How to Flash Windows and Doors (Correct Layer Order)

Sill pan, jambs, head, then house wrap, step by step with diagrams, plus tape rolls and cost for every opening.

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Flashing sequence, step by step

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Windows

Doors

Avg window width (in)

Avg window height (in)

Jamb and head tape

4 in flashing tape: 114 ft; 2 rolls of 75 ft
9 in sill tape: 62 ft; 1 roll
Primer and sealant: 1 qt primer, 3 tubes
Est. cost: $227

Save this as a project: a checklist for each of your 7 openings with all 8 steps, and a photo of every layer before trim covers it, for the inspector and for resale. The tape, sill tape, primer and sealant come along as a shopping list.

Project plan · built from your numbers

Flash 7 openings (6 windows, 1 door)

Flash 7 rough openings in the right order: sill pan, unit, sides, head, then housewrap flap. About 114 ft of 4" tape and 62 ft of 9" sill tape.

Materials: $227
Time: 1 day
Level: Easy

7 steps 8 tools 6 materials

1
Cut the housewrap
Cut a modified I in the wrap: across the top, down the center, and across the bottom. Then cut a 45 degree flap above the head and tape it up out of the way.
- Modified I cut
- Head flap cut at 45 degrees
- Flap taped up temporarily
2
Prep the opening
Fold side wrap into the opening and staple it. Sweep dust off the sheathing and prime if it is cold or dusty.
- Sides folded in
- Sheathing clean and dry
- Primer where needed
3
Install the sill pan
Run 9" sill tape across the sill, lapping 6" up each jamb and 2" down the face. Use corner patches or flexible tape so no corner has a pinhole.
- Sill sloped out
- 6" up each side
- Corners sealed

01 · Your numbers

Openings: 7
Flashing tape: 114 ft
Sill tape: 62 ft
Est. cost: $227

02 · The plan

7 steps · 8 tools · 6 materials

4 more steps, the full tool list and a shopping list you can check off as you go.

03 · Finished

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Flash 7 openings (6 windows, 1 door)

Finished in 1 day · $227 DIY

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Why Proper Flashing Matters

Water Damage Prevention: Improper flashing is the #1 cause of water intrusion around openings

Rot Prevention: Water trapped in walls causes structural rot and mold

Energy Efficiency: Proper sealing reduces air infiltration

The key principle: Water flows downhill - each layer must overlap the one below it like shingles

The Critical Installation Sequence

Always Bottom to Top!

Each piece overlaps the one below so water sheds outward, never inward

Step 1: Sill Pan

Install first, before anything else
Extend up jambs 6-8"
Back dam to prevent water entry
Slope toward exterior for drainage

Step 2: Side Jamb Flashing

Overlaps sill pan flashing
Extends full height of opening
Wraps onto face of WRB 2-3"

Step 3: Install Window/Door

Set unit into prepared opening
Shim and level
Fasten per manufacturer specs

Step 4: Head (Top) Flashing

Install LAST - overlaps everything
Extends over side jamb flashing
Tucks under WRB above

Sill Pan Detail

Materials Options

Peel-and-stick membrane: Most common, self-adhering
Liquid-applied: Good for complex shapes
Preformed plastic/metal pans: Easy, reliable

Critical Features

Back dam: Turns up at interior to stop water
End dams: Turns up at jambs to contain water
Slope: Pitched toward exterior (1/8" per foot minimum)
Drainage: Must allow water to exit at front

Common Mistakes

Flat sill with no slope (water pools)
No back dam (water enters wall)
Sealing bottom of nailing fin (traps water)

Integrating with House Wrap (WRB)

Proper Integration

WRB should be installed before windows
Cut WRB in "I" or "modified-I" pattern at opening
Fold WRB flaps into opening
Tape flaps to rough opening

WRB Integration Sequence

WRB installed on wall
Cut opening and fold flaps in
Install sill pan (over bottom WRB flap)
Install side flashing (over side WRB flaps)
Install window
Install head flashing (tucks UNDER top WRB)
Tape top WRB over head flashing

Key Principle:

Head flashing goes UNDER WRB above, all other flashing goes OVER WRB

Flashing Tape Application

Surface Preparation

Clean, dry, dust-free surface
Temperature above 40°F (most products)
Prime OSB/plywood if required
No wrinkles or bubbles

Application Tips

Use J-roller to press firmly
Work out air bubbles immediately
Overlap seams minimum 2"
Fold corners carefully - no fish mouths

Corner Technique

Cut relief at inside corners
Fold and press firmly into corners
Use corner patches for extra protection
No stretched or pulled material

New Construction vs. Replacement

New Construction (Nailing Fin)

Full access to rough opening
Flash before window installation
Tape nailing fin to WRB (sides and top only)
Never tape/caulk bottom fin - drainage path

Replacement (No Nailing Fin)

Often can't access rough opening
Use exterior trim/casing for weatherproofing
Caulk perimeter (except bottom weeps)
May need to flash over existing siding
Consider insert windows if frame is sound

Door-Specific Considerations

Sill/Threshold

Most vulnerable point for water entry
Sill pan absolutely critical
Ensure threshold slopes away from door
Check adjustable threshold for proper seal

Outswing vs. Inswing

Inswing: Threshold exposed to weather, must shed water
Outswing: Better water protection, but check hinge security

Patio/Sliding Doors

Wide openings need robust sill pans
Track must drain properly
Weep holes must remain clear

Common Flashing Materials

Material	Best For	Notes
Peel-and-stick membrane	Most applications	Self-sealing around fasteners
Flexible flashing tape	Jambs, head flashing	Stretchable for irregular surfaces
Liquid-applied	Sill pans, complex details	Seamless, but requires dry time
Metal flashing	Head flashing, drip caps	Durable, good kickout
Preformed plastic pans	Sill pans	Easy installation, reliable

Testing & Inspection

Visual Inspection

Check all overlaps are correct (shingle style)
No gaps, fish mouths, or lifted edges
Corners properly sealed
Bottom not sealed (drainage path open)

Water Testing (Before Siding)

Use hose at low pressure
Start at bottom, work up
Hold at each level for 5 minutes
Check inside for leaks

Documentation

Photograph each step before covering
Document materials used
Keep records for warranty claims

Critical Don'ts

❌ Never seal the bottom of nailing fin - traps water

❌ Never install head flashing over WRB - water enters behind

❌ Never stretch flashing tape - will pull back and fail

❌ Never skip the sill pan - most leak-prone area

❌ Never rely on caulk alone - it fails over time

❌ Never flash out of sequence - defeats the purpose

❌ Never install on wet/dirty surfaces - won't adhere

Additional Information

Flashing tape types

Adhesive	Min. apply temp	Primer	OSB and plywood	ZIP System	Notes
Butyl	About 25 to 40 F	Below 40 F or on dusty wood	Good	Good	Stays tacky, works with most sealants; the usual pick
Rubberized asphalt	About 40 to 50 F	Often required	Good with primer	Check the maker	Cheapest; can bleed into some sealants and vinyl, softens in heat
Acrylic	Down to 0 F	Rarely	Excellent	Excellent	Best in cold and on ZIP; more expensive

Widths: 4 in on jambs and heads of most windows, 6 in for wider flanges and doors, 9 in or wider for sill pans.

Flashing mistakes: wrong vs right

Wrong	Right
Head tape tucked under the jamb strips (reverse lap)	Head over jambs, wrap flap over head: every upper layer laps over the one below
Sealant along the bottom fin	Sides and head only; the bottom stays open to drain the pan
Flat or back-sloped sill	Sill sloped out about 1/8 in per ft, or a beveled shim under the pan
No back dam at doors	A 1/2 in back dam and end dams on every door pan

ZIP System, stucco, masonry and replacement windows

ZIP System sheathing: no wrap to I-cut. Wrap the rough opening with ZIP stretch tape or liquid flash, sill first, sides, then head, all shingle-lapped, then tape the flange the same way.
Stucco: two layers of WRB, a weep screed and a head flashing with end dams; the window flange goes over the inner layer and under the outer layer at the head.
Brick and masonry: a sill pan plus a steel or copper head flashing with end dams and weeps above the lintel; never rely on sealant at the brick.
Replacement (insert) windows: you cannot reach the WRB, so flash the old sill with flexible tape, keep the sill weep path open, and seal the sides and head with backer rod and sealant.

About This Door and Window Flashing Guide

Improper flashing around doors and windows is the number one cause of water intrusion in buildings. Unlike roof leaks that show immediately, wall leaks from poor flashing often remain hidden for years while rot and mold develop inside wall cavities. Proper flashing technique is simple but critical - each layer must overlap like shingles so water always flows outward.

This guide covers the essential flashing sequence (sill pan first, head flashing last), proper integration with house wrap, material selection, and the critical details that prevent leaks. You'll learn the difference between flashing new construction with nailing fins versus retrofit installations, and the specific considerations for doors versus windows.

Whether you're installing a new window, replacing a door, or checking existing flashing for problems, this reference ensures you understand the principles that keep water out of your walls. Proper flashing is invisible when done right - and obvious when done wrong.

Why Use This Reference Guide?

Water damage from window and door leaks causes thousands of dollars in structural repairs and mold remediation. This guide helps you flash correctly and avoid these costly problems.

Understand the critical bottom-to-top flashing sequence
Learn proper sill pan construction with back dams and slope
Integrate flashing correctly with house wrap (WRB)
Select appropriate flashing materials for each application
Avoid common mistakes that guarantee future leaks
Test installations before closing up walls

Frequently Asked Questions

Does flashing tape go over or under the house wrap?

Both, in order. The sill tape and jamb tape go over the wrap folded into the opening, and the head tape goes under the wrap flap above the window. Every layer laps over the one below it so water always runs out.

Do you flash before or after installing the window?

Both. The sill pan and corner patches go in before the window; the jamb and head tape go on after it is set, over the flanges.

What width flashing tape for windows?

4 in for jambs and heads on most windows, 6 in for wider flanges and doors, and 9 in or wider for the sill pan so it can wrap up the jambs and over the face.

How do you flash a door sill?

Slope the rough sill out, then install a rigid or tape sill pan with end dams and a 1/2 in back dam, patch the corners, and set the door on sealant at the back of the pan only so water can still drain out the front.

Can you flash windows in cold weather?

Yes with the right tape. Butyl tapes apply down to about 25 to 40 F with primer, acrylic tapes to about 0 F; rubberized asphalt usually needs 40 to 50 F. Keep rolls warm and roll every inch.

Why shouldn't I seal the bottom of a window nailing fin?

The bottom of the nailing fin must remain unsealed to allow any water that enters the window frame to drain out. Windows are not waterproof - they have weep holes specifically designed to drain infiltrating water. If you seal the bottom fin, water gets trapped behind the window, rots the rough sill, and enters the wall cavity. Only tape the sides and top of nailing fins; the bottom must drain freely onto the sill pan flashing.

What is a sill pan and why is it so important?

A sill pan is a waterproof membrane or preformed tray installed at the bottom of a window or door opening before the unit. It catches any water that gets past the window and directs it to the exterior. Critical features include: a back dam (turned up at interior), end dams (turned up at jambs), slope toward exterior, and drainage path at the front. Without a sill pan, water collects on the rough sill and wicks into the wall - the most common cause of window-related water damage.

Should head flashing go over or under the house wrap?

Head flashing goes UNDER the house wrap above the window. This is the opposite of all other flashing locations and the most commonly made mistake. Water running down the wall hits the house wrap first - if the head flashing is over the house wrap, water flows behind the flashing and into the wall. The correct sequence: head flashing tucks under the house wrap, then tape the house wrap over the top of the flashing.

How do I flash a replacement window without a nailing fin?

Replacement windows without nailing fins rely on exterior trim and caulk for water protection. Install the window, then apply brickmould or flat casing trim. Caulk the top and sides of the trim to the siding - but leave the bottom uncaulked for drainage. Use high-quality exterior caulk. For better protection, install a metal head flashing (drip cap) above the window that tucks under the siding. This isn't as reliable as proper new-construction flashing but works when done carefully.

Can I use spray foam to seal around windows?

Use only low-expansion foam designed for windows and doors. Standard expanding foam can bow window frames, preventing proper operation and breaking seals in double-pane glass. Apply foam sparingly in 1/3 of the gap depth; don't fill completely. Foam helps with air sealing and insulation but does NOT replace proper flashing. Foam is not waterproof when exposed to UV and is not a substitute for membrane flashing at jambs and sill.

Why is my door threshold leaking?

Common causes: missing or poorly installed sill pan flashing, threshold not sloped away from door, worn door sweep or weatherstripping, or threshold set below exterior surface allowing water to pool. Inswing doors are more vulnerable because the threshold is exposed. Solutions include installing a proper sill pan with back dam, adjusting the threshold height and slope, replacing weatherstripping, and ensuring the exterior walkway slopes away from the door. For persistent leaks, the door may need reinstallation with proper flashing.

Next: install the unit with the window installation guide or the exterior door installation guide, seal gaps with the weatherstripping and air sealing sheet, and check the rest of the house with the energy savings audit.

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Plan the whole project

Save this as a project: a checklist for each opening

All 8 steps for every window and door, a photo of each layer before trim covers it, and the tape, primer and sealant on one shopping list. The calculators and cheat sheets stay free, no account required.

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