How to Build and Install Window Boxes

In this video, This Old House general contractor Tom Silva shows how to build window boxes.

In one This Old House project, a homeowner had looked at premade window boxes at local hardware stores and in catalogs but found the plastic options looked cheap while the catalog versions were extremely expensive. Building custom window boxes from scratch proved to be a more affordable path to a high-quality result.

Pro Tip: As Tom explains in the This Old House project episode: “You know what makes a good quality window box expensive is the type of material that is made of, you need a wood that will withstand the weather—redwood, for example, will hold up really well. But it’s expensive.” He recommends medium density overlay (MDO) plywood as a cost-effective alternative that still delivers a polished look.

Another material option to consider is cellular PVC, which looks and works like wood but won’t rot. While it typically costs more upfront than MDO plywood, it can save money over time by eliminating the need for replacement due to weather damage. For homes with vinyl siding, mounting may be simpler since the siding can be temporarily removed to locate the wall’s structural framing.

Steps:

1. Use circular saw and clamped-in-place straightedge guide to cut window-box parts to size from medium-density overlay.

2. Use drill/driver to drill counterbore and screw-pilot holes, then assemble the box parts with polyurethane glue and 2-inch-long ceramic-coated deck screws.

3. Attach two galvanized-steel right-angle brackets to the house wall directly below a window. On vinyl siding, be sure to insert two small wood blocks behind each bracket to avoid crushing the siding.

4. Drill pilot holes through the siding and secure each bracket to the house wall with 2-inch-long ceramic-coated deck screws.

5. Drill six ⅜-inch-diameter weep holes through the bottom of the window box.

6. Fill all screw holes with auto-body filler, then sand flush with random-orbit sander.

7. Prime and paint the box with exterior-grade house paint.

8. Set window box onto brackets and secure by driving ¾-inch galvanized or stainless steel screws up through the brackets and into the bottom of the box.

Pro Tip: For window boxes longer than 48 inches, add a third, center bracket to prevent the box from sagging under the weight of soil and plants. As noted in This Old House Magazine, two brackets are standard, but that extra support is critical for wider windows. Also consider that if you’re mounting to vinyl, aluminum, or fiber-cement siding, you should bore a pilot hole through the siding itself so the screw bites securely into the sheathing behind it.

Sizing Tip: When using a circular saw with a clamped straightedge guide, account for the offset between the blade and the saw’s base plate. Tom Silva notes that on his saw, “the relationship of the blade to the plate is an inch and a quarter,” meaning you need to add that distance to your measurement when positioning the straightedge. For a 38-inch window box, that means clamping the guide at 39¼ inches.