Prefab toilets: how they are installed and plumbed
Contents
In short
- A prefab toilet needs three things: a water supply, a drain, and somewhere for waste to be treated or stored.
- Household soil pipes are often 100 mm, and should fall at least 1 cm per 1 m.
- Choose a spot close to the connection, with short pipes and access for a pump truck.
- Finish the pipes and tank before the toilet is delivered.
Types of prefab toilet
Before looking at pipes, decide where the waste goes, because the site work differs a lot.
| Type | Suits | Prepare |
|---|---|---|
| Connected to a septic tank | Homes and long-term use | Buried tank, pipes and an outlet |
| Built-in holding tank | Work sites, or land where a tank cannot be buried | Access for a pump truck |
| Connected to an existing system | Land with a house already | Check it can take the extra load |
How the pipes run
Water supply
Ask the seller where the inlet is and what size pipe it uses. Without mains water you need a storage tank and pump, plus space, power and an overflow. Fit a shut-off valve close to the toilet.
Waste and soil pipes
A sample Thai house design uses a 100 mm soil pipe and a separate 55 mm waste pipe. The fall should be at least 1:100, or 1 cm per 1 m of length. On a long run, work out the height from the toilet floor to the tank inlet, or the pipe will not fall enough. Allow for a cleaning point too.
Vent pipe
Run the vent high enough to clear areas people use, so smells escape and do not blow back into the house. Each fitting needs its own trap to stop smells.
Choosing a septic tank
Household tanks on the market start from around 600 litres. Choose from the manufacturer's table of users for that model, and ask where the outlet should go, because rules vary between local authorities.
- Put the tank where a pump truck can reach it and vehicles will not drive over it.
- Keep the lid above ground, or easy to find.
- Keep it away from wells, and ask local officials about the required distance.
- Check that groundwater will not float the tank in the rainy season.
Choosing a spot that is easy to maintain
- Close to the connection: short pipes make it easier to keep the fall.
- Not in a low spot: the floor should sit above any standing water.
- Access for repairs: leave room at the back or side to reach the pipes.
- Smell and wind: avoid spots near bedrooms or a neighbour's kitchen.
- Light and power: the path at night needs lighting and a non-slip surface.
Order of work before installation
- Get a pipe drawing from the seller, with distances from the toilet's edges.
- Make the supports and set the floor level.
- Bury the tank, run the pipes, and test the flow before backfilling.
- Place the toilet and have a plumber connect the pipes.
- Turn on the water and check for leaks under the floor and around the toilet.
See Preparing your site: ground, supports and truck access.
Looking after it
Do not put oil, thick paper, sanitary products or strong chemicals down the toilet. Follow the manual for checking the tank, and pump out sludge as often as the maker advises. If water drains slowly or it smells when used, check the fall and the vent first.
Frequently asked questions
Can I use one without mains water?
Yes, with a storage tank and pump, or a raised tank that feeds by gravity. Check that the pressure suits your fittings, such as the toilet and water heater.
What causes bad smells?
Common causes are a dry trap, too little fall, or no vent pipe. If that does not fix it, ask a plumber.
Do I need permission?
It depends on the area and use, and there may be rules on waste water. Ask local officials, and see Do you need a permit to place a knock-down house on your land?
Can a prefab toilet be moved?
Yes, if a crane can reach it, but the pipes must be cut and a new tank or pipes made at the new spot. Include that cost before moving.