Floor drains are easy to forget about. They sit in a basement, garage or laundry room doing nothing for months, and then one day the room smells like a sewer or the drain backs up during a heavy rain. In almost every case the P-trap under that drain is involved. Here is how floor drain traps work, why they dry out, what kind of trap suits each location, and how to keep one working.
How a floor drain trap works
Like any fixture trap, a floor drain trap holds a seal of water, typically 2 to 4 inches deep in residential work, that stops sewer gas from escaping. The difference is that a floor drain rarely gets water. Sinks and showers refill their traps every day, but a basement floor drain might only see water when the water heater leaks or someone mops the floor. Over a few weeks or months that seal evaporates, and the open pipe lets gas straight into the room.
Most floor drain traps are 2 or 3 inches in diameter and set into the concrete directly below the grate. Many older homes have the trap cast into the slab with a cleanout plug on the side, and some basement drains connect to a sump or to the sewer line, depending on how the house was built. Our guide on where basement floor drains go explains the difference.
Floor drain trap options compared
| Option | What it does | Typical cost | Notes |
|---|---|---|---|
| Standard P-trap below the drain | Holds a water seal | About $10 to $40 for parts | Installed when the slab is poured or opened |
| Trap primer valve | Adds a little water to the trap automatically | About $50 to $150 for the valve, $300 to $800 installed | Required for some floor drains by code |
| Waterless trap seal insert | One-way flapper or membrane in the drain | About $15 to $40 | Easy DIY, check local approval |
| Mineral oil top layer | Slows evaporation | Under $5 | Cheap stopgap |
| Backwater valve | Stops sewer backflow | About $1,000 to $2,500 installed | Different job from a trap, often paired |
For the different drain styles, from area drains to trench drains, see our guide to types of floor drains.
How to refill and protect a floor drain trap step by step
- Lift the grate. If it is stuck, pry gently with a flat screwdriver around the edge.
- Look and sniff. A dry trap often shows a visible dry bend or a strong sewer smell.
- Pour about a gallon of water slowly into the drain to refill the seal.
- Add about a quarter cup of mineral oil or cooking oil on top. It floats and slows evaporation for months.
- If the drain is in a laundry room or near a water heater, consider a trap seal insert that drops into the drain body and seals when dry.
- Replace the grate and set a reminder to pour in water every month or two.
- If the smell returns within days, the trap may be cracked or the cleanout plug may be missing. Shine a flashlight in and look for a side opening with no cap.
When the trap needs more than water
- Backups: water coming up through the drain points to a blocked line or a sewer backflow, not the trap. A plumber can snake the line, typically for about $150 to $400.
- Missing cleanout plug: older cast-in traps have a cleanout on the side. If the plug is gone, sewer gas bypasses the seal entirely. A replacement test plug costs a few dollars.
- Cracked or broken trap: in old cast iron this often needs concrete work, typically about $800 to $2,500.
- Adding a new drain: installing a drain in a garage or basement slab is a bigger project. Our guide on installing floor drains in existing garage floors covers it.
If you have a smell elsewhere in the house too, the cause may be venting. See why a P-trap smells.
Floor drain traps versus other traps
Floor drain traps follow the same basic rules as sink and shower traps, but they are larger and far more likely to dry out. They are also usually not reachable without lifting a grate or breaking concrete, much like a tub trap. Compare our guides on tub P-trap access and the P-trap for a shower drain if you are planning work in a basement bathroom. And if you are curious whether your floor drain trap is deep enough, see how deep a P-trap should be.
Safety and code notes
Most codes require every floor drain connected to the sanitary sewer to have a trap. Many require a trap primer or an approved trap seal device where the drain will not get regular water, so check what your local code accepts before choosing a waterless insert. Floor drains connected to storm or sump systems follow different rules. Never pour harsh chemical openers into a floor drain, since they can damage old cast iron and splash back. Sewer gas can contain hydrogen sulfide, so ventilate a basement with a strong smell, and wear gloves and eye protection when working in the drain.
Frequently asked questions
Does a floor drain need a P-trap?
Yes. Any floor drain tied to the sanitary sewer needs a trap to block sewer gas.
Why does my basement floor drain smell?
Usually the trap has dried out. Pour in a gallon of water and a little mineral oil to keep the seal.
How often should I fill a floor drain trap?
Every month or two for most homes, or install a trap seal insert or trap primer.
What is a trap primer?
A valve that feeds small amounts of water into a floor drain trap automatically so it never dries out.
Can a floor drain trap be replaced without breaking concrete?
Sometimes a seal insert or cleanout plug fixes the problem, but replacing the trap itself usually means opening the slab.