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