I used to think window sill planters were just for people with too much time and a Pinterest addiction.
Turns out, building a DIY planter box for your window sill is one of those projects that sounds complicated but actually makes you feel unreasonably accomplished when you’re done. I’ve watched my neighbor Janet spend three weekends measuring, cutting, and swearing at cedar planks, only to end up with something that holds six herbs and makes her kitchen smell like a farmers market. The thing is, window sill construction isn’t really about carpentry—it’s about figuring out what your specific window can handle, weight-wise, and then not overthinking it. Most interior sills can support roughly 15-20 pounds if they’re properly braced, give or take, though I’ve seen older homes where the wood is so dried out you wouldn’t want to lean on it hard. The basic approach involves measuring your sill depth (usually 3-6 inches), deciding if you want the box to sit inside or extend outward, and then choosing materials that won’t rot when you inevitably overwater your basil. Cedar and redwood are the go-to woods because they handle moisture better than pine, but honestly, even treated pine works if you line it with plastic sheeting.
You’ll need a saw, wood screws, drill, and some kind of waterproof liner. Maybe sandpaper if you’re fancy. The construction itself is basically making a rectangular box with no top—four sides and a bottom with drainage holes drilled through.
Measuring Your Window Sill Geometry and Load-Bearing Capacity Before You Start Cutting Anything
Here’s the thing: most DIY planter fails happen because people skip the measuring part and just start building. I watched a YouTube tutorial once where the guy built this gorgeous 36-inch box and then discovered his window sill was only 32 inches wide. The frustration was palpable. You need to measure the interior width of your window frame first, then subtract at least half an inch on each side so the box actually fits without jamming. Depth matters even more—if your sill is shallow, you’ll need to build brackets or supports that extend underneath, which means screwing into the wall or window frame. That’s where things get complicated, because you’re now dealing with load distribution and whether your landlord will lose their mind over holes in the trim. Interior sills are usually attached to the framing studs below the window, so they can handle weight, but exterior sills are often just nailed-on trim that wasn’t designed to support a 15-pound box of wet soil. I’ve seen external planter boxes held up with metal brackets anchored into the brick or siding, which works great until you realize you need a masonry bit and suddenly this simple project requires three trips to the hardware store.
Anyway, the weight calculation is roughly: soil weighs about 75 pounds per cubic foot when wet, so a box that’s 24 inches long, 6 inches wide, and 6 inches deep holds about 0.4 cubic feet, meaning around 30 pounds when saturated. Add the wood weight (maybe 5 pounds) and you’re at 35 pounds total—way more than most people estimate.
Cutting and Assembling the Box Structure Without Losing Your Mind or Any Fingers
The actual construction is straightforward but requires patience I definitely don’t have on most days. You’re cutting four side pieces and one bottom piece—the two long sides are the full length you want, the two short ends fit between them, and the bottom sits inside all four walls. I use 1-inch thick cedar boards from the home improvement store, which come in standard widths like 4, 6, or 8 inches. For a typical sill, 6-inch width works because it gives you enough soil depth for roots without making the box so heavy it becomes a structural hazard. The cuts need to be square or the whole thing sits crooked, which bugs me more than it probably should. I clamp everything together first before drilling pilot holes, because if you just drive screws straight into cedar without pre-drilling, the wood splits and you get to start over. Use exterior wood screws, maybe 1.5 inches long, two per corner. The bottom piece needs drainage holes—I drill five or six quarter-inch holes spaced evenly, which seems like enough to prevent root rot but not so many that soil pours out like a sieve.
Sand the edges if you want it to look professional. I usually skip this step and just accept the rustic vibe.
Waterproofing, Drainage Systems, and Preventing Your Window Sill From Rotting Out in Two Seasons
This is the part everyone underestimates. Even rot-resistant wood will eventually degrade if it’s sitting in constant moisture, and window sills—especially interior ones—weren’t designed to be perpetually damp. I line the inside of the box with heavy-duty plastic sheeting, the kind used for pond liners, and staple it to the top edges so it creates a waterproof barrier between the soil and the wood. You still need those drainage holes in the bottom, but now you’re drilling through both wood and plastic, which means the water drains into—well, that’s the problem. If this is an interior sill, you need a tray underneath to catch runoff, or you’ll end up with water stains on your wall and possibly mold growing in places you can’t see. I use shallow plastic boot trays from the dollar store, which are ugly but functional. For exterior boxes, drainage is easier because water just drips outside, though you might annoy the neighbors below if you’re in an apartment. Some people install a false bottom—a second layer of wood with spacers underneath—so excess water pools below the soil level and evaporates slowly. That works, but it adds complexity and weight. Another option is using a coco coir liner instead of plastic, which lets the wood breathe better but also dries out faster, meaning you water more often. I’ve tried both and honestly can’t decide which annoys me less. The key is recieving that you’re creating a mini ecosystem where moisture, drainage, and airflow all have to balance, and if you get it wrong, you’ll either drown your plants or watch the wood swell and crack within six months.
Oh, and seal the exterior of the box with linseed oil or exterior wood stain—something that repels water without being toxic to plants. I forgot this step once and the wood turned gray and weathered in about three weeks. Looked cool in a distressed way, but definately not what I intended.
