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

  1. WRB installed on wall
  2. Cut opening and fold flaps in
  3. Install sill pan (over bottom WRB flap)
  4. Install side flashing (over side WRB flaps)
  5. Install window
  6. Install head flashing (tucks UNDER top WRB)
  7. 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
MaterialBest ForNotes
Peel-and-stick membraneMost applicationsSelf-sealing around fasteners
Flexible flashing tapeJambs, head flashingStretchable for irregular surfaces
Liquid-appliedSill pans, complex detailsSeamless, but requires dry time
Metal flashingHead flashing, drip capsDurable, good kickout
Preformed plastic pansSill pansEasy 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

Free Door & Window Flashing Guide - Simple Waterproof Sequence | DIYProject.ai