Door & Window Flashing Guide

Proper flashing sequence and techniques to prevent water infiltration around openings. Critical for avoiding rot and mold in exterior walls.

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

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

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.

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