Window Drip Cap: Install Head Flashing Over Windows and Doors

Rotted window headers are one of the most common surprises when old siding comes off, and the cause is usually a missing strip of metal no wider than your hand. A window drip cap is a bent piece of flashing that sits on top of the head trim and kicks water out past the face of the siding. Without it, rain runs down the wall, sits on the flat top of the trim, and soaks into the joint between the window and the framing.

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Installing one is not hard. The work comes down to sizing the cap to your trim, slipping its vertical leg behind the housewrap rather than on top of it, and leaving a small gap so water can escape. The same detail applies to exterior doors, garage doors and any trim board that interrupts siding.

Here is how drip caps work, what to buy, how to install them on new and existing windows, and how to handle doors and siding transitions.

What a Window Drip Cap Does

Think of a window drip cap as a tiny roof over the head casing. It has three parts: a vertical back leg that goes up the wall behind the weather barrier, a short horizontal top that covers the trim, and a front lip that turns down over the face of the trim so water drips clear. Some builders call it head flashing, window drip edge or window drip edge flashing, and on older homes you may hear it described simply as window metal flashing. All describe the same job.

Water that runs down siding hits the cap and is redirected outward. Without the cap, water tracks along the top of the trim, wicks into end grain, and eventually reaches the rough opening. Even windows with a nailing flange need a drip cap whenever there is head trim, because the trim creates a ledge the flange cannot protect.

Materials and Sizes

Drip caps are sold as preformed lengths, usually 10 feet long, in several profiles. Choose based on trim thickness and exposure.

• Aluminum drip cap: The most common choice. Light, rust-free, available in white, brown and other colors to match trim. Avoid direct contact with copper-treated lumber.
• Galvanized steel: Stiffer and a bit cheaper, but can rust at cut edges if the coating is scratched.
• Vinyl: Used with vinyl siding systems. Inexpensive but can become brittle in cold climates.
• Copper: Durable and attractive on historic homes, but pricey and incompatible with aluminum or steel nearby.

Profiles are sized by the depth of the top, which should match the thickness of your head casing plus any band or crown molding. Common depths are 5/8, 1 1/8 and 1 5/8 inches. The front lip should drop at least 1/4 to 3/8 inch over the trim face, and the back leg should rise 1 1/2 to 2 inches or more up the wall. If you cannot find a stock size, bend your own from trim coil on a sheet metal brake.

Tools and Materials

• Preformed drip cap or trim coil and a brake
• Aviation snips and a hand seamer
• Utility knife for slitting housewrap
• Flashing tape (self-adhered, compatible with your housewrap)
• Stainless or aluminum siding nails, or galvanized for steel caps
• Exterior sealant compatible with metal and trim
• Pry bar and putty knife for retrofit work
• Level and tape measure
• Ladder or scaffold, gloves and safety glasses

How to Install Drip Cap on New Windows

On new construction or a full re-side, installing drip cap on windows happens after the window and head trim are in, and before the siding goes on.

1. Confirm the housewrap flap. Proper window installs leave a flap of housewrap at the head, cut at 45 degrees from each top corner and folded up. If yours does not have one, cut it now with a utility knife and tape it up temporarily.
2. Install the head trim. Nail the head casing in place over the window flange and side casings.
3. Cut the cap to length. Measure the head trim and add 1 inch on each side so the cap extends past the trim ends.
4. Fold the end returns. Snip the top and front lip at each end and fold the front lip around the trim end, creating a small closed end that prevents water from running sideways behind the trim. This step is often skipped and is worth the extra five minutes.
5. Set the cap. Slide the back leg against the sheathing, under the housewrap flap, with the top resting on the trim. Leave a 1/8 inch gap between the bottom of the front lip and the trim face if possible so water drips free.
6. Fasten high. Nail through the back leg only, near the top, so the nails will be covered by housewrap and siding. Never nail through the horizontal top.
7. Tape the back leg. Run flashing tape across the top of the back leg onto the sheathing, then fold the housewrap flap down over it and tape the diagonal cuts.
8. Install siding with a gap. Leave about 1/4 inch between the bottom edge of the siding and the top of the drip cap. Do not caulk this gap; it is the drainage path.

Retrofitting a Drip Cap Above an Existing Window

Adding a drip cap above window trim on an existing house is a common repair after leaks show up. With lap siding, you can usually work it in without stripping the wall.

1. Remove or loosen the course of siding directly above the head trim. Use a pry bar and putty knife, or a siding removal tool for vinyl.
2. Inspect the top of the trim and sheathing. Probe for soft wood. Replace any rotted trim before continuing.
3. Slit the housewrap horizontally about 2 inches above the trim if it is not already loose.
4. Slide the drip cap’s back leg up into the slit so the wrap laps over the leg.
5. Nail high on the back leg, tape the slit closed with flashing tape over the top of the leg, and refasten the siding with the drainage gap.

If your siding is stucco, brick or fiber cement panels that cannot be loosened, a retrofit becomes more involved. Some installers cut a shallow kerf and use a surface-mounted cap sealed at the top, but that relies on sealant and needs regular inspection.

Drip Cap for Doors and Siding Transitions

An exterior door drip cap follows the same steps as a window. Doors are often more exposed because they lack the overhang a window may get from a soffit, and the head trim is wider. A drip cap for door trim should extend past both side casings and have folded end returns. If you have a storm door, install the cap before mounting the storm door’s top rail.

Garage doors need door drip edge flashing too, especially where wide wood or composite jamb trim meets siding. Use a longer piece or overlap two lengths by at least 2 inches, sealing the overlap with a bead of sealant.

Siding drip edge, sometimes called a Z flashing or water table flashing, protects horizontal transitions such as the joint between two siding materials or between siding and a band board. It is installed the same way: back leg behind the wrap, front lip out over the lower material, and an open gap above.

Moisture, Mold and Hidden Damage

When you open up a wall around a window, look for dark stains, soft sheathing, or a musty odor. These indicate long-term water entry. Fix the source first: install the drip cap, correct the housewrap laps, and replace damaged sheathing and trim before closing the wall. Let any damp framing dry fully.

Small areas of surface mold on wood can be cleaned with detergent and water while wearing gloves, eye protection and an N95 respirator. If mold covers more than about 10 square feet, or if it extends into insulation or interior drywall, hire a mold remediation professional. Covering wet or moldy wood with new siding only traps the problem.

Troubleshooting

• Water still stains the trim: The back leg may sit on top of the housewrap, sending water behind it. Reinstall under the wrap.
• Drips at the trim ends: Add end returns or a dab of sealant at the ends, and make sure the cap extends past the trim.
• Cap sags or bows: The trim may be uneven, or the cap too thin. Shim the trim or use heavier stock.
• Paint peeling on head casing: Siding caulked tight to the cap can hold water. Cut the caulk out and restore the gap.
• Rust streaks: Galvanized cap with scratched coating. Replace with aluminum or prime and paint the exposed edges.

Cost to Add or Replace a Drip Cap

A 10-foot length of preformed aluminum drip cap usually costs about 5 to 20 dollars depending on profile and finish, so materials for a typical window are a few dollars. Flashing tape adds 15 to 40 dollars per roll. Done during a re-side, drip cap adds little to labor.

Retrofitting existing windows typically runs 75 to 250 dollars per opening when a contractor must remove and reinstall siding, more if rotted trim or sheathing needs replacing. Upper-floor windows cost more due to ladder or scaffold setup.

Ladder Safety and When to Call a Pro

Set ladders on firm, level ground at a 4-to-1 angle and keep your belt buckle between the rails. Do not lean out to reach the end of a long cap; move the ladder. For second-story windows, use scaffolding or hire a pro.

Call a siding or window contractor when you find rot in the header or framing, when the siding is stucco or brick, when windows lack a proper flange, or when you are unsure how your weather barrier is lapped. A pro can also replace damaged sheathing and integrate new flashing with the existing wall system so the repair lasts.