Inside and Outside Corner Treatments for Metal Siding
Corners show every error in a wall — out of plumb framing, missing furring, panels pinned at both ends. The corner options available in metal, and what each one demands behind it.
The corner is the hardest thing on a siding job to hide. It is a straight vertical line seen against the sky from a long distance, it is where two planes meet at an angle that reveals any deviation, and it is where the panel run terminates and thermal movement has to go somewhere. Everyone looks at corners without realizing they are doing it.
Outside Corners: Four Approaches
Metal cladding offers a range of corner treatments, from simple to demanding, and they look genuinely different.
Over-Corner Trim
A formed corner piece with two legs is applied over the panel ends after the panels are installed. Fast, forgiving, and hides a multitude of small inaccuracies in the panel cuts underneath. The corner reads as an applied element with a visible width, which suits traditional and transitional designs. It needs a hem or return on each leg so the edge is stiff and straight; a raw folded edge will wander.
Receiver or Under-Corner
A corner post with receiving channels is installed first, and panels slide into it from both directions. The corner reads as a crisp recessed line and there is no exposed panel end. It is less forgiving than an over-corner because panel lengths must be cut accurately to land in the receiver with movement clearance and without falling short of the channel.
Wrapped or Mitred Corner
The panel itself is folded around the corner so the profile continues uninterrupted — a batten wraps the corner, or a standing seam pan turns the return. This is the highest-craft option and looks the most integrated, particularly on contemporary designs where an applied trim would read as a break in the surface. It requires shop-formed corner panels or skilled site brake work, precise dimensions, and no tolerance for out-of-square framing.
Reveal Corner
The two planes stop short of each other and a deliberate shadow gap is left at the corner, with a drained corner detail and a dark surface behind. This is the corner treatment that suits reveal-joint flush panel systems, and it is the most demanding to keep straight because the eye reads the gap width along the entire height of the building.
A corner that is 6 mm out of plumb over a two-storey wall will be visible from the street, because the eye compares it against the vertical of the building next door and against the roof line. Plumb and shim the corner backing before any cladding goes on, and check it with a level or a laser rather than trusting the framing.
The Backing Problem Nobody Plans For
Here is the practical failure that shows up on rainscreen jobs again and again: there is no furring at the corner. Strapping gets laid out at regular spacing across the field of the wall and simply runs out before it reaches the end. Then the corner trim has nothing solid to fasten to, and the last panel on each face has no support at its edge — which is exactly where it most needs one.
- ✓Plan furring layout from the corners inward, not from a fixed spacing outward. Put a strip hard at each corner on both faces before filling in the field.
- ✓The corner needs backing on both planes, not one. A trim fastened on one leg only will move, and the free leg will drum in wind.
- ✓Add blocking behind the corner in the cavity where the corner detail requires it, without blocking the cavity's drainage and ventilation path.
- ✓Check that the corner backing is plumb and that both faces are in their respective planes before the panels start.
Inside Corners
Inside corners are geometrically simpler — two planes meeting concave — but they collect water rather than shed it. Rain running down two intersecting walls concentrates at that line, and on a north-facing inside corner it may stay damp for long stretches of an Ontario winter.
The correct treatment is a formed inside corner trim with a return leg on each side, installed so that the drainage plane behind it is continuous and so that water reaching the corner has a path down and out at the base. Sealing the inside corner solid at the bottom is the mistake: it makes a vertical trough with a closed end. The base of an inside corner needs the same drainage exit as the rest of the wall, and it is worth checking specifically because it is easy to miss when the base trim is run.
Corners and Thermal Movement
Corners are where panel runs end, and a panel run needs at least one free end. The failure to watch for is a panel whose top is fixed at a soffit trim and whose bottom is jammed into a base trim, or a panel captured in receivers at both corners of a narrow wall with no clearance left.
The corner trim itself moves too. A two-storey corner post is a long piece of metal in full sun, and if it is lapped and pinned at both ends it will bow visibly. Lap corner trim pieces with a slip joint, fasten so only one section is fixed, and put the lap where it is least conspicuous — typically near a horizontal line the eye already reads, such as a floor band.
Out-of-Plumb and Out-of-Square Walls
Older Ontario houses are not square, and even new framing has tolerance in it. A corner trim can hide a small deviation, but only a small one — as the gap between trim and panel varies, the shadow line varies, and the eye picks it up immediately.
- ✓Measure the corner at top, middle and bottom before ordering trim, and check whether the two walls actually meet at ninety degrees.
- ✓Furring is where you correct plane and plumb. Shim it; do not try to correct at the trim.
- ✓Where a corner is genuinely out of square, an over-corner trim with generous legs hides more than a tight receiver detail will.
- ✓On a wrapped or mitred corner, out-of-square framing is not correctable at all — the folded panel has a fixed angle. Verify before ordering shop-formed corners.
Edge Quality at the Corner
Every panel end at a corner is a cut end. Raw cut edges are sharp, will show a bright line of exposed substrate on a painted panel, and on steel will eventually mark with rust. Hem the panel end where the system allows it, use the manufacturer's cut-edge treatment where it does not, and keep the cuts straight — a wandering cut behind a receiver channel that is slightly narrow will show itself.
Cut ends should also be kept clean of steel filings. A corner is where offcuts, swarf and hardware collect on the scaffold, and filings left on a finished panel face will rust into permanent orange marks that no amount of washing removes.
Frequently Asked Questions
Should the corner trim go on before or after the panels?
It depends on the system. Receiver-style corner posts are installed first and panels slide into them. Over-corner trims go on last, covering the panel ends. Mixing the two approaches — installing a receiver post and then trying to cover it — produces a heavy, awkward corner.
How do I keep a two-storey corner looking straight?
Set the corner backing plumb with a laser or a long level, shim it, and string a line down the finished face as the trim goes up rather than relying on the panels to keep it straight. Corners are checked against the whole building by anyone looking at it, so this is time well spent.
Do inside corners need to be caulked?
Not sealed solid. The inside corner trim needs a continuous drainage plane behind it and an open path down to the base drainage. Sealant may be used selectively as a secondary line, but a fully caulked inside corner with a closed bottom becomes a water trap.
Can panels be mitred at a corner instead of using trim?
A wrapped or brake-formed corner is the metal equivalent of a mitre and it looks excellent when executed well. It requires accurate, square framing, shop-formed or carefully brake-bent pieces, and a crew comfortable with the work. It is not a place to improvise on site.
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