p trap too low

How Low Can A P Trap Go?

If you are renovating or upgrading your sink and are not lucky enough for the plumbing to line up or are working with a tight space underneath your sink, you are likely to find that your p trap could end up too low. 

Which is not ideal because there is a plumbing code for how low a p trap can be below the drain.

A p trap cannot be lower than the drain pipe simply because you will be going against gravity which forces the water out to the exit drain naturally. Rather in this case the force of gravity which allows water to flow naturally will be limited hence increasing the chances of a clogging

To understand this further we will examine the maximum and minimum height of a P-trap. And what you can do to fix a case when the p trap is too low.

How Low Can P Trap Go

Based on the International Residential Code, 24 inches (2 feet) is the most vertical distance separating the p-trap entrance and the sink drain. On the other hand, according to the Universal Plumbing Code, 24 inches or below must be the developed length of all the fittings, arms, and tailpieces connecting the p-trap to the drain. 

The Building Codes do not stipulate the minimum vertical distance needed between the p-trap and sink drain. However, multiple factors direct the tailpipe’s minimum length for a specific installation.

If the vent pipe has already been installed with a sink connection, the connection’s location has the most significant impact on the P-trap’s location. The trap arm horizontally connecting the p-trap and vent pipe must have a 1/4” per foot slope sloping upwards towards the drain. 

For a sink with a pop-up drain, the tailpipe’s length must be sufficient to accommodate the drain mechanism. However, the distance from the trap connection top to the fittings and trap arm pipe’s top will help you determine the least tailpiece’s length if you haven’t installed the vent pipe and the drain mechanism isn’t of concern. 

See also Can A P Trap be higher than Drain Pipe

What Happens if P Trap Is too Deep

The maximum height of a p trap is supposed to be 4 inches. In the case where a p trap is too deep then the water will rush through the trap too fast which in turn will siphon the water in the trap seal and leads to building up of sludge and eventually clogs.

On the other hand if the p trap is too shallow then the trap seal that remains behind is highly likely to be too little hence could evaporate. This then allows the sewer gas access into your home.

Does the Height of the P Trap Matter

Yes the height of the p trap does matter. The international plumbing code requires that the trap seal must be between 2 inches and 4 inches. However, if you are installing a sink and find that the p trap will end up deeper than 4 inches you will have to fix that.

As we have stated if the p trap height is too deep, water will rush p0too fast and end up siphoning the trap seal water and lead to build up of sludge. On the other hand, if it is too shallow the trap seal water will be shallow and easily evaporate.

If you find that the trap seal is deeper than 4 inches then you will have to open up the wall and relocate the connection of the trap arm at the drain connection.

In addition, the trap arm length must be two diameters long and should slope ¼ inch per foot towards the vertical discharge.

See also No Room for P trap What To Do

Does A P Trap Have to be Level

p trap too low

Yes p traps have to be level otherwise chances of waste backing from the drainage line will be high.

A P-trap’s height isn’t measured using its U-bend but rather determined by the distance from the trap arm’s center line to the floor. The trap arm refers to the straight pipe running parallel to the floor, joining the trap’s U-bend to the primary drain line. 

The vertical space between the trap arm center line and the floor is the P-trap height. You will get this figure by measuring the distance from the trap arm underside to the floor.

For the most accurate distance, add half of the diameter of the trap arm pipe. If the pipe you are using is 2 inches, add an inch to the space between the trap arm underside and the floor.

See also How Deep Can A P Trap be?

Conclusion

If you find that the drain does not line up with your p trap, then the p trap could end up being too low which is not ideal. The maximum vertical distance from the p trap should be 24 inches.

If the p trap is too low you will limit the free flow of water by gravity. The only way to solve the issue is to open up the wall and raise the drain outlet to align with your p trap.

See also Can a P Trap Go Either Way?

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