A backwater valve, for the drain that floods from below
When the street canal is full, the water in it can come back up your waste pipe and out of the lowest drain in the house. A backwater valve is a one-way flap on that pipe: your water goes out, theirs does not come in.
Fitting one, and finding out whether the water is really coming from outside, is drain work, and that is part of our drain and toilet declogging service. Start by checking which kind of backup you have, below, because a blocked pipe looks the same from the CR floor and needs a different fix.
Rain, or a blockage?
Think back to the last time it happened.
- Only in heavy rain, and it went away when the rain stopped: the water is coming from outside. That is what a backwater valve is for.
- On a dry day too, or when the washing machine empties: something in your own pipe is blocked, and a valve will not fix it. See unclogging a shower or floor drain and where your washing machine drains.
- Several drains at once, worse after rain, and it smells: it can be a full septic tank. Read septic tank trouble before anything else.
What the valve does
Drag the drawing sideways to see all of it.
It goes on the pipe that carries waste water out of the house to the canal, the sewer or the septic tank. Inside is a flap. Water flowing out pushes it open. Water flowing back pushes it shut, and the pipe is sealed until the level outside drops.
There are two kinds. One sits open and closes when water comes back. The other sits closed and only opens when your own water pushes through it. Either works. What matters more is that it is on the right pipe and that you can get to it to clean it.
Where it goes in a house with no basement
Drag the drawing sideways to see all of it.
Most guides online are written for houses with basements, so they tell you to dig up the basement floor. Most houses here have none. The low point is the ground floor CR, the kitchen and the laundry area, and those are the drains the water comes out of.
On the main line out
One valve on the pipe leaving the house protects every drain behind it. It needs a place you can open later: a cleanout box in the yard, or an access cover in the floor.
On one branch
If only one drain ever backs up, a valve on that branch alone can be enough, and it is a smaller job. The rest of the house keeps draining normally while that drain is shut.
What it cannot stop
Floodwater coming in through the door. A valve only seals the pipes. If the street floods above your floor level, the water is coming in anyway, and pumping it out is a different job, covered on our sump pump page.
Check where your roof water goes first
A common reason a house drain fills up in rain has nothing to do with the street. The downspout from the alulod has been piped into the waste line, so every storm sends a whole roof of rain down the same pipe as the toilet. The Revised National Plumbing Code of the Philippines does not allow rain and surface water into a line meant for sewage without approval, and it is the first thing worth looking at. Our guide to the alulod and downspout covers where that water should go.
On the day it rains hard
- Hold off on laundry, long baths and washing up. With the valve shut, the water you use stays in your pipes.
- If water is already coming up and you have no valve, a sealed bag of sand or a plug over the floor drain buys time. Take it out once the water outside drops.
- Keep children out of the water. What comes up a floor drain is sewage, not rain. Our emergency first steps page covers shutting things off safely.
Backflow on the clean water side is a different thing
If you searched "backflow" you may have found pages about something else: stopping dirty water being drawn back into the clean supply, for example through a hose left in a pail. That is a backflow preventer on the water line. It protects your drinking water. It does nothing for a floor drain filling up in a storm.
Questions about backwater valves
- Why does water come up through my floor drain when it rains hard?
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Because the drain your house empties into is full. When the canal or the line outside fills with rainwater faster than it empties, the water has nowhere to go but back up your pipes, and it comes out at the lowest opening in the house, which is usually a floor drain on the ground floor.
- Is a backwater valve the same as a backflow preventer?
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No. A backwater valve sits on the waste pipe and stops dirty water coming back into the house. A backflow preventer sits on the clean water supply and stops dirty water being drawn back into it. Both are one-way, but they protect different pipes.
- Can I use the toilet when the valve is closed?
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Only a little. While the valve is shut nothing leaves the house either, so the water you use has nowhere to go. In heavy rain, hold off on laundry, long baths and anything that sends a lot of water down the drain until the water outside drops.
- Do I need one if my house has never flooded?
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Not necessarily. If water has never come up through a drain, you may never need it. The houses that do are the ones that have seen it at least once, are lower than the street, or sit where the canal overflows every heavy rain.
- Does it need looking after?
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Yes. A flap that is jammed with grease or grit does not close when it should. Open the cover and clean it out a couple of times a year, and before the rains start.
If you would rather chat
A written message reaches us with the detail already in it, so we know what is wrong before we ring you back. For a quick question first, the chat button sits in the corner of every page.
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Drains backing up in the rain?
Finding where the water comes from and fitting a valve on the right pipe is part of our drain and toilet declogging service. Send us the problem and say which drain it comes out of.
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