Window Flashing and Waterproofing: How a Replacement Window Keeps Water Out

A window opening is a hole cut through every layer that keeps your house dry. Flashing is the set of materials and laps that rebuilds those layers around the window so water running down the wall is carried past the hole and back to the outside.

It is the least visible part of a window job and the part most responsible for whether the job lasts. This page is part of our California window guide.

The principle: assume water gets in, and give it a way out

No cladding is waterproof. Stucco absorbs water, lap siding lets wind-driven rain behind the boards, and every sealant joint cracks eventually. Walls are built with that in mind. Behind the cladding is a water-resistive barrier, and its job is to catch what gets through and drain it down and out.

The California Residential Code states the idea in Section R703.1.1: the exterior wall envelope is to be built so that water does not accumulate within the wall assembly, by providing a water-resistive barrier behind the cladding and a means of draining water that penetrates the cladding back to the exterior.

A window interrupts that barrier. Flashing reconnects it.

What the code requires

Three sections of the 2025 California Residential Code do the work:

• Section R609.1 requires windows to be installed in accordance with the fenestration manufacturer's written instructions, and window openings to be flashed in accordance with Section R703.4.
• Section R703.4 requires approved corrosion-resistant flashing applied "in a manner to prevent entry of water into the wall cavity," with overlapped flashing "applied in shingle fashion." Exterior window and door openings are the first location on its list.
• Section R703.4.1 covers window openings specifically. Flashing is to extend to the surface of the exterior wall finish or to the water-resistive barrier for drainage. It is to be installed according to the window manufacturer's installation and flashing instructions, or the flashing manufacturer's instructions where the window maker's are silent. Where no flashing instructions or details are provided, pan flashing is required at the sill, sealed or sloped to direct water out, with flashing or protection at the head and sides. The section also calls for air sealing around the opening on the interior side of the rough opening gap.

Notice what the code does and does not do. It does not draw one mandatory detail. It makes the manufacturer's instructions the requirement. That is why a good installer has the instructions for your specific window and frame type on site, and why "that is how we always do it" is not an answer.

The standards behind the instructions

Manufacturer instructions are not written from scratch. They follow a small set of industry practices:

• ASTM E2112, Standard Practice for Installation of Exterior Windows, Doors and Skylights. Its scope covers installation in new and existing construction for residential and light commercial buildings. It also states that where the manufacturer's instructions conflict with the practice, the manufacturer's instructions prevail.
• AAMA 2400, from the Fenestration and Glazing Industry Alliance, for windows with a mounting flange (nail fin) in new construction.
• AAMA 2410, for flush-fin retrofit windows installed over an existing frame.

Milgard's installation page tells installers to follow AAMA 2400 for new-construction applications and AAMA 2410 for replacement applications. Which one governs your job depends on the frame type, covered on our page on nail fin, flush fin, and block frame windows.

The layers, from the bottom up

For a full-frame window with a nail fin, the opening is flashed in a set order. The order is the point.

1. The sill pan

The bottom of the opening is where water ends up, so it gets the most protection. A sill pan lines the rough sill, turns up at the interior edge to form a back dam, and turns up at each end. If water gets past the window, it lands in the pan and runs out over the face of the barrier below.

Two details matter here. The rough sill should be level or sloped to the exterior; JELD-WEN's instructions say it should not be crowned or sagged. And the pan must drain. JELD-WEN's instruction is to always allow water to drain out of the pan onto the building wrap, the drainage plane, or the exterior.

2. Sill flashing below the opening

AAMA 2400 starts with a strip of flashing applied horizontally immediately below the sill, extending past each side of the opening. Its top edge is fastened, and its bottom edge is left loose. The reason is specific: the water-resistive barrier installed later is slipped up underneath that loose edge, so the flashing laps over the barrier.

3. The window, bedded in sealant

The window is set, leveled, and shimmed, with a continuous bead of sealant behind the mounting flange. AAMA 2400 says the installer should look for the sealant to squeeze out along the edge of the flange, which shows the bead made contact all the way around.

4. Jamb flashing

Strips go up each side, over the flange, running from below the sill flashing to above the head of the window. The bottom of each jamb strip laps over the sill flashing.

5. Head flashing

The top strip goes on last and extends past the jamb strips on each side. Where there is a water-resistive barrier above, it laps over the head flashing. Water coming down the wall hits the barrier, runs onto the head flashing, onto the jamb flashing, onto the sill flashing, and onto the barrier below. At no point does it meet an edge facing uphill.

That is what "shingle fashion" means, and it is the test for any flashing job: trace a drop of water from above the window to below it. If there is a place where it could run behind a layer, that lap is backward.

6. A drip cap at the head

Many installations add a rigid drip cap over the head of the window or the head trim. It kicks water away from the top of the frame. The code's list in R703.4 also calls for flashing continuously above all projecting wood trim. JELD-WEN's instructions add a point installers get wrong: if sealant is applied above the drip cap, the bead should be discontinuous so water can drain.

7. The interior air seal

The gap between the window frame and the rough opening is sealed from the inside with low-expansion foam or backer rod and sealant. This is the air barrier, not the water barrier, and the code asks for it in R703.4.1. Milgard requires that any foam used be the low-expansion type meeting AAMA 812, and warns that frame distortion caused by foam is not treated as a warranty issue.

Tape versus sealant versus liquid flashing

There are several kinds of flashing material, and the code names a standard for each:

These are not alternatives to choose between. Tape and sealant do different jobs. Sealant beds the window flange and closes joints. Flashing membrane laps and sheds. A job that uses only sealant has no drainage layer. A job that uses tape stuck over a dusty, damp, or cold surface has tape that will let go.

Compatibility matters too. AAMA 2400 requires that coatings, flashings, and sealants that touch each other be compatible, and JELD-WEN recommends using flashing, primer, sealant, and foam from the same manufacturer and verifying compatibility. Some sealants attack some membrane adhesives. Installers who mix whatever is on the truck are taking a risk you will not see for a few years.

Weep holes: the drains built into the window

Separate from the wall flashing, the window frame has its own drainage. Sliding and hung windows collect rain in the sill track, and small slots on the exterior face of the frame let it out. Milgard describes this weep system as vented slots on the exterior bottom of the frame and says it is normal for water to build up in the sill or track during rain.

Two installation rules follow:

• Never seal over weep holes. JELD-WEN's instructions say it in so many words: do not seal weep holes or channels. Milgard notes that blocking the weep system can void part or all of the product warranty.
• On a retrofit, keep the old frame's weep path open too. AAMA 2410 calls for a gap of at least one inch in the sill sealant in line with each of the existing frame's weep holes, both in the primary seal and in the secondary seal around the flange. Where the old frame has no weep openings, it calls for two such gaps, one near each corner.

If you see a perfectly continuous bead of caulk along the bottom of a newly installed retrofit window, ask about it. Keeping weep holes clear over the years is covered in our window maintenance guide.

How a retrofit differs

A flush-fin retrofit does not build the layered system above, because the cladding stays on. Instead it relies on the original flashing around the old frame and adds two sealant seals. AAMA 2410 is explicit that the practice assumes the seal between the pre-existing window and the wall is adequate, and it requires corrective measures before installation if there is evidence of water penetration.

That is a reasonable trade on a sound stucco wall, and it is the reason retrofit became standard on California stucco houses. It is not a fix for an opening that already leaks. See replacing windows in stucco homes for when each approach fits.

Flashing and new siding

If siding is being replaced along with the windows, the order of work decides whether the flashing can be done properly. Windows go in and get flashed before the cladding, and trim details at the head and sill need their own flashing. Our page on how installers handle corners, windows, and trim walks through those details for fiber cement and other durable sidings, and the exterior window trim guide covers trim choices.

Questions to ask before the job starts

1. Which manufacturer installation instructions will you follow for this window and frame type, and can I see them?
2. For full-frame openings: what is the sill pan made of, and how does it drain?
3. What flashing membrane and sealant are you using, and are they compatible with each other and with the window frame?
4. How does the new flashing tie into the existing water-resistive barrier?
5. For retrofit openings: how are the old frame's weep holes kept open?
6. Will you photograph each opening before it is covered?
7. Who checks the old opening for water damage once the window is out, and what happens if you find some?

The general sequence of a window installation is on our window installation process page. What to check when the job is finished is on the window installation inspection checklist.

Sources

• 2025 California Residential Code, Sections R703.1.1, R703.2, R703.4 and R703.4.1
• 2025 California Residential Code, Section R609.1
• ASTM E2112-23, Standard Practice for Installation of Exterior Windows, Doors and Skylights (summary page)
• AAMA 2400-10, installation of windows with a mounting flange (licensed copy published by Milgard)
• FGIA, AAMA 2410-13, flush-fin installation over an existing frame (licensed copy published by Anlin)
• Milgard, Installation Information
• JELD-WEN, Installation Instructions for Vinyl Windows with Integral Flush Fin (JII-90024), retailer-hosted copy
• Milgard, Window and Door Care Tips and Frequently Asked Questions (weep system)

Working with Heritage Exteriors

We install siding as well as windows, so flashing an opening into the wall behind it is familiar work for us. If an old opening shows water damage once the window is out, we will show you and talk through the repair before going further. Licensed in California, CSLB #1055919. Call (916) 546-2991.