Sawdust, with a carbon-to-nitrogen ratio of about 400:1, is the best cover for a composting toilet because a small amount covers the waste and soaks up moisture, while chopped straw at about 80:1 and dry leaves at about 50-80:1 are good but need more volume. Whatever you use must be dry, clean and untreated, and stored under cover, because wet cover fails and sawdust from treated wood can carry poison.
Part of Composting Toilet Systems
Sawdust, chopped straw and dry or composted leaves are the carbon-rich cover materials used on a waterless toilet. Each differs in how much carbon it carries, how well it soaks up moisture and how much of it a household must find and store.
What Each Cover Offers
The ratios are from the parent page; absorbency and notes from Composting Toilet.
| Cover | C:N | Absorbency | Notes |
|---|---|---|---|
| Sawdust | about 400:1 | Excellent | Best all-round; fine texture covers fully |
| Wood shavings | about 300:1 | Good | Coarser, slower to decompose |
| Dry leaves, crushed | 50-80:1 | Good | Seasonal; crush for better coverage |
| Straw, chopped | about 80:1 | Good | Seasonal; chop short for better packing |
| Rice hulls | about 80:1 | Excellent | Where grown |
How much of each is needed to reach a composting ratio is worked out in Carbon-to-Nitrogen Ratio: a weaker cover such as straw needs a larger multiple of the waste’s carbon than sawdust does.
Leaf Mold: Leaves Left To Break Down
Leaf mold is leaves that have been piled and left to decompose into a dark, crumbly, soil-like material. Fresh dry leaves are a cover in their own right (C:N 50-80:1 above). Partly decomposed leaves are less bulky and more absorbent but have lost some of their carbon, so treat them as a softer, lower-carbon cover and use more. The parent page gives no separate ratio for leaf mold, so check the pile by the smell and heat test rather than assuming one.
Where To Get Each
- Sawdust and shavings: sawmills, carpentry workshops and furniture makers. Fresh and dry is best.
- Straw: bales left from threshing or salvage; chop it with a knife or blade into short pieces.
- Leaves: autumn leaves raked and kept dry; crush or chop them.
What To Refuse
- Treated or painted wood. Sawdust from pressure-treated or preserved timber can carry the preservative, which can include arsenic or copper compounds, as I recall, into the compost and the garden. Refuse it.
- Glued board and plywood. Binders and glues do not belong in food-garden compost.
- Wet or mouldy material. Damp stock cannot absorb liquid and breeds mould spores that irritate lungs.
- Oily or chemical-soaked sawdust, such as from a machine shop.
Method: Keep The Stock Usable
Step 1: Dry it. Spread fresh sawdust or leaves in the sun or a covered, airy place until dry to the touch.
Step 2: Store off the ground, in sacks or a roofed bin, away from rain and ground damp.
Step 3: Keep the day’s supply in a lidded container by the seat.
Step 4: Check weekly for damp, mould and rodent nests; discard anything spoiled.
Common Mistakes
- Using only coarse straw. It does not pack over a deposit; chop it, or mix with sawdust.
- Storing outside uncovered. One rain ruins a month’s stock.
- Taking sawdust from a joinery without asking what wood it is. It may be treated.
Safety
Dry fine cover is a fire and breathing hazard; treated sawdust is a poison hazard
Keep stock away from stoves, fires and sparks. Wear a cloth over nose and mouth when pouring dusty material, and never use mouldy straw or leaves, because mould spores can cause serious lung disease. Never put treated-timber sawdust on a toilet pile whose compost may go on food crops.
What’s Next
- Learn next: Ash & Lime Applications — when a drying alkali helps and when it harms
- Combine with: Cover Materials & Management — how much to use and stock