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Exterior Trim and Window Capping: Aluminum Capping at $9 a Foot, PVC and Fibre-Cement Trim at $14, Corner Posts, What Capping Hides, and When It Rots the Wood Under It
Exterior Finishes
Window Capping and Exterior Trim: $9 a Foot to Never Paint the Brickmould Again, and the Top Edge That Decides Whether the Wood Under It Lives
Capping is aluminum sheet bent on a brake to wrap the wood trim around a window or a door, so the trim never needs paint again. It is the cheapest exterior trim line there is, $9 a linear foot, and on most Ontario houses with wood-trimmed windows it is the right call. It is also, done badly, the most reliable way I know to turn a painted brickmould into a rotten one without anyone seeing it happen, because the aluminum looks the same over sound wood and over mush. Here is what capping is, what it costs against replacing the trim in PVC, and the one detail at the top of the window that separates the two outcomes.
The numbers
$9aluminum capping over wood trim, per lf
$14PVC or fibre-cement trim board, per lf
$85flashing kit per opening, on a re-side
$1,627ten windows capped, with HST
| Line | Rate | What it covers |
|---|---|---|
| Aluminum capping on existing wood trim | $9 per lf | Prefinished aluminum bent on site to the trim profile, nailed at the edges, sealed at the joints. Brickmould, window and door casings, garage door jambs, fascia returns. |
| PVC or fibre-cement trim boards | $14 per lf | New trim, 1×4 to 1×6, replacing the wood: cellular PVC that cannot rot, or fibre-cement trim to match Hardie siding. Includes the flashing behind the head. |
| Corner posts | $12 per lf | Outside and inside corner trim for siding, PVC, aluminum or matched to the siding. |
| Window and door flashing kit, re-side | $85 per opening | Sill pan, head flashing, self-adhered tapes at the jambs, done when the siding is off and the opening is exposed. |
| Exterior painting of wood trim | $3.50 per sq ft | Two coats, the alternative to both. On trim that is a few inches wide, this is a labour job with a ladder, and it comes round again in six years. |
Minimum job $900 for a capping-only call. Capping and trim are usually done with the eaves or the siding, and the siding cost page carries the corner posts and trim boards beside the cladding rates.
How capping is done, and the part that matters
The installer measures the trim, bends a piece of 0.019 in. prefinished aluminum on a portable brake to a U that fits over it, cuts the corners, and fits the pieces: sill first, then the two sides, then the head. Each piece is nailed with small coloured nails at its outside edge, the joints are lapped so water runs over and not into them, and the laps and nail heads are caulked. A good capper does a window in under an hour.
The part that matters is the top. The head piece sits against the siding above the window, and whatever the siding does with water at that line, the capping has to deal with. On a house with J-channel above the window, water running down the wall lands in the J-channel and runs out the ends, onto the top of the capping, which is fine. On a house where the siding butts the trim with a bead of caulk and no channel, the caulk fails in five years and the water goes behind the head capping, down the back of the side pieces, and onto the sill wood, which is sealed in by the capping and never dries. That is how capped trim rots.
The fix is a head flashing, a bent strip of aluminum tucked up behind the siding and lapped out over the top of the head capping, so water that comes down the wall is thrown out over the window instead of behind it. It is a 5 minute piece on the brake. The $85 a window flashing kit on a re-side includes it with the sill pan and the tapes; on a capping-only job over existing siding, ask for a head flashing on every window and look for it when it is done. The wall protection hub has the photographs to take.
Before capping, check the wood. Push a screwdriver into the bottom corners of every brickmould and the sill. Soft wood gets replaced, not capped, and the honest capper tells you so on the day. Capping over rot is the same mistake as the fascia page describes, at a window instead of an eave.
Capping or new trim? A ten-window house priced
Ten windows averaging 3 x 5 ft, wood brickmould, sound. Capping goes around the outside of the brickmould, so each window is about 16 lf of capping: 160 lf. Capping 160 at $9 is $1,440 before HST, $187.20 of HST, $1,627.20. Done in two days, no paint, no ladder for the next twenty years, as long as the head detail is right.
The same ten windows, trim replaced in PVC. The brickmould comes off, 1×4 cellular PVC goes on with a head flashing behind it: 160 at $14 is $2,240 before HST, $291.20 of HST, $2,531.20. Nine hundred dollars more than capping. What you get for it is a trim that can be cut, mitred and painted to any profile, that cannot rot, and that does not depend on a bead of caulk staying soft. On a house getting new fibre-cement siding, the fibre-cement trim at the same $14 is the match.
The same ten windows, on a re-side. Siding off, openings exposed. Now the flashing kit goes in at $85 a window, $850, under either trim choice: that is the sill pan and the tapes that protect the wall, not the trim, and it is the best $850 on the job. Capping on a re-side also gets easier, because the head flashing goes in behind the new siding as it should. The window leaks page explains what the $850 is preventing.
Where capping goes wrong
- No head flashing. The one above. Caulk at the top of a window is a five-year seal; flashing is a lifetime one.
- Capping over soft wood. It hides the problem and keeps it wet. Rotten brickmould is replaced at the $14 a foot trim rate first.
- Nails through the face. A capper in a hurry nails through the flat of the sill piece, and every nail is a hole in the one surface that holds water. Edges only, and the sill piece should be bent so it sheds, not pools.
- Caulk as structure. Capping that depends on a bead of caulk to keep water out of a joint, instead of a lap, is capping that leaks when the caulk does. Laps face down and out.
- Capping the weep holes. Vinyl and aluminum windows have small slots at the bottom of the frame that drain the sill track. A sill piece that covers them traps water in the window.
- Colour from the wrong chart. Capping aluminum comes in the standard eaves colours. Matching a custom siding colour is possible from some suppliers and takes weeks; match the eaves instead, which is what the eye reads it against anyway.
Corner posts and the rest of the trim
Siding needs trim at every corner, every opening and every change of material, and on a re-side those lines add up before the first panel goes on. Corner posts at $12 a foot: a two-storey with four outside corners at 20 ft is 80 lf, $960. Trim boards at $14 a foot where the siding needs a frame: frieze under the soffit, a band between a stone base and the siding above, the trim around a garage door. A house with a lot of corners and bump-outs can carry $2,000 to $3,000 of trim before the cladding, and this is one reason a house with dormers and bays costs 15 to 25% more per square foot than a plain box.
On the ICF houses we finish in stucco, the trim is in the stucco itself: foam bands and frieze boards coated with the finish, priced per foot on the stucco details page. There is nothing to cap, which is one of the quieter arguments for it.
Questions people ask
How much does window capping cost?
$9 a linear foot of trim, installed, before HST, with a $900 minimum job. An average window is about 16 ft of capping, so about $145 each; ten windows are about $1,440. New PVC or fibre-cement trim instead is $14 a foot.
Does aluminum capping cause rot?
Not by itself. Capping with no head flashing lets water in at the top and then stops it drying out, and that rots the wood. Capping over wood that was already soft hides rot that was already there. With a head flashing and sound wood, capped trim lasts decades.
Is PVC trim better than capping?
It costs $5 a foot more and it cannot rot, dent or depend on caulk. On a re-side, on a house getting fibre-cement siding, or on a window where the wood is already gone, it is the better buy. On sound wood trim under existing siding, capping is the sensible one.
Can you cap windows without removing siding?
Yes, that is the usual job. The capping goes over the brickmould and tucks under the J-channel or the siding edge, and a head flashing is slid up behind the siding above each window. If there is no J-channel and the siding is caulked to the trim, ask how the head will be flashed.
Should I cap or replace my windows?
If the glass is sound, the sashes seal and only the exterior wood is tired, cap it. If the units are fogged or the frames are rotten past the brickmould, capping is a cosmetic over a window that is leaving anyway; the windows section covers that call.
Get your own number
The soffit, fascia and eavestrough calculator carries capping and trim boards as lines beside the eaves, so a trim job and an eaves job can be priced in one sheet by the foot. If you have a capping quote and it does not mention head flashing or what happens to soft wood, the $99 review will ask the questions for you.
Sources and further reading
- Ontario’s Building Code, Section 9.27 Cladding: the first and second planes of protection, sheathing membrane, flashing at openings and transitions, and the clearances from grade and roofs that every siding and stucco job has to meet.
- James Hardie installation instructions: the clearances, fastening, flashing and primed-cut rules a fibre-cement warranty depends on; the installer is held to this sheet.
- Canada Mortgage and Housing Corporation: the Canadian wall-assembly and moisture research behind rainscreens and the second plane of protection.
