Short answer
To properly flash and seal a window, build a sloped, waterproof sill pan, install the window plumb and square, and integrate self-adhered flashing with your weather-resistive barrier (WRB) in a shingle-lap sequence: sill first, then jambs, then head, plus a rigid drip cap for flangeless or certain claddings. Leave the bottom flange untaped so any water that gets in can drain out. Finish with low-expansion foam around the frame and a backer rod and sealant joint at the exterior perimeter.
Why this matters
Water leaks around windows usually come from skipped steps, backwards laps, or relying on caulk alone. Good flashing redirects water to the exterior even if sealant fails. Following AAMA/ASTM-style sequencing (sill, jambs, head) protects the opening long term.
Tools and materials
- Self-adhered flashing: butyl preferred; 4 to 9 inch widths
- Flexible sill flashing or liquid-applied flashing (for corners)
- Housewrap/WRB tape and primer (if your WRB needs it)
- Sloped sill shim or beveled siding strip, or preformed sill pan
- Polyurethane or silyl-terminated sealant; color to match trim
- Backer rod (closed-cell)
- Low-expansion window and door spray foam
- Drip cap/head flashing (for flangeless windows or over trim)
- Corrosion-resistant screws or nails for flanges
- Tools: utility knife, shears, stapler, level, framing square, tape measure, J-roller, caulk gun, oscillating multi-tool, speed square, pry bar, PPE
Rough cost: 50 to 150 dollars per window for flashing/foam/sealants. Time: 2 to 5 hours per window for a careful DIY install.
Key specs at a glance
Overlaps: minimum 2 inches tape laps; extend 6 inches past corners
Sill slope: about 1/8 inch per foot toward exterior
Frame gap: 1/4 to 3/8 inch around window for foam and movement
Sealant joint: 3/8 to 1/2 inch wide with backer rod; tool to hourglass shape
Do not tape bottom flange on flanged windows; leave a drain path
Step-by-step: flanged new-construction window
1) Prep the opening
- Check the rough opening for plumb, level, and square. Plane or shim as needed.
- Cut the WRB in an inverted Y or I pattern at the opening. Fold and temporarily tape the head flap up.
- Install a sloped sill: use a beveled strip or composite shim to pitch the sill toward the exterior.
- Create a sill pan: line the sill with flexible self-adhered flashing, running it up the jambs at least 6 inches and out onto the face of the wall sheathing 6 inches. Form end dams at the jambs by folding the flashing up. Use a J-roller for full adhesion. Prime dusty OSB if required by the tape manufacturer.
2) Dry-fit and bed the window
- Dry-fit the window; confirm reveal and clearance.
- Run a continuous sealant bead at the head and jambs behind the flange (skip the bottom so water can drain). If the manufacturer specifies full-perimeter sealant, add weep gaps at the bottom corners.
- Set the window in the opening from the exterior. Shim at hinge points and under the jambs to keep the frame square and plumb. Check diagonal measurements.
3) Fasten
- Fasten through the flange per manufacturer spacing. Do not overdrive. Re-check operation of the sash before finishing.
4) Flash the flanges in shingle-lap order
- Sill: some pros skip taping the bottom flange entirely to allow drainage. If you tape it, notch the lower corners or leave weep gaps.
- Jambs: apply vertical strips overlapping the sill flashing at the bottom and running 4 to 6 inches above the head.
- Head: install a rigid metal or PVC drip cap over the top flange. Then apply head flashing tape that laps over the drip cap and jamb flashing.
- WRB integration: pull down the WRB head flap over the head flashing and tape the diagonal cuts. This completes the shingle lap from top to bottom.
5) Insulate and seal
- From the interior, fill the gap with low-expansion foam in light passes to avoid bowing the frame. Allow to cure, trim flush.
- At the exterior perimeter (for flangeless or trim interfaces), insert backer rod and apply a tooled sealant joint. Avoid three-sided adhesion by using backer rod.
Flangeless or retrofit windows
- Use a robust sill pan: preformed pan, flexible flashing with corner boots, or liquid-applied flashing. Carry the pan up the jambs 6 to 9 inches.
- Install the window per manufacturer clips or screws. Maintain the 1/4 to 3/8 inch perimeter gap.
- Install a drip cap above the head trim, tucked behind the WRB or into a kerf in the cladding where possible.
- Flash jambs and head to the WRB using the same shingle-lap sequence. Finish with backer rod and sealant around the exterior perimeter joint.
Safety
- Eye protection, cut-resistant gloves, and long sleeves when handling flashing and metal caps.
- Respirator or good ventilation for spray foam and sealants.
- Ladder safety and a helper for large units.
- Lead-safe practices for homes built before 1978; test paint before disturbing.
Tips for best results
- Prefer butyl-based flashing tapes; they stick better to colder, damp surfaces and many WRBs. Avoid asphalt tapes against vinyl flanges.
- Prime OSB and dirty substrates for maximum adhesion.
- Use a J-roller to eliminate fishmouths and bubbles in tape.
- Label and save manufacturer instructions; some require full-perimeter bedding or specific fasteners for warranty.
- In coastal or high-wind zones, verify fastener schedule and add additional head flashing per local code.
Common mistakes
- Relying on caulk alone instead of a true sill pan.
- Backwards laps: head tape under jamb tape, or head flashing under WRB.
- Taping the bottom flange without a drainage path.
- Over-foaming and bowing the frame, causing sticky sashes.
- Skipping drip caps over flangeless windows or thick trim stacks.
When to call a pro
- Stucco, brick veneer, or stone facades where WRB integration is hard to reach.
- Structural rot discovered at the sill or studs.
- Large, heavy units or upper-story installs needing staging.
- High-performance assemblies using liquid-applied membranes or strict warranty requirements.
Do the steps above patiently and in the right order, and your windows will shed water reliably for years. If you get stuck, a qualified installer familiar with ASTM E2112 or AAMA InstallationMasters can help dial in the details.