Installing a Garden Window

Remove the existing window's casing and perhaps the stool (inside sill) to measure the rough opening. Order a garden window custom-made to fit your opening or plan to modify your framing to accommodate a standard-size unit.

Also check the opening to make sure it is square and reasonably straight on its sides. Use a framing square (shown) or measure from corner to corner diagonally.

Most manufacturers require that the rough opening be 1/2 inch wider and taller than the window. If you need to make the opening a bit smaller, add pieces of 2x lumber or strips of plywood as needed. Check again for square and straightness.

Unbox the garden window and inspect it for flaws. Make sure any operable parts work smoothly and seal tightly. Remove shelves, but keep in place any packing that keeps the unit from warping as you install it.

Set the window temporarily in place. Use notched pieces of lumber, set at a slight angle toward the house, to hold the window in place. You may need to tack (partially drive) two nails or screws as well.

From the inside check the window for level, plumb, and square, and tap in shims as needed. Note: If you will be installing a continuous support and thick flashing, as shown in steps 9 -- 11, take the thickness of those materials into account when setting the window.

On the outside, mark the siding for cutting. If you are adding molding or the window has brick molding, hold it in place and mark.

Remove the window. Tack a 1x4 strip as a guide and set the circular saw blade to cut just through the siding . Cut the lines and finish the cuts using a hammer and chisel. If your window requires it, install a drip cap at the top.

Wrap the opening with roofing felt or building wrap. Seal the rough opening as instructed by the manufacturer. For the system shown the first step is to install self-stick flashing to the sill. You may need to spray the area with spray adhesive first.

Attach the support with nails. This window requires a continuous support, a strip of 1/4-inch plywood slightly shorter than the rough sill.

Cut a piece of plastic drain screen to the sill length plus 2 inches, center it, and attach with staples. Cut and attach another piece of self-stick flashing over the drain screen.

Set the window in place, making sure the flange or brick molding rests firmly against the flashing. From the inside adjust with shims as you did in step 6. Drive screws through the flange. If your window does not have a flange, attach through the jambs.

Apply self-stick flashing pieces as directed. First install the bottom horizontal, then the sides, then the top piece. (This arrangement ensures that any accumulation of moisture will flow down over the joints and not be able to seep in.)

Cut and attach the exterior trim pieces using 8d galvanized casing nails. If fastening brickmold, use 10d galvanized casing nails.

Caulk all the joints with high-grade exterior caulk. Paint the new trim soon after installation.

While not always required by manufacturers, two metal or decorative wood brackets are good insurance if you have plenty of plants. Locate framing members (don't just attach to the siding) and drive 3-inch screws.

Patch drywall as needed. If the window's jamb does not come flush with the interior wall, prepare rip-cut pieces to fit.

Custom-cut a stool (inside sill) if needed.

Cut and install the apron and casing and prep for finishing.