Journal

How to Install a Window.

Installing a replacement window correctly comes down to four things done in order: prepare the rough opening, set the sill pan, install and shim the window, then flash and seal. The sequence matters more than any single step; doing them out of order is the most common cause of water leaks years later. This is the standard residential approach; new construction with a nailing fin adds a couple of steps but follows the same logic.

Updated May 31, 2026

The order.

Step 1: prepare the rough opening. Remove the old window completely; expose the framing. Verify the opening is square (diagonals equal), plumb (sides vertical), and the right size (1/2 inch over each window dimension). Repair any rotted framing before going further.

Step 2: set the sill pan. A sill pan is a waterproof tray at the base of the rough opening that catches any water that gets past the perimeter seal and drains it back out. Pre-formed pans or site-built flashing tape over a backer both work; the pan must extend up the sides of the rough opening at least 4 inches.

Step 3: install and shim the window. Set the window in the opening, shim under the corners until it sits plumb and square (check both diagonals match), and fasten through the side jambs. Do not over-tighten; the window must not bow.

Step 4: flash and seal. Apply flashing tape over the top of the side jambs, then over the head, lapping the head flashing over the side flashing. Seal the exterior perimeter with backer rod and sealant. Insulate the interior gap between the window frame and the rough opening framing with low-expansion foam, then air-seal at the interior plane.

Common questions.

Can I install a window myself?
For a single straightforward replacement on a ground-floor opening with no structural changes, a handy homeowner can install a window. For multiple windows, upper floors, or anything that requires reframing, a trade installer is the right call. The cost of fixing a poorly installed window after years of water damage exceeds the install labor by a wide margin.
Do I need a sill pan?
Yes for code compliance in most jurisdictions, and yes for long-term performance. The sill pan is the second line of defense against water; the perimeter seal is the first. Without the pan, any seal failure means water enters the wall framing.
What flashing tape should I use?
Self-adhering modified bitumen or butyl flashing tape rated for window installation. Sheet sizes vary; 6-inch and 9-inch are common. Use the same brand as the WRB (water-resistive barrier) if possible to ensure compatibility.

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