Window Installation—Single/Double-Hung and Casement

Why Proper Window Installation Matters

A window that is out of level, out of plumb, or poorly flashed will cause problems that outlast the warranty: sashes that bind or won't latch, air and water infiltration, rot in the rough framing, and callbacks that damage your reputation. Taking the time to set each unit correctly and flash it in the right sequence is not optional—it is the craft.

Understanding the Rough Opening

Manufacturers size their windows to fit a rough opening (RO) that is typically 1/2 inch to 3/4 inch larger than the window unit in both width and height. Always confirm the required RO in the manufacturer's installation instructions before cutting or framing. The extra space allows you to shim the unit into level and plumb position without forcing it. Before the window goes in, the RO must already be flashed: a sill pan of self-adhered membrane installed with the ends turned up the jack studs, and flexible tape sealing the corners so no water can pond on the rough sill and wick into the framing.

Installing a Single or Double-Hung Window

Step 1 – Prepare and inspect the opening. Confirm the RO dimensions, check the rough sill for level, and verify the sill pan flashing is in place and fully adhered with no fish-mouths or gaps. Remove any protective shipping film from the nailing fin before fastening.

Step 2 – Set the unit. With a helper, tilt the window into the opening from the exterior. Rest the sill on the rough sill. Center the unit side to side so the gap is roughly equal on both jamb sides.

Step 3 – Level the sill. Place a 4-foot level on the sill. Slide cedar shims under the sill corners until the bubble reads level. A level sill is the foundation of everything that follows; a sill out of level by even a small amount will cause the sashes of a double-hung to drift open or closed on their own.

Step 4 – Plumb and square the unit. Hold the level against each jamb and adjust shims until both jambs read plumb. Then measure the diagonals—corner to corner in both directions. Equal diagonal measurements confirm the unit is square. Adjust shims until diagonals are within 1/8 inch of each other.

Step 5 – Fasten through the nailing fin. Drive fasteners through the nailing fin in the pattern specified by the manufacturer—typically starting at the top corners and working toward the center, alternating sides to avoid racking the frame. Use the fastener type and length called out in the instructions; substituting fasteners can void the warranty and compromise holding strength in wind uplift.

Step 6 – Recheck level, plumb, and square after fastening, since driving fasteners can shift the unit slightly. Trim shims flush with the framing interior once the unit passes inspection.

Flashing Integration—The Shingled Sequence

Water management follows one rule: every upper layer laps over the top of the layer below, just like shingles on a roof. For a window with a nailing fin, the correct sequence is:

1. Sill pan – already installed before the window goes in.

2. Jamb flashing – self-adhered membrane strips applied over the nailing fin on each side jamb, starting at the sill pan and running up past the head. The membrane must adhere fully to the nailing fin and the WRB (weather-resistive barrier) with no air pockets.

3. Head flashing – the membrane strip at the top laps over the tops of the jamb strips. Finally, a metal drip-cap or pre-formed head flashing is installed above the head nailing fin, sloping water outward, and the WRB above the window laps over the top of the metal flashing.

Document each layer with a dated photo before it is covered by the next layer. This photo log is your proof of correct installation if a leak issue arises years later.

Installing a Casement Window

A casement window swings outward on vertical hinges operated by a crank mechanism. The installation sequence for setting, leveling, plumbing, squaring, and fastening is the same as for a hung unit. The critical additional checks are:

• Crank operation: Operate the crank through its full range. The sash should swing smoothly without binding. Binding usually means the jambs are racked; re-check plumb and adjust shims.
• Latch engagement: Close the sash and engage the latch handle. It should turn to the closed position without forcing. If it is stiff, the sash is not square in the frame.
• Weatherstripping compression: With the window latched, push gently on each of the four sides of the sash. There should be no movement—the weatherstripping should be uniformly compressed all the way around. A gap on any side allows air and water infiltration.

Safety

• Wear safety glasses whenever cutting shims or operating a screw gun.
• Wear work gloves when handling metal nailing fins and metal head flashing—edges are sharp.
• When working at elevated wall mockups or on scaffolding, follow all fall protection requirements per OSHA standards for your working height.
• Never lean out of a window opening to reach the exterior nailing fin from the inside; reposition your body or use proper staging.
• Keep the work area clear of shim scraps and packaging to prevent trips and falls.

Lab Documentation Requirement

For each installation, your lab log must contain dated photos of: (1) the sill pan before the window is set, (2) the level reading on the sill, (3) the plumb reading on each jamb, (4) the diagonal tape measurement, (5) each flashing layer before it is lapped, and (6) the finished unit. Photo logs are standard practice on professional job sites and are part of your grade.