Wall Trim · Metal building trim guide
Metal Inside Corner Trim for L-Shaped Buildings and Lean-Tos
Inside corner trim seals the inward-facing corners of metal walls, like where a lean-to wall meets the main building.
Also called: inside corner, inside corner flashing, inside angle trim, interior corner trim
Metal inside corner trim closes the corner where two metal walls meet and point inward. You'll find these corners on L-shaped shops, on buildings with a notched porch, and where a lean-to's end wall runs into the side of the main building. An inside corner leaves a gap full of panel ribs and raw edges. This trim seals it.
A plain box building has no inside corners, so many people forget this piece until the walls are up. This guide shows where inside corners happen, how to measure them, and the two common ways to install the trim.
What is metal inside corner trim?
Inside corner trim (also called inside corner flashing or inside angle trim) is sheet metal bent so its two legs lie flat against two walls that meet at an inward 90-degree corner. It is bent the opposite way from outside corner trim, so the two are not interchangeable.
It keeps rain from running into the corner and behind the panels. It blocks the open ribs so insects, birds and rodents can't move in. And it hides uneven panel cuts so the corner looks crisp.
Where inside corner trim goes on a metal building
Look at your floor plan. Any corner where you stand outside and the two walls wrap around you is an inside corner.
- Lean-to with end walls: where each lean-to end wall meets the main building's sidewall.
- L-shaped or T-shaped shop: the inward corner where the two wings meet.
- Notched porch or recessed entry: the corners at the back of the notch.
- Closed-in carport added to a shop: where the new wall meets the old wall.
Inside corner trim sizes, gauge, profile and colors
Most metal inside corner trim is 26 gauge steel to match R-panel and PBR walls. 29 gauge versions exist for lighter systems. Legs are commonly a few inches each way, but leg size and hems vary by panel maker and by building, so ask us what fits your walls.
It comes in Galvalume or painted colors. Most people order it in the trim color to match the outside corners, doors and eaves. Pieces are typically 10 ft long. Ask about longer lengths. Custom bends are available when two walls meet at something other than 90 degrees or a lean-to wall is out of square.
How to measure inside corner trim and how many pieces to order
Formula: add up the wall height at every inside corner. Multiply by 1.10 for laps and waste. Divide by the piece length and round UP to whole pieces. Each lap joint uses about 2 to 4 in. Measure each corner from the slab to where the roof or roof flashing covers it.
Worked example: our standard 30 x 40 gable building (12 ft walls, 3:12 pitch) has no inside corners by itself. Add a 12 x 40 lean-to along one sidewall with end walls closed in. The lean-to roof ties into the main sidewall about 11 ft up. That makes 2 inside corners, front and back, each 11 ft tall.
2 x 11 = 22 ft. Add 10%: 22 x 1.10 = 24.2 ft. With 10 ft pieces, 24.2 / 10 = 2.4, so order 3 pieces.
Corner check: each corner takes one full 10 ft piece plus a short piece of about 1 ft 3 in. (1 ft plus a 3 in. lap). Both short pieces cut from the third piece. 2 + 1 = 3 pieces. If you can get 12 ft pieces, each corner is one piece with no joint.
How to install inside corner trim
There are two common methods, and practice varies by crew and panel maker. Method A: fasten the trim to the framing first, then lap the wall panels over its legs. Method B: hang the panels first, then screw the trim over the panel faces. Method A hides the panel edges best. Method B is easier on a retrofit. Pick one before you start.
- Install base trim along both walls so it runs into the corner.
- Start at the bottom. Set the first piece tight into the corner and check it for plumb.
- Fasten with color-matched #12 tek screws into the framing (Method A) or #14 x 7/8 in. lap screws into the panels (Method B), about 12 to 24 in. apart on each leg.
- Lap the next piece OVER the lower one by 2 to 4 in., shingle-style, so water runs over the joint. Add butyl tape in the lap for extra protection.
- At the top, tuck the trim under the roof-to-wall flashing or eave trim so the roof trim laps over it.
- Brush off all drill shavings so they don't rust on the paint.
Common inside corner trim mistakes to avoid
- Ordering outside corner trim by mistake. The bend is reversed and it won't fit.
- Forgetting inside corners exist until the lean-to walls are already up.
- Measuring to the main eave instead of where the lean-to roof meets the wall.
- Lapping the lower piece over the upper piece, which channels water into the joint.
- Leaving the top of the corner open under the roof flashing. Seal it or tuck it under.
- Mixing Method A and Method B on the same building, which looks uneven.
What to order with inside corner trim
- Outside corner trim for the outward corners of the building
- Roof-to-wall flashing where the lean-to roof meets the main wall
- Base trim along both walls
- Color-matched tek screws and lap screws
- Butyl tape or sealant
- Inside foam closures at the wall base
- Touch-up paint
Get metal inside corner trim in Houston from Ironrod Steel
Ironrod Steel rolls its own R-panel and PBR and supplies matching metal inside corner trim, outside corners, closures, screws and butyl tape. Send us your building size, your lean-to size and a few photos, and we'll help you build a complete trim list. Pick up at 8122 Hillsboro St, Houston, TX 77029, or ask about delivery across Houston and Southeast Texas. Call or text (281) 902-7619.
Inside Corner Trim FAQ
Can I use outside corner trim on an inside corner?
No. Inside corner trim is bent the opposite direction so it sits flat on both walls. An outside corner won't lie flat and will leave gaps. Order the right piece for each corner.
Does a lean-to need inside corner trim?
Only if the lean-to has end walls. Where each end wall meets the main building's sidewall, you get an inside corner. An open lean-to with no end walls doesn't need it.
Does inside corner trim go on before or after the panels?
Both methods are used. Fastening it to the framing first and lapping panels over it hides the edges best. Screwing it over the panels is easier on a retrofit. Ask your panel maker or your crew which they prefer.
How tall does inside corner trim need to be?
Measure from the slab to where the roof or roof flashing covers the corner. On a lean-to, that is where the lean-to roof meets the main wall, not the main building's eave.