Aluminum capping wraps the exterior wood trim around a window in bent coil stock, so there is no exposed wood left to paint. It costs $100 to $250 per opening and is one of the most commonly excluded items in a cheap quote.
Capping is the part of a window project people know least about and it accounts for a meaningful share of the price difference between quotes.
It is also the part most likely to be done badly in a way you will not notice for years.
What capping actually is
Aluminum coil stock is a flat roll of pre-finished aluminum, typically about 0.019 inches thick, in a range of standard colours. During a window installation, the installer measures each piece of exterior wood trim around the opening, bends the coil stock on a portable brake to match that profile, and fastens and seals it over the wood. The result is that the exterior trim is permanently covered in a finished metal skin. The wood underneath is still there and still structural, but it is no longer exposed to weather or UV, which means it no longer needs painting and no longer rots from the face. On an older Capital Region home where the exterior trim has been painted twelve times and is starting to fail, this is frequently the difference between a window project that ends and one that generates maintenance forever.
How it gets done badly
- Capping over rotted wood. The rot continues underneath, invisible, until the trim fails structurally. Wood should be sound or replaced before capping.
- Face nailing through the flat of the cap, which creates a penetration on a horizontal surface where water sits. Fasteners belong on the underside or in a lapped joint.
- Butt joints at corners rather than folded and lapped, which relies on sealant instead of geometry.
- No drip edge at the head, so water running down the wall gets behind the cap.
- Caulking the bottom edge continuously, which traps any water that gets behind it.
- Loose or oil-canned flat surfaces, which is cosmetic but signals rushed work.
- Colour mismatch with existing capping elsewhere on the house.
What happens to interior trim
| Replacement type | Interior trim | What to expect |
|---|---|---|
| Insert replacement | Undisturbed | Casing, stool and apron stay in place |
| Sash replacement kit | Undisturbed | Only the removable stops come off |
| Full frame replacement | Removed | Reinstalled if it survives, replaced if not |
| Full frame, historic home | Removed carefully | Old trim often does not survive, budget for matching |
| Full frame with drywall return | None | Drywall wrapped into the opening instead of casing |
On a full frame replacement in a pre-1940 home, plan for some trim not surviving removal. Milling a matching profile is normal and should be in the quote.
Should you cap or paint?
Capping costs $100 to $250 per opening and eliminates exterior painting on that trim permanently. Painting costs less up front and needs redoing every five to eight years in this climate, sooner on south and west elevations. Over twenty years capping wins on cost comfortably, and it wins on convenience immediately. The argument against it is appearance on a historic home, where flat aluminum over a detailed profile can flatten the look and where historic review may not permit it. On a designated property in Albany, Troy, the Schenectady Stockade or a Saratoga Springs review area, confirm before assuming capping is an option. Everywhere else, it is close to a default.
Ranges reflect typical Capital Region projects and are for planning only. Every Sundog quote is written, itemized, and free.
Frequently asked questions
What is window capping?
Pre-finished aluminum coil stock bent on site to match the profile of the exterior wood trim around a window, then fastened and sealed over it. The wood stays in place and stays structural but is no longer exposed to weather, which means it never needs painting again.
How much does capping cost?
Typically $100 to $250 per opening, depending on how much trim there is and how complex the profile is. It is one of the most commonly excluded items in a cheap quote, so confirm explicitly whether it is included and in what colour.
Is capping a good idea on an old house?
On most older homes, yes, because the exterior trim is usually the component in worst condition and capping ends the paint cycle permanently. The exception is a designated historic property, where flat aluminum over a detailed profile may not be approved and may flatten the appearance. Confirm with the review body first.
Can capping hide rot?
It can, and that is the main way it gets done badly. Capping over rotted wood means the rot continues invisibly until the trim fails structurally. The wood should be sound, or repaired or replaced, before anything is wrapped over it. A contractor who caps without checking is storing up a problem.
Will my interior trim be damaged during a window replacement?
Not on an insert replacement or a sash kit, where the interior casing is never disturbed. On a full frame replacement the trim comes off, and in a pre-1940 home some of it typically does not survive removal. Milling a matching profile is normal and should be priced in the quote rather than discovered mid-project.
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