Measure Opening

Determine the size of the window by estimating the rough opening — take the measurement from the inside extension jamb on one side to the inside of the extension jamb on the other side and add two inches. Order the window.

Note: You need to plan enough time to order the window. The units can take several weeks to manufacture and deliver.

Some companies custom make garden windows to fit the exact size you need. Other companies have stock sizes and you will need to order dimensions that are close to your existing rough opening.

Note: The garden window is an all-vinyl window with energy-efficient insulated glass. These windows come in wood and other forms, but the vinyl is the most popular choice because it stands up to weather well.

Pro Tip

Check with your local municipality to see if a permit is needed for this project.

Step 1

loosen and remove pieces of trim with pry bar

loosen and remove pieces of trim with pry bar

Remove the Old Window

Gather all the tools and materials. Use the reciprocating saw to cut through the side jambs and remove the old window. Cut to the edge of the trim, being careful not to cut into the siding. Slice through the caulk between the trim and the siding. With a pry bar, loosen and remove the pieces of the trim. Remove the aluminum nailing fin.

Pro Tip

Do not save the trim, it is easier to replace it.

Step 2

Replace Any Rotten Wood

Pry off the siding carefully to see if there is any rot and if so, how far the rot extends. If the panels are attached with special ring shank nails that are hard to remove, you can push them through the siding with a nail punch (Image 1). Pull off the housewrap and insulation. Remove the rotten wood (Image 2), cut out any sheeting and replace it — and replace any triple studding that may be present.

Pro Tip

The last thing you want to do when installing a new garden window is to cover up a bunch of rotten wood. This is the kind of thing to watch out for when you hire people to do work for you.

Step 3

Frame the Window

First, frame in the exterior wall by using pressure treated wood. Nail it to the existing sill plate by using a power nailer (Image 1). Cut the trimmer or jack stud. Make sure it is plumb and then nail it along the top. Toe nail the king stud at the bottom (Image 2). Now that the trimmer is in, you can go back to the king stud next to it. You'll need to fill in the gap that was created when the rotten boards were cut and removed. Do this by sawing two studs and nailing them together for support. Toe nail them into place. Nail another two studs of the same length together that will support the header for the left side (Image 3). Make sure the studs are plumb.

The next step to repair the outside wall is to put in two of the shorter studs, called cripple studs, (Image 4) on each side. Lay the sill cross the top and put the third cripple stud under the middle. Note that the size of these studs are dictated by the height of the rough opening. Put in a 2x4 riser block for the inside wall. Use 3/4 plywood to replace the tongue-and-groove sheathing.

Pro Tip

It's vital when installing a garden window that it be plumb, level and square to ensure that the side panels open and close smoothly. 

You can use OSB sheathing, if you prefer. Lay on roofing felt (Image 5) to give the wall extra protection.

Step 4

Seal the Opening

An innovative special flashing system was used to seal and protect the window from leaking for this particular project. Apply this double-sided adhesive along the surface of the window frame, starting along the top. Overlap the next strip at the corner (Image 1), making sure there are no gaps, and press into place.

Lay pan flashing down on the sill using flashing tape. You can create this pan flashing with window tape. Start the flashing on the sill on one side of the window and extend it to the other side (Image 2). Then make two cuts so you can fold the flashing over the top of the sill. The adhesive creates a watertight seal and helps protect the sill from condensation or leaks, which should prevent any future rotting.

Note: The outside of the tape has a polyethylene film that helps keep the water out. Add a second layer of flashing over the first layer. Cut the outside edges so this layer can lay flat against the sheathing.

Pro Tip

This is a very sticky asphalt based product that will create a positive seal once the window goes in. It's pliable.

Step 5

Install the Window

Dry fit the window first to see if you need to make any adjustments. For this particular project, the window was a little low, so we used two small shims (Image 1) in each corner to raise the window to the proper height.

Remove the window, exposing the sticky tape. While someone holds the window in place for you, drive a couple of 3" screws to hold the unit temporarily until you shim it in place. Check the level. While someone continues to secure the window, drive a 4" lag screw in through the center top of the window frame (Image 2). Adjust the plumb by drilling in screws at the top and bottom and then add the shims to hold everything into place. Level and plum the window.

Pro Tip

Make sure the unit is centered left and right so you can put insulation on both sides.

When installing a window this heavy, safety is important. Don't walk away from the window until it is secured.

Step 6

Seal the Window

Put flashing tape up and put on the drip cap (Image 1). Put flashing along the bottom and sides. Lay the wrap so that it covers the window about a quarter of an inch. You do not want the flash to show above the trim or the siding. Put the outside wall back together. Drywall the return on the inside by cutting the drywall pieces and then putting them into place (Image 2). Assemble the drywall corner bead. Put on joint compound.