How to Build a Tree Bog (A Beginner's Guide to Woodland Composting Toilets)
If you have a bit of land and want an off-grid toilet that genuinely works with nature rather than against it, a tree bog is one of the most satisfying things you can build. It requires no chemicals, no electricity, and — done properly — no emptying. The woodland looks after itself, and the tree or trees planted nearby do the work of processing the waste.
Here's a practical guide to building one from scratch.
What Is a Tree Bog?
A tree bog is a simple composting toilet structure built over a shallow pit, typically positioned around a living tree or a post planted specifically for the purpose. Human waste falls into the pit, where the tree roots access the nutrients directly. In a true tree bog with a well-established tree, there is no need to empty the pit — the tree does the processing.
The concept comes from Wales, developed by the Centre for Alternative Technology in Machynlleth. It's low-tech, low-cost, and surprisingly effective. The structure above ground can be as basic or as beautiful as you like — some people build simple rustic shelters from poles and corrugated tin, others create genuinely lovely little buildings.
Site Selection: Getting This Right First
Where you place your tree bog matters more than almost anything else about how it's built. Get this wrong and you'll have problems no amount of clever carpentry can fix.
Distance from water sources: Position your tree bog at least 30 metres from any watercourse, pond, or ditch, and at least 50 metres from any borehole or drinking water well. These are the standard guidelines from the Environment Agency, and they're sensible minimums — further is always better if you have the space.
Slope and drainage: A gently sloping site is ideal. You don't want the pit sitting in a hollow where rainwater collects, and you don't want it at the top of a steep slope where run-off could travel downhill toward a water source. A slight slope away from the structure helps drainage without creating any run-off risk.
Which trees work well: Fast-growing trees with vigorous root systems are the best choice. Willows are the classic option — they're extraordinarily greedy for water and nutrients, and a willow planted near a tree bog will grow visibly faster than its neighbours. Alder, elder, and ash are also good choices. Fruit trees are less ideal as the proximity to waste isn't appropriate for edible crops.
Existing trees vs. new planting: You can build around an existing mature tree, positioning the pit within the drip line where the roots are most active. Alternatively, plant a young tree at the same time you dig the pit, though you'll need to rely more on the composting process in the early years while the tree establishes.
Digging the Pit
The pit needs to be large enough to function for several years without filling up. A good starting size is roughly 1.2 metres square and 1 metre deep. Line the lower sides with permeable material — wooden boards with gaps, or open-weave mesh — to allow root access while preventing the sides from collapsing. Leave the base unlined so roots can penetrate freely.
The top of the pit should be roughly level with the ground, with the toilet structure built to sit over it. You'll want a drop of at least 300–400mm between the toilet seat level and the surface of the pit to avoid any splashback and to give the contents room to build up over time.
Building the Structure
The structure above ground is largely a matter of taste, budget, and carpentry skill. At its simplest, a tree bog just needs four posts, a roof, walls on three sides, a door, and a seat. Here are the essentials:
The platform: Build a sturdy platform over the pit using treated timber joists — this is what carries the weight of the seat and the user. Allow adequate ventilation around the pit through gaps in the platform; this is important for the composting process to work aerobically.
The seat: A simple box seat with a round hole cut in the top is all that's needed. Hardwood is preferable for durability — oak or sweet chestnut will last decades outdoors if kept reasonably dry. Make the seat lid hinged so you can add cover material easily and inspect the pit periodically.
The shelter: Walls and a roof keep the rain out of the pit (wet pits smell and compost more slowly) and give the user privacy and shelter. A corrugated metal or polycarbonate roof is perfectly functional. If you're building something more attractive, a living roof of sedums or even turf works beautifully on a tree bog and feels right in a woodland setting.
Ventilation: A vent pipe rising from the pit through or alongside the seat box and out above the roof helps draw smells upward and out rather than letting them linger inside the structure. A simple 110mm soil pipe with a fly-proof mesh cap at the top is sufficient. Position it on the sunward side of the structure — solar heating of the pipe creates a natural draw upward.
Urine Separation — Why It's Worth Doing Even in a Tree Bog
A true tree bog in ideal conditions can handle combined waste without separation. In practice, though, we'd always recommend fitting a urine separator in any composting toilet, including tree bogs. Keeping the solid waste dry makes the composting faster and more effective, and significantly reduces any smell inside the structure.
With urine separated out, the pit stays drier, the composting is aerobic and fast, and the tree gets the nutrients it needs from the solids rather than being overwhelmed by liquid. The urine itself can go to a simple soakaway nearby or be diluted and used as liquid fertiliser elsewhere in the garden.
Maintenance Through the Seasons
A well-built tree bog needs surprisingly little maintenance, but a bit of attention goes a long way.
Cover material: After each use, add a small handful of cover material — wood shavings, sawdust, dried leaves, or even newspaper torn into strips. This prevents flies, adds carbon to balance the nitrogen in the waste, and speeds up composting. Keep a small lidded bucket of cover material next to the seat.
Seasonal checks: In spring, check the pit depth — if it's filling faster than expected, it may be that the tree needs more time to establish, or that the system is being used more intensively than anticipated. In a new build, you may want to add a commercial compost activator in the first year to get the biology going.
When the pit does fill: Even with a mature tree, eventually you may find the pit needs attention. The simplest solution is to cap the existing pit (cover it with soil and leave it to finish composting) and dig a new one nearby. Alternatively, if the contents have been in place for a year or more, they can often be dug out as finished compost — dark, crumbly, and with no smell — and used in the garden away from food crops.
A Genuinely Natural System
There's something deeply satisfying about a tree bog that works well. You're closing a nutrient loop that industrial sanitation broke — returning what the land grew, back to the land. The tree grows faster, the woodland is richer for it, and you've built something that works without any inputs at all beyond a handful of wood shavings.
If you're getting started and want to fit a urine separator or source a good quality composting toilet insert, we can help.
Browse our composting toilet products and urine separators →
