Building a Simple Timber Compost Bin at Home

Why Three Bays Beat a Single Bin

Most of us start with one bin, fill it to the brim, and then spend the next two years staring at it while it slowly subsides. The problem is that a single bin forces you to add fresh material on top of half-finished compost, and every time you turn it you're mixing the two together. A three-bay system solves that at a stroke. You fill the first bay, turn it into the second when it's full, and turn the second into the third. By the time the third bay is full, the first is ready to use, and you have a proper rotation rather than a permanent heap.

Each bay needs to be roughly one cubic metre — small enough to turn comfortably with a fork, large enough to hold the heat that drives decomposition. Three bays in a row give you about three metres of frontage, which fits neatly along a boundary fence or the back of a shed.

Choosing Timber and Siting the Bin

Use untreated timber. Treated and tanalised timbers can leach preservatives into the compost you're going to spread on your vegetables, and they aren't necessary here. Larch, Douglas fir, oak or sweet chestnut will last ten years or more outdoors without any chemical help, and reclaimed scaffold boards or old decking work well if you can find them. If you want extra protection, brush the timber with raw linseed oil or a plant-based wood stain.

Site the bin on bare soil or grass, never on concrete — you want worms and soil microbes to move in from below, and you need drainage. A spot in dappled shade is ideal, as full sun dries the heap out, and full shade keeps it cold. Leave enough room in front of the bays to swing a fork and wheel a barrow. And keep it away from the neighbours' kitchen window; a well-run heap doesn't smell, but a neglected one does.

A Simple Cutting List

For three bays, each one metre wide, one metre deep and one metre high:

• 8 uprights — 75 x 75mm sawn timber, 1.4m long (300mm goes into the ground, leaving 1.1m above)
• 6 rails — 75 x 50mm timber, 3m long, for the top and middle rails front and back
• Roughly 30 linear metres of 150 x 22mm board for slats — feather-edge or gravel board is perfect
• Exterior-grade or stainless screws, 60mm, plus a handful of 100mm coach screws

Building the Bays, Step by Step

• Mark out a straight line and set your posts at one-metre centres, so there are four positions front and four back.
• Dig each hole about 300mm deep and 200mm across. Drop 50mm of gravel in the bottom for drainage, then set the post. Check it for plumb with a spirit level and ram the soil back firmly in layers.
• Screw the 3m rails across the tops of the front and back posts, then a second set of rails about 500mm down. These tie the whole structure together and stop the posts wandering.
• Fix the slats to the back and the two outer sides, leaving a 25mm gap between each board. Those gaps are the lungs of the heap — they let air in and keep the contents from turning anaerobic.
• On the internal dividing walls, board both faces of the shared posts, or simply fix slats on either side. Don't skip the dividers; they're what makes the rotation work.
• For the fronts, fix pairs of vertical battens to each post with a 30mm gap between them, forming a channel. Cut your front boards to length and drop them into the channels from above. You can then remove boards as the heap drops, and lift the whole front out when it's time to turn.

A lid of old carpet, a sheet of heavy-duty plastic or a hinged timber cover keeps the rain out and the heat in. Just remember to remove it when you turn the heap.

Filling, Layering and Turning

Aim for roughly three parts browns to one part greens by volume. Browns are the dry, carbon-rich materials — cardboard, shredded paper, straw, dead leaves, woody prunings. Greens are the wet, nitrogen-rich ones — grass clippings, kitchen peelings, coffee grounds, spent plants. Mix them as you go rather than layering them in stripes, and chop anything thicker than your finger.

Fill bay one until it's full, then fork the whole lot into bay two, watering it if it's dry. Turn it again into bay three a month or so later. Each turn introduces air and restarts the heating process. In a good summer, you'll have crumbly, dark-brown compost from the third bay in three to six months.

Keeping It Sweet: Common Problems

• Slimy and smelly — too wet and short of air. Turn it and add handfuls of shredded cardboard.
• Bone dry, nothing happening — water it as you turn, and add grass clippings or a bucket of nettle tea.
• Cold and inert — the heap is too small or too woody. Gather a bigger batch in one go rather than adding a bit each day.
• Rats or foxes — keep cooked food, meat, dairy and fish out entirely, and bury fresh additions in the middle of the heap.

Finally, don't worry about getting it perfect. Compost is forgiving, and even a mediocre heap will give you something worth spreading. Build the bays once, and you'll wonder why you ever put up with a single bin.