Exterior Window Capping: Wrapping Trim in Aluminum Coil

Most peeling window trim is not a paint problem at all, it is a water problem hiding under a paint problem. Exterior window capping solves the repainting cycle by wrapping wood casings, sills and brick molding in custom-bent aluminum trim coil, so rain hits prefinished metal instead of bare grain. Done well, a capped window sheds water for 20 to 30 years with nothing more than an occasional caulk touch-up.

Part of our Windows & Doors guide — explore the full guide →

Done badly, capping seals soft, wet wood inside a metal box where it keeps rotting out of sight. That is why the job starts with probing the trim, not bending metal. You will need a brake (or a lot of patience with hand seamers), coil stock in a color that matches your windows, and a steady approach to ladder work on anything above the first floor.

Plan on 1 to 3 hours per window for a first-timer once the brake is set up, less as you settle into a rhythm. Sills take the longest because they need a sloped top, a drip edge and clean end caps.

What Exterior Window Capping Involves

Capping, sometimes called cladding or wrapping, means forming flat aluminum coil into profiles that match each piece of the existing trim: the head casing across the top, the two side casings, and the sill with its apron underneath. Each profile slips over the wood and is fastened with color-matched trim nails along hidden edges, then sealed where it meets the window frame and the siding.

The wood stays in place as the structural backer. That is the big difference from replacing trim with cellular PVC boards or a prefab trim kit, which removes the old casing entirely. Capping keeps the original proportions, costs less in materials and works well on older homes where pulling trim would damage plaster or siding around the opening.

Standard trim coil is about 0.019 inch thick and comes in 24-inch wide rolls, usually 50 feet long. Thicker stock (around 0.024 inch) resists oil-canning on wide flat runs but is harder to bend by hand. Painted finishes on both sides are worth the small upcharge, since the back face shows on some returns.

Check for Rot and Moisture Before You Wrap Anything

Push an awl or a thin screwdriver into the sill, the bottom 6 inches of each side casing and the corners where joints meet. Sound wood resists; rotted wood lets the tip sink with light hand pressure or feels spongy. Pay special attention to sills, since they collect standing water and almost always fail first.

• Surface softness under 1/4 inch deep: dig out, let dry, consolidate with a wood epoxy and fill.
• Soft spots deeper than that or spreading across the board: cut out and replace the section with rot-resistant lumber or PVC stock.
• Rot in the framing or rough opening behind the trim: stop and open the wall up. Capping over a leaking window just hides structural decay.

Also look for the source of the water. Failed caulk at the head, missing head flashing, a cracked sill or a gutter overflowing onto the window will all keep wetting the wood under new metal. Fix those first, and make sure there is a head flashing or drip cap above the top casing before you cover it.

Lead Paint and Ladder Safety

Homes built before 1978 very often have lead paint on exterior trim, sometimes many layers down. Test before you scrape, sand or pry, using an approved lead test kit or a certified inspector. If the result is positive, follow lead-safe work practices: mist surfaces instead of dry sanding, lay plastic sheeting on the ground 10 feet out from the wall, wear a P100 respirator and disposable coveralls, keep kids and pets away, and clean up with a HEPA vacuum and wet wipes. Capping is actually a common way to encapsulate sound lead-painted trim, as long as you minimize disturbing it and handle any loose chips safely. Very old homes may also have asbestos in some window glazing compounds or siding; test anything suspect before cutting into it.

Most capping happens on a ladder with sharp metal in one hand. Set extension ladders at a 4-to-1 angle (1 foot out for every 4 feet of height), on firm level ground, extended 3 feet above any landing, and keep three points of contact. Do not overreach; move the ladder instead. For second-story windows, the safer call is scaffolding, a lift or a pro crew with proper fall protection. Two-story capping work is a job worth paying for.

Tools and Materials

• Aluminum trim coil (estimate about 6 to 10 linear feet of 24-inch coil per average window, more for wide trim)
• A portable sheet metal brake, rented or purchased, for straight crisp bends
• Hand seamers (bending tongs) for end tabs and small returns
• Coil cutter or utility knife with sharp blades, plus left and right aviation snips
• Color-matched aluminum trim nails (1 1/4 inch) and a light hammer
• Exterior sealant rated for aluminum and paintable surfaces, in a matching color
• Tape measure, speed square, fine marker, cut-resistant gloves and safety glasses

A portable brake is the single tool that separates tidy, factory-looking capping from wavy, hand-bent metal. It clamps the coil along a straight line and folds it to an exact angle in one stroke. Rental yards stock 8 to 10 foot units, which handle every piece of a standard window.

Step-by-Step: Bending and Installing Window Capping

1. Repair and prime. Fix rot, reset loose trim with exterior screws and seal open joints. The wood should be dry and solid.
2. Measure each face. For every piece, record the face width, the depth of the edge (return) and the length. Add about 1 inch to each length for overlap and end tabs.
3. Cut strips from the coil. Score and snap with the coil cutter or run the coil through the brake and slit it. Keep the finish side protected.
4. Cap the sill first. Bend the sill profile so the top slopes away from the window, folds down over the nose, and ends with a small outward kick or hem that acts as a drip edge. Cut the ends long, then notch and fold tabs to close the horns of the sill.
5. Wrap the side casings. Bend each side so it covers the face and the outside edge. Tuck the inner edge into a narrow hem or 1/4-inch return that sits against the window frame. Let the bottom overlap onto the sill cap so water runs over, not into, the joint.
6. Cap the head last. The head piece laps over both side casings. Slide its top edge up under the existing head flashing or siding so water above stays on the outside of the metal.
7. Fasten on hidden edges. Nail into the returns and the edges that face the siding, roughly every 12 to 16 inches. Avoid face-nailing wide flat areas; fasteners there dimple the metal and leak.
8. Seal selectively. Run a neat bead where the capping meets the window frame on the sides and head. Do not seal the bottom edge of the sill or the drip hem, so any moisture that gets in can escape.

Work shingle-style from bottom to top, always lapping upper pieces over lower ones. That one rule prevents most capping leaks.

Troubleshooting Common Capping Problems

Wavy or oil-canned faces

Aluminum expands in the sun, and a wide flat face nailed tight will ripple. Bend a small stiffening crease along long faces, leave a little play in nail holes, and choose thicker coil on wide boards.

Gaps at the corners

Corners show every sloppy cut. Leave tabs about 1 inch long and fold them around the adjoining face instead of butting cut edges together. A dab of matching sealant behind the tab keeps it tight.

Water stains inside the window

Often the head piece was tucked under the siding wrong, or the sill end caps were sealed shut and trapping water. Recheck the laps and open a drainage path at the sill.

Peeling sealant

Wipe the metal with denatured alcohol before caulking and use a sealant labeled for aluminum. Generic latex caulk lets go within a season or two.

Exterior Window Capping Cost Ranges

DIY materials typically run about $25 to $60 per window for coil, nails and sealant, plus a brake rental in the range of $50 to $100 per day. Buying a portable brake makes sense only if you plan to wrap a whole house or fascia as well.

Professional exterior window capping usually lands between roughly $150 and $400 per window, depending on trim width, sill complexity, height and how much rot repair is needed first. Second-story windows that need staging, and old windows with deep sills or decorative casings, sit at the top of that range. Many siding contractors price it as an add-on during re-siding, which is often the cheapest time to do it.

When to Call a Pro

Call a siding or trim contractor when windows are on a second story or higher, when rot reaches the sill framing or wall sheathing, or when lead test results are positive and you would rather not handle containment. Certified lead-safe renovators are required by law for paid work on pre-1978 homes in many situations, so ask any contractor for that certification. A pro is also worth it on historic homes, where local review boards may restrict metal cladding over original trim.