What is a curb trap and why do Philadelphia homes have them?

What does a curb trap do?

Every fixture in your house has a trap, the curved pipe under a sink that holds water and blocks sewer gas. A curb trap does the same job for the whole house, on the main sewer lateral. It is a U-shaped fitting, usually clay or cast iron in older homes, set near the front property line, most often under the sidewalk or just inside the front wall of the basement. Water sits in the bend at all times. Gas from the city sewer hits that water seal and stops.

Philadelphia plumbing rules have long called for a trap on the house sewer, which is why nearly every rowhome and twin in the city has one, and many older suburban homes do too. The fitting comes with a vent to the outside air, sometimes a small cap in the sidewalk, and one or two cleanout openings with plugs. When you see a small round metal cover in the sidewalk in front of a Philly rowhome, that is usually the curb trap access. Confirm current code requirements with L&I, since rules on new installations change.

Curb trap, house trap, cleanout: what is the difference?

People use these names loosely, so here is how we use them:

• Curb trap: the trap on the lateral out near the curb or property line, usually under the sidewalk, with its own access box.
• House trap: the same kind of trap but located inside the foundation, in the basement floor near the front wall. Many Philadelphia homes have one or the other, and some have both.
• Cleanout: an access opening with a removable plug that lets a plumber get a cable or camera into the line. Traps have cleanouts built in. A line can also have a separate straight cleanout with no trap.
• Fresh air inlet: the vent pipe on the house side of the trap that lets air in so the water seal is not siphoned out. It often ends at a small grate on the front wall or in the sidewalk.
• Knowing which one you have matters because it tells a plumber where to enter the line for snaking, hydro jetting, or a camera inspection, and it tells you where a clog is most likely to sit.

Why do curb traps cause problems?

The same shape that holds water also holds everything else. Paper, wipes, grease, and grit settle in the bottom of the U. Roots find the joints on either side. Over decades, clay traps crack, cast-iron traps rust through, and the water seal leaks away into the soil, which lets sewer gas up and lets soil fall in. A trap that is full of debris is one of the most common causes of a whole-house backup in Philadelphia.

Signs your curb trap is the problem:

• Backups start at the lowest drain in the front of the basement.
• A sewer smell near the front wall or from the sidewalk grate.
• Water standing in the sidewalk access box, or the cover sinking.
• A snake from inside the house cannot get past a point near the front wall.
• A camera shows a full trap, a cracked trap, or a missing water seal.
• PWD inspectors also flag broken or missing curb traps, and a Notice of Defect for a trap is common.

Can I remove a curb trap instead of replacing it?

Sometimes, and it is worth asking about. Modern plumbing codes in many places no longer require a house trap, because every fixture already has its own trap and a whole-house trap adds a clog point. When we replace a lateral with new PVC, we can often install a straight cleanout in place of the old trap, or a new trap if the city requires it for your situation. Whether removal is allowed depends on the current Philadelphia plumbing code and how your line is vented, so we confirm with L&I on the permit before deciding.

If the trap is sound and the problem is debris or roots, cleaning it out and running a camera through it may be all that is needed. If it is cracked, a new trap or a cleanout is set in a small excavation at the sidewalk. The estimate is driven by depth, what is on the surface, and the sidewalk and street permits.

What should I do if I think my curb trap is clogged?

Do not pull the sidewalk cover and start digging in it. The plugs are often seized, the box may be full of sewage, and prying on an old clay trap can crack it.

Request service and tell us where the backup is showing up. We will open the trap from the proper cleanout, clear it, and run a camera through the trap and out to the main so you can see whether the trap is the whole problem or just the first symptom of a lateral that needs attention. If a repair is needed, we pull the Philadelphia L&I plumbing permit and the street permit, our own underground crew does the excavation and the sidewalk restoration, and we schedule the inspection. You get the options in writing before we start. No high-pressure sales.