Super Simple DIY Windowsill Shelf for Cats, Kids and Plants

Measure your windowsill's width and depth to determine the dimensions for your shelf. My windowsill was about 3 feet wide, and I wanted the new shelf to be approximately 10-18 inches deep to provide adequate space for both my daughter and our cat.

Find a suitable piece of hardwood. I used spalted maple that I had left over from another project. Spalted maple has beautiful dark lines from partial decay that give it character. When selecting wood, check for soft spots and make sure it's fully dried (kiln-dried is best). For this project, any hardwood at least 1 inch thick will work well.

Cut your wood to the proper length to fit your windowsill. If your window has edges or trim that the shelf needs to fit around, mark these locations and cut notches as needed. I used a scroll saw to cut notches at each end of my shelf, but a band saw or even a careful hand with a hack saw would work. Test fit the wood on the windowsill to ensure it fits properly.

Start with 80-grit sandpaper on an orbital sander, following the grain direction. For spalted maple, you can create a refined 'live edge' by sanding in random half-moon patterns along one edge. After the initial sanding, 'pop the grain' by wiping the wood with water and letting it dry (this raises the wood fibers). Then sand again with 120-grit for a smooth finish.

Maple and similar woods can blotch when finished. To prevent this, apply a pre-stain wood conditioner. For warm tones, use an oil-based conditioner; for cooler tones, use water-based. Apply according to the manufacturer's instructions. This step is crucial for an even finish, even if you're not staining the wood.

Once the wood conditioner has dried, apply polyurethane for protection. I used an oil-based clear semi-gloss polyurethane to bring out the warm tones in the spalted maple. Apply thin, even coats with a high-quality brush or foam applicator, sanding lightly with fine-grit sandpaper between coats. Two to three coats are typically sufficient.

Clean the existing windowsill thoroughly. Apply a generous amount of wood glue to the top of the old sill. Place your new shelf on top and weigh it down with heavy objects (I used paint cans). Pre-drill and install wood screws through the top of the new shelf into the old sill - I used two screws at each end. These can be hidden with plants or decorative items later.