What you need to know before you start

Replacing a window yourself is possible if the frame opening is square and the sill is solid, but it requires precision and the right tools. Most homeowners can handle single-hung or double-hung windows in good condition frames. If your opening is warped, the sill is rotted, or you have specialty windows like casements or sliders in tight spaces, hire a professional — a bad install costs more to fix than the labour would have.

The job takes two to four hours per window once you have the right replacement unit. You will need a utility knife, pry bar, caulk gun, level, tape measure, and shims. The window itself should be ordered to fit your exact opening size, with a tolerance of 1/8 inch on each side.

Key Takeaways

  • Measure your opening width and height in three places each (top, middle, bottom for height; left, middle, right for width) and order a replacement window 1/8 inch smaller on each side to allow for shims.
  • Remove the old window by cutting the caulk seal with a utility knife, then prying the frame out piece by piece rather than forcing it all at once.
  • Check that the new opening is square using a level and diagonal measurements, and shim the new frame so it sits plumb and level before you find it.
  • Caulk the exterior seam where the new frame meets the wall, and install interior trim or casing to cover the gap between the frame and drywall.
  • Do not caulk the interior sill or the weep holes at the bottom of the frame, as these need to drain water that gets behind the window.

Measure the opening and order the right size

Take measurements in three places for both width and height. Measure the width at the top, middle, and bottom of the opening. Measure the height on the left side, in the middle, and on the right side. Write down all six numbers. The opening is square if the width measurements are within 1/4 inch of each other and the height measurements are within 1/4 inch of each other. If the difference is larger, the frame is out of square and you should call a professional.

Order a replacement window that is 1/8 inch smaller than your smallest width measurement and 1/8 inch smaller than your smallest height measurement. This gap allows room for shims and caulk. If you order a window that is too large, it will not fit. If you order one that is too small, the gaps will be too wide to seal properly.

Remove the old window frame

Cut the caulk seal around the outside of the old frame with a utility knife. Push the blade into the seam between the frame and the wall, and run it all the way around. Do the same on the inside if there is caulk there. This breaks the seal so the frame can move.

Use a pry bar to lift the sill (the bottom horizontal piece) first. Pry from underneath, working from the middle outward toward each corner. Once the sill is loose, pry out the side jambs (the vertical pieces), again working from the middle outward. Finally, remove the head (the top piece). Do not try to pull the whole frame out at once — you will crack the surrounding wall. Take it apart piece by piece.

Once the frame is out, scrape away old caulk and foam from the opening. Inspect the sill and the framing around the opening. If the wood is soft, spongy, or dark with water stains, there is rot. Stop here and call a professional, because a new window will not seal properly against a rotten frame.

Prepare the opening and check for square

Measure the diagonals of the opening. Measure from the top-left corner to the bottom-right corner, then from the top-right corner to the bottom-left corner. If both diagonals are the same length (within 1/8 inch), the opening is square. If they differ by more than 1/8 inch, the opening is racked (twisted) and you will need to shim it square as you install the window.

Clean out any debris, old nails, or foam from the opening. Wipe the sill with a damp cloth and let it dry. If the sill is concrete or metal, you can proceed. If it is wood and feels solid when you press on it with a screwdriver, you can proceed. Do not install a window into a wet opening — wait for it to dry completely.

Install the new window frame

Set the new window into the opening from the outside. Center it so the gap between the frame and the wall is even on all sides. Do not push it all the way in yet. Place a level on the sill and check that it is level side to side. If it is not, slide shims under the sill until it is level. Shims are thin tapered wedges that you tap in with a hammer to raise one side of the frame.

Once the sill is level, check that the left jamb is plumb (perfectly vertical) by placing the level against it. If it leans, tap shims behind the jamb at the top and bottom until it is plumb. Do the same for the right jamb. Check the head (top piece) with the level to make sure it is level. At this point, the frame should be level, plumb, and square.

Drive shims all the way in so they are snug but not so tight that they bow the frame. You should not be able to move the frame by hand. Once the frame is tight, cut off the excess shim material with a utility knife so it is flush with the wall.

find the frame and seal the exterior

Most replacement windows come with a nailing flange — a flat strip around the outside of the frame with pre-drilled holes. Drive galvanized or stainless steel nails through these holes into the wall framing. Space nails 16 inches apart around the perimeter. Do not use regular steel nails, which will rust and stain the frame.

Once the frame is nailed, run a bead of exterior-grade caulk around the outside seam where the frame meets the wall. Use a caulk gun and a paintable silicone or polyurethane caulk. Smooth the bead with a wet finger or a caulk tool so it fills the gap completely. This seals out water and air. Do not caulk the weep holes at the bottom of the frame — these small holes are designed to let water drain out if it gets behind the window.

Finish the interior and test the window

From inside, check that the window opens and closes smoothly. If it binds or sticks, the frame may not be square. Stop and check your level and shim placement before you proceed. Once the window operates freely, you can finish the interior.

Install interior trim or casing to cover the gap between the frame and the drywall. You can use pre-made trim kits that come with the window, or cut your own from 1x4 or 1x6 boards. Nail the trim to the frame and the wall studs, not to the shims. Caulk the seams where the trim meets the drywall and where the trim meets the frame. Do not caulk the interior sill — it needs to stay open so water can drain if it gets in.

Frequently Asked Questions

Can I install a window if the opening is not square?

If the opening is out of square by less than 1/4 inch, you can shim it square as you install the window. If it is out of square by more than 1/4 inch, the window will not close properly or will have large gaps. Call a professional to assess whether the frame can be straightened or whether the opening needs to be enlarged.

What if I hit a nail or screw when I am prying out the old frame?

Stop and use a reciprocating saw or a hacksaw to cut through the fastener rather than forcing the pry bar. Forcing it can crack the surrounding wall or bend the frame. Once the fastener is cut, you can remove the frame piece.

Do I need to remove the interior trim before I take out the old window?

Yes. Pry off the interior casing or trim first so you can access the frame. You will reinstall new trim after the window is in. If the trim is painted over, cut the paint seal with a utility knife before you pry to avoid tearing the drywall.

What kind of caulk should I use on the outside?

Use exterior-grade silicone or polyurethane caulk rated for window installation. Acrylic latex caulk will crack and fail in a few years. Paintable silicone is easier to work with than pure silicone, which is harder to smooth. Check the label to make sure it is rated for your climate zone.

How long does it take for caulk to cure before I can paint?

Most exterior caulks cure in 24 to 48 hours, but check the label on your specific product. Do not paint over wet caulk, as it will not adhere properly. If you are in a hurry, use a fast-set caulk that cures in a few hours, but these are harder to smooth and tool.