A sewer smell that appears during hot, dry weather and fades when it rains is one of plumbing's most common mysteries — and one of its cheapest fixes, most of the time.
The number-one cause: dry traps
Every drain has a U-shaped trap holding water that blocks sewer gas. In guest bathrooms, basement floor drains and laundry sinks that go unused, summer heat evaporates that water in a couple of weeks. The fix costs nothing: pour a quart of water down every rarely-used drain, and add a spoonful of cooking oil to slow evaporation in floor drains.
Cause two: a tired wax ring
If the smell lives in one bathroom and the toilet rocks even slightly, the wax seal under it has likely failed — gas escapes at the base every time pressure changes in the line. That's a proper repair, because a rocking toilet is usually a flange problem, not just wax.
Cause three: vent stack issues
Your plumbing breathes through roof vents. A bird nest, leaves, or in older Roanoke and Martinsville homes a rusted-through cast iron vent lets gas take shortcuts into the house. Smells that appear upstairs or seem to come from walls point here.
When it's serious
A smell that persists after you've refilled traps, doesn't track with weather, or comes with gurgling and slow drains can mean a cracked sewer lateral under or beside the house. That's camera-inspection territory — cheap to diagnose, and far cheaper to fix at one joint than after it undermines a slab.