How to Install a Flanged Window
Measure, flash, level, and fasten a new flanged exterior window into a rough opening with proper weatherproofing, shimming, and low-expansion foam insulation.
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🧰 Required Tools & Equipment
- ✓ 4-foot spirit level
- ✓ Tape measure
- ✓ Cordless drill / driver
- ✓ Caulking gun
- ✓ Hammer
📦 Materials & Supplies
- ✓ New flanged window unit
- ✓ Self-adhering flashing tape
- ✓ Exterior grade polyurethane sealant
- ✓ Composite or cedar shims
- ✓ 1-1/2 inch exterior roofing nails or pan-head screws
- ✓ Low-expansion window foam sealant
⚠️ Safety Checkpoints & Warnings
- • Wear safety glasses when cutting flashing, driving fasteners, or applying pressurized foam.
- • Ensure a secure work platform or ladder when working on second-story exterior rough openings.
- • Use two people to lift and set heavy double- or triple-pane window units.
📋 Step-by-Step Guide (6 Steps)
Prepare and Flash the Rough Sill
Clean sawdust and debris from the rough opening sill and framing. Cut a strip of self-adhering flashing tape at least 12 inches longer than the rough opening width. Press the tape firmly across the sill, extending it at least 6 inches up each vertical jamb and rolling it down over the exterior house wrap below.
- • Sill wood is completely covered by tight, wrinkle-free flashing tape
- • Flashing tape wraps at least 6 inches up both vertical side jambs
- • Bottom edge of tape overlaps the exterior weather barrier cleanly
- • Leaving air bubbles or wrinkles in the flashing tape
- • Failing to turn the flashing up the side jambs to create a waterproof pan
Apply Sealant Bead to Framing
Load a tube of exterior hybrid or polyurethane sealant into the caulking gun. Apply a continuous 3/8-inch bead of sealant along the top (header) and side (jamb) exterior framing faces around the rough opening. Leave the bottom sill flange area unsealed so any accumulated water can escape.
- • A thick, unbroken 3/8-inch bead of sealant covers the top and side exterior perimeter
- • Bottom sill framing line is completely free of sealant
- • Sealing the bottom sill edge, which traps water inside the wall assembly
- • Applying too thin of a bead that fails to bridge gaps against the window flange
Set Window and Secure Top Corner
Lift the window assembly into the rough opening from the exterior, resting the bottom edge on the sill first. Tilt the top of the window inward until the side and head nailing flanges press firmly against the sealant beads. Drive a single 1-1/2 inch fastener through the top corner of the nailing flange into the framing to hold the unit temporarily.
- • Window rests squarely in the opening with nailing flanges flat against framing
- • Sealant visibly squeezes out along the top and side flanges
- • A single fastener holds one upper corner securely in place
- • Dropping the window onto the sill too hard, cracking the vinyl weld or glass
- • Fastening all four corners before checking for level, plumb, and square
Level, Plumb, Square, and Shim
Place a 4-foot level on the bottom sill, side jambs, and across the face of the window frame. Measure diagonally from corner to corner in an 'X' pattern to verify squareness; both measurements must match within 1/16 inch. Insert shims between the rough jambs and the window frame behind hinge and latch points until the frame is plumb, level, and square with equal reveals.
- • Level bubble is centered across the interior sill and along both vertical side jambs
- • Diagonal corner-to-corner measurements match exactly
- • Shims are snugly placed behind side jamb mounting points
- • Over-shimming, which bows the window frame inward and binds the sash
- • Shimming the middle of the sill, which causes the bottom frame to bow upward
Fasten the Nailing Flange
Drive exterior fasteners through the pre-punched slots in the nailing flange every 8 to 12 inches around the perimeter. Fasten through the center of the slots rather than the edges to allow for thermal expansion and contraction. Verify that the window sash opens, locks, and operates smoothly before driving the final fasteners.
- • Fasteners driven flush into pre-punched flange slots every 8 to 12 inches
- • Flange sits flat against the exterior sheathing without buckling or warping
- • Window sash opens, closes, and latches effortlessly
- • Driving fasteners through solid vinyl instead of the pre-punched slots
- • Over-driving fasteners, which bends or cracks the vinyl nailing fin
Flash Exterior Flanges and Insulate Interior
Apply self-adhering flashing tape over the vertical side flanges, extending 2 inches above the head flange and overlapping the sill tape below. Apply a final piece of flashing tape across the top head flange, overlapping both side tapes. From the inside, apply a continuous bead of low-expansion foam into the gap between the window frame and rough framing.
- • Side flashing tape completely covers side nail fins and overlaps sill flashing
- • Top flashing tape completely covers the head flange and extends over side tapes
- • Low-expansion foam fills interior framing gap to approximately 50 percent depth
- • Taping the head flange before the side flanges, which creates an upside-down water lap
- • Using standard high-expansion foam, which expands aggressively and bows the window frame
Credit: inspired by an original tutorial by HAUS PLANS ®️ — watch the original on YouTube.
Don’t have everything?
- No Cedar shims? Composite plastic shims
- No Exterior roofing nails? Corrosion-resistant exterior pan-head screws