What does a drip cap do on a window?

A drip cap redirects water that runs down the wall surface outward and away from the window frame. Its Z-shaped profile creates a physical barrier that prevents water from entering the gap between the window head and the rough framing above it.

Is drip cap required on windows?

Most building codes and window manufacturer installation instructions require flashing at window heads. Drip cap is the standard method. Omitting it voids many window warranties and is a common cause of water damage around window openings.

What material should I use for drip cap flashing?

Aluminum is the most common choice for residential windows because it does not rust, bends easily, and is compatible with most siding types. Galvanized steel is also used but can corrode near dissimilar metals. Vinyl drip cap is available for vinyl siding installations.

How is drip cap installed?

Drip cap slides under the housewrap or building paper above the window and sits over the top edge of the window frame or casing. The upper leg of the Z tucks behind the wall membrane and the lower leg extends over the window head. Siding installs over it to hold it in place.

What happens if I skip the drip cap?

Water running down the siding will find the joint at the window head and wick into the wall cavity. Over time this causes rot in the rough framing, mold in the wall assembly, swelling or warping of the window frame, and potentially damage to interior finishes below the window.

Drip Cap — Window and Door Head Flashing for Homeowners

A drip cap is a Z-shaped metal flashing installed at the top of a window or door opening to redirect water away from the frame and prevent it from penetrating behind the siding or casing.

Drip Cap diagram — labeled parts and installation context

What It Is

Drip cap flashing tucks under the siding or housewrap above the window head and extends over the top edge of the window frame or casing. Its Z-profile creates a physical break that deflects water outward and down rather than allowing it to run behind the window trim and into the rough opening. A standard drip cap profile has three bends: an upper leg that slides behind the wall membrane, a horizontal shelf that spans the thickness of the siding, and a lower lip that kicks water away from the trim face.

Without a drip cap, water that runs down the wall surface can find its way into the joint between the window head jamb and the rough framing. Over time this leads to rot, mold, and eventual failure of the window installation. Drip cap is inexpensive insurance against one of the most common water intrusion paths in residential construction. Studies by window manufacturers consistently rank missing or improperly installed head flashing as the leading cause of premature window assembly failure.

Most drip caps are formed from 0.019-inch aluminum, 26-gauge galvanized steel, or rigid vinyl. Aluminum and vinyl are common in residential work because they resist rust and are easy to cut with tin snips. Stock drip cap is sold in 10-foot lengths at most building supply stores and costs roughly one to three dollars per linear foot. The flashing is installed before siding is applied, so it is hidden in the finished assembly.

Types

Flat-back drip caps are the simplest profile and are used with housewrap and fiber cement siding where the siding sits flat against the sheathing. Nailing-fin drip caps have a perforated flange for fastening to the sheathing with roofing nails every 8 to 12 inches. Sloped drip caps include a small positive pitch built into the horizontal shelf to ensure drainage even when the window header framing is perfectly level.

Some windows include a factory-installed head flashing as part of their nailing-fin system, eliminating the need for a separate drip cap. Copper drip caps are specified on high-end projects or historic restorations where exposed flashing must match existing copper trim. Custom-bent drip caps are fabricated on a sheet-metal brake to match unusual trim profiles or extra-thick wall assemblies.

Where It Is Used

Drip cap is installed at the head (top) of every window and exterior door in wood-frame construction. It is also used above garage door openings, large fixed windows, and any exterior opening where water running down the wall surface could enter the rough opening. In regions with heavy wind-driven rain, drip caps are especially critical because water can be pushed horizontally into gaps that would stay dry under calm conditions.

Drip caps are also specified above through-wall vents, exterior electrical boxes, and hose bib penetrations where a horizontal joint in the wall creates a water entry risk.

How to Identify One

Drip cap is hidden under siding in a finished wall. It may be visible as a thin metal strip — typically 1/2 to 3/4 inch wide — at the top of the window casing where the siding begins, or it can be seen in a cross-section drawing. During installation, it appears as a bent metal flashing tucked under the first course of siding above the window. On stucco walls, the drip cap lower lip may be concealed under a bead of sealant or stucco termination, making it invisible from the exterior.

Replacement

Drip cap replacement is usually prompted by visible water staining above or inside a window, peeling paint at the window head, soft or spongy trim, or rot discovered when siding is replaced. The repair involves removing the siding course above the window, pulling out the old flashing, inspecting the housewrap and sheathing for moisture damage, installing new flashing with a minimum 2-inch overlap onto the housewrap above, resealing all laps with compatible flashing tape, and reinstalling siding. If the rough framing or sheathing behind the old drip cap has rotted, that material must be replaced before the new flashing is installed.