Sewer Smell in the Basement

Check the floor drain first

Every floor drain has a trap under the grate that holds a few inches of water to block gas from the sewer. In a basement where nobody runs water near the drain, that seal evaporates over a dry winter with the furnace running. Pour a full pail of water into the floor drain, then a cup of mineral oil on top to slow evaporation. If the smell fades within an hour, that was it. The same goes for an unused basement shower or a laundry standpipe behind the washer.

Then the toilets and cleanouts

• Wax ring. A toilet that rocks has broken its wax seal. You may see a stain on the subfloor from below. We pull the toilet, scrape the flange, fit a new ring and new closet bolts.
• Cleanout plug. The threaded plug on the main stack or the front-wall cleanout sometimes gets left loose after a drain call. Snug it with a wrench and check the gasket.
• Rough-in caps. Basements roughed in for a future bathroom have capped pipes in the slab. A cracked test cap or a missing plug lets gas straight out.

Harder ones we find

A cast-iron stack in an older Milton house can crack along its length or rot through at the bottom bend; we see these in pre-war houses near the old downtown. A blocked roof vent, often from ice or a bird nest, lets traps get siphoned dry every time a toilet flushes, so you hear gurgling and then smell gas. A failed air admittance valve under a basement sink does the same. For these we run a smoke test: we cap the drain at the cleanout, push harmless theatrical smoke into the system and watch where it comes out.

When it is urgent

If the smell comes with water backing up from the floor drain, the main line is blocked and the trap seals are being pushed out. Stop running water upstairs and call. If the smell is rotten egg near a gas appliance rather than near a drain, leave the house and call your gas utility from outside, not us.