Waterproofing an opening is six layers in a set order, not one product. What goes where on a window, what changes at a door threshold, and how the head jamb is kept dry.
In short: "waterproofing" around a window or door is not one product. It is a sill pan under the unit, flashing at the jambs, flashing and a drip at the head, the wall membrane lapped over all of it, a sealant joint outside and an air seal inside, each with a different material doing a different job. Doors add a threshold seal that windows do not need, and the head jamb is where most searches about "jamb waterproofing" should be looking.
When a client asks what waterproofing we use on windows, the honest answer is six things in a set order. Water that hits a wall runs down. Every layer around the opening is arranged so that water running down lands on something that carries it further out, never behind. That is the shingle principle, and it is why a single bead of caulk around a frame is not waterproofing: caulk has no lap, it only blocks, and when it fails there is nothing behind it.
The diagram is the order we install in: sill pan first, then the window, then jamb flashing over the frame edge, then head flashing over the jambs, then the wall membrane lapped over the head and jambs, then sealant outside and the air seal inside. The Woodmere site visit is what happens when step four is skipped.
Each layer uses a product suited to its position. The table is what we specify and what we supply; the installation materials themselves can be seen at nyceglass.com.
| Type | What it is | Where we use it | Watch for |
|---|---|---|---|
| Self-adhered membrane (rubberized asphalt or butyl, e.g., Henry Blueskin, Soprema) | Peel-and-stick sheet with a factory adhesive | Jamb and head flashing, sill pans, tying the window into the wall membrane | Primer on the substrate, every edge rolled, temperature limits; an unrolled edge is a leak |
| Fluid-applied flashing | Trowel- or gun-applied membrane that cures to a seamless film | Rough openings with odd geometry, masonry openings, corners where sheet membrane would wrinkle | Film thickness, cure time before the window goes in, compatibility with sealant |
| Formed sill pan | Rigid PVC, aluminum or stainless pan with end dams and a back dam | Under every window and door sill | Back dam height, slope to the outside, pan not sealed at the front edge |
| Housewrap (e.g., DuPont Tyvek) | Mechanically fastened sheet, water-resistive barrier | The wall field; it is lapped over the window flashing and taped | It is not a flashing; untaped seams and wrap tucked behind the flashing are the usual failures |
| Sealant (silicone, hybrid, polyurethane) | Gunned joint with backer rod | Exterior cladding joint, interior air seal, threshold-to-sill | Joint sized for movement, compatible with the membrane, not used as the only line of defense |
| Metal head flashing / drip cap | Bent aluminum or stainless over the head trim | Above doors and windows under siding, stucco and panels | End dams, lapped under the membrane above, kick-out at the ends |
A window frame is a box set in a hole, and the hole has four edges. The sill is where water ends up, so the sill gets a pan: sloped to the outside, with a back dam at the interior so water cannot cross into the room, and end dams turned up the jambs. The front edge of the pan is never sealed; it is the exit. The jambs get self-adhered flashing from the frame onto the wall, lapped over the pan's end dams. The head gets flashing over the frame and over the jamb flashing, and the wall membrane is then lapped over the head flashing. Outside, a backer rod and sealant joint closes the gap to the cladding on the jambs and head only. Inside, the frame is air-sealed with low-expansion foam or backer rod and sealant so warm interior air cannot reach the cold cavity and condense.
Aluminum windows with nailing fins are flashed over the fin. Block-frame windows in masonry, which is most of what we install in New York, have no fin, so the flashing laps onto the frame face and the opening is often done with fluid-applied membrane where sheet membrane will not conform to the masonry. See waterproofing and new construction window installation.
A door has everything a window has plus a threshold that people walk on and that sits at or near the floor and the outside grade. That changes the sill detail in three ways:
Grade matters as much as the detail. A threshold close to a stoop or a deck that drains toward the door will be wet no matter what is under it. We look for the threshold to sit above the outside surface with that surface sloping away, and where it cannot, we recommend a channel drain in front of the door.
People search "jamb waterproofing" and usually mean the top of the opening, because that is where the stain on the drywall appears. The head jamb is the one edge where gravity works against you: water running down the wall above arrives at the top of the frame and has to be sent out past the face of the window, not into the gap behind the trim. Three parts do that:
The common mistakes are the reverse lap (wall wrap tucked behind the head flashing), a drip cap without end dams so water runs off the ends into the jamb, and a continuous bead of caulk along the top of the trim that seals the exit shut. Any one of them puts water into the head jamb, where it soaks the framing, rots a wood jamb from the top down and shows up as the familiar stain at the top corner of the window.
On an existing house with wood door or window jambs, rot at the bottom of the jamb means water has been getting in for years, usually from a missing sill pan or a threshold that was caulked shut. The repair is not a new coat of paint. The casing comes off, the extent of the rot is checked into the sill and framing, the rotten section is cut out and replaced, and then the opening is flashed the way it should have been: pan, jamb and head flashing, drip cap, open sill joint. Where the jamb is replaced, a cellular PVC or composite jamb removes the wood from the wet zone entirely. We do this with the window or door replacement; it is the part of a window replacement that the price of the window does not show.
We flash and waterproof the windows, doors and storefronts we install, inspect openings on new construction before cladding, and repair water problems at existing openings across the five boroughs of New York City, Long Island, northern New Jersey and the Jersey Shore.
A sill pan under the window, self-adhered or fluid-applied flashing at the jambs and head, the wall membrane lapped over the head flashing, an exterior sealant joint on the jambs and head, and an interior air seal. Each layer laps over the one below it.
A door has a threshold that carries traffic and sits near grade. It needs a rigid formed sill pan, the threshold bedded to the pan with the front edge left open to drain, a separate weatherstrip and sweep system, and an outside surface that slopes away from the door.
Head flashing membrane over the frame, the wall membrane lapped over its top edge, a drip cap with end dams above the trim, and an open or weeped joint at the head so water can exit above the opening. Never caulk the top joint shut.
No. Sealant closes a joint and manages movement, but it has no lap. It is the last layer, not the only one. Flashing and the sill pan are what keep the opening dry when the sealant fails.
A formed pan in rigid PVC, aluminum or stainless with a back dam and end dams for doors; a membrane or formed pan for windows. The pan must slope out and must not be sealed at the front edge.
Yes. We supply the membranes, sill pans, foam, backer rod and sealants we install with; see nyceglass.com.
Waterproofing Leaking windows or failed membrane: Woodmere site visit Good and bad membrane adhesion New construction window installation Window replacement Entrance door alignment, Somerset NJ Request an inspection
About this page. Written by Alex GC Inc. d/b/a Alex Glass Construction and reviewed before publication by Aleksandr Chernovolenko, Managing Partner, with our design and estimating lead and a field foreman. Our foremen hold the NYC DOB Site Safety Training supervisor card and our crews the 40-hour worker card; we are certified to fabricate, install and service Alutech and Everest aluminum systems. Facts are drawn from our own projects, manufacturer test data and the applicable codes; where we are giving an opinion, we say so. Last reviewed September 2026. Credentials
Send photos of the window or door, inside and out, and tell us whether the building is new or existing. We will tell you which layer is missing and price the fix.
