How to Install a Window
Learn how to prepare the rough opening, flash the sill, set and level the window unit, shim for squareness, and secure it with weatherproof seals.
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🧰 Required Tools & Equipment
- ✓ Cordless drill and driver bits
- ✓ 4-foot spirit level
- ✓ Tape measure
- ✓ Caulk gun
- ✓ Utility knife
- ✓ Rubber mallet
📦 Materials & Supplies
- ✓ New window unit
- ✓ Self-adhering flashing tape
- ✓ Exterior-grade silicone sealant
- ✓ Cedar or composite shims
- ✓ 2-1/2 inch window/construction screws
- ✓ Low-expansion window & door spray foam
⚠️ Safety Checkpoints & Warnings
- • Wear cut-resistant gloves when handling exposed glass panels and sharp metal flashing edges.
- • Use safety glasses when drilling into framing and spraying expanding foam.
- • Ensure stable, level ladder footing or scaffolding if installing from an elevated exterior position.
📋 Step-by-Step Guide (6 Steps)
Inspect and Prepare the Rough Opening
Clear away any old nails, debris, and rotted wood from the rough opening. Measure diagonally from corner to corner in both directions to verify squareness within 1/8 inch. Check the bottom sill with a level to confirm it is flat and level.
- • Bare, clean framing lumber with no protruding fasteners
- • Tape measure showing identical diagonal dimension measurements across opposite corners
- • Failing to remove old fasteners, causing the new frame to bow
Apply Sill Flashing Tape
Cut a strip of self-adhering flashing tape 12 inches longer than the rough opening width. Lay it across the bottom rough sill, extending 6 inches up each vertical side jamb. Smooth the tape firmly down onto the sill and over the exterior face to create a watertight drain pan.
- • Wrinkle-free flashing membrane running continuously across the sill
- • Tape folding cleanly 6 inches up both left and right side jambs
- • Stretching the tape tightly into corners, causing it to tear or pull away later
Apply Caulk Beads to the Flange or Stop
Load exterior sealant into the caulk gun and cut the tip to yield a 3/8-inch bead. Lay a continuous bead along the interior backside of the top and side exterior nailing flanges. Leave the bottom flange dry and uncaulked so trapped moisture can weep outward.
- • Continuous 3/8-inch bead of sealant along top and side exterior flanges
- • Completely dry, uncaulked bottom flange
- • Caulking the bottom flange, which traps water inside the wall assembly
Set, Plumb, and Shim the Window
Lift the window unit into the opening from the exterior, resting the bottom edge first before tipping it in flat. Insert pairs of cedar shims from opposing sides behind the side jambs to center the unit. Place your 4-foot level against the side jambs and header to verify level, plumb, and square reveal.
- • Window frame centered with an even 1/4-inch to 3/8-inch gap on all sides
- • Spirit level bubble centered when held against both vertical jambs and sill
- • Over-driving shims, which bows the jambs inward and binds the sash
Fasten the Window Frame
Drive 2-1/2 inch screws through the mounting flange or pre-drilled side jamb holes directly into the studs through the shims. Start with the top corner on the hinge side, check squareness across diagonals, and secure the opposite bottom corner. Complete all remaining fastener locations without overtightening.
- • Screws seated flush against the flange or jamb counter-sinks
- • Window sash slides or cranks open and closed smoothly without sticking
- • Driving screws at an angle, which pulls the window out of plumb
Insulate and Weather-Seal
From the interior, fill the gap between the window jamb and rough stud with low-expansion foam, filling the cavity roughly 50% deep to allow for expansion. Score and snap off protruding shims flush with the wall plane using a utility knife. On the exterior, tape over the side and top nailing flanges with flashing tape in shingle fashion.
- • Trimmed shims perfectly flush with the wall framing studs
- • Uniform ring of expanded foam filling the interior gap without spilling onto frame face
- • Exterior flashing tape applied side-first, then top-flange overlapping the sides
- • Using high-expansion regular foam, which exerts enough pressure to warp the window frame
Credit: inspired by an original tutorial by CR Carpentry & Remodeling — watch the original on YouTube.
Don’t have everything?
- No Cedar shims? Composite plastic shims
- No Self-adhering flashing tape? Pre-formed plastic sill pan with exterior sealant