels and slip the wall leg up behind them. On brick, stucco or wood, cover the top of the wall leg with a counterflashing or set it under the siding or wrap.
- Fasten the roof leg through the closure into each major rib with #14 x 7/8 in. lap screws or roofing screws. Fasten the wall leg into the wall framing or girt, keeping screws high where the covering will protect them.
- Lap pieces 2 to 4 inches with sealant in the joint. On sidewall flashing, start at the eave and work up so each upper piece laps over the lower piece.
- At the bottom of a sidewall run, turn the end out (a kick-out) so water dumps into the gutter or off the eave, not down behind the wall.
Common Lean-To Flashing Mistakes to Avoid
- Face-nailing the wall leg on top of the siding and relying on caulk to keep water out.
- Ordering endwall flashing without the roof pitch.
- Leaving out outside closures at the high end of the panels.
- Lapping sidewall pieces backward so water runs into the joints.
- No kick-out at the bottom, so water runs down behind the wall.
- Using high-side eave trim against a wall where flashing belongs.
What to Order With Lean-To Flashing
A lean-to roof needs more than just the wall flashing. A typical list:
- Outside foam closures for under the endwall flashing.
- Counterflashing if the wall is brick, stucco or wood.
- Eave trim or gutter for the low edge, with inside closures.
- Rake trim for the open sides.
- Butyl tape, sealant, color-matched lap screws and roofing screws.
Get Roof to Wall Flashing in Houston From Ironrod Steel
Ironrod Steel Co. rolls its own R-panel and PBR roofing and supplies matching endwall and sidewall flashing, custom bends, closures, butyl tape and screws for lean-tos and add-ons. Send us the lean-to size, roof pitch and a photo of the wall it ties into, and we will help you build the trim list. Pick up at 8122 Hillsboro St in Houston or ask about delivery across Houston and Southeast Texas. Call or text (281) 902-7619.
Roof-to-Wall Flashing FAQ
What is the difference between endwall and sidewall flashing?
Endwall (headwall) flashing goes where the top of a roof slope runs into a wall, straight across. Sidewall flashing runs up the slope where the side of the roof meets a wall. Many lean-tos need only endwall flashing.
How do I flash a metal lean-to roof to an existing building?
Use endwall flashing bent to your roof pitch, with outside closures and butyl tape under the roof leg. Tuck the wall leg behind the wall panels or under a counterflashing so water cannot run behind it.
Can I just caulk the top of the flashing to the wall?
Caulk alone will fail over time from sun and movement. Use it as a backup only. The wall leg should be tucked behind the siding or covered by a counterflashing.
Does roof to wall flashing need closures?
Endwall flashing does. Outside foam closures go under it on top of the panels to fill the ribs. Sidewall flashing usually sits on a full panel rib with butyl tape and does not need closures.