Install the New Garden Window
Before installing the new garden window, find and mark the center of the window opening. Also determine the installation height of the garden window, which may be different from the old window and the rough opening, and mark a level line across the opening.
If the rough opening is taller than needed, it’s generally better to install the new window to the top of the opening and fill in below it. This will help to maintain the home exterior’s defense against rain and water runoff, and any siding replacement will be less noticeable below the window than above it.
If necessary, add spacer blocks to the rough opening sill to elevate the new window up to the level line (Image 1).
Install an aluminum drip cap above the window opening even if the manufacturer claims the garden window does not require it. Tuck the drip cap underneath the siding so the drip edge extends out over the top of the window (Image 2).
Lift the garden window up to the scaffolding and place it into the opening. Ensure that the window is level, vertically plumb and centered in the opening.
Insert tapered wood shims between the window and house framing to firmly position the window without forcing or bowing in the window sides. Double-check the window alignment in all dimensions, then screw through the window sides and shims into the house framing (Image 3).