If you notice a foul sewer smell in your basement, here are the five possible causes in order of probability:
- You have a water trap under a floor drain, laundry tub or wash basin that has dried out from lack of use. Water in any trap under unused drains will eventually evaporate. That would allow sewer gas to come up through the drain into the room. Solve that problem just by dumping a pitcherful of water into the drain to restore the trap water. If you follow that up by pouring a couple of tablespoons of cooking oil into the drain, a floating seal of oil will keep the water from evaporating so fast next time.
- Check for a cleanout plug inside the floor drain. Remove the grate that covers the opening and make sure there's a plug inside the drain bowl. If the plug is missing, there's a direct path for sewer gas to bypass the water trap. Sometimes, the plugs are removed to clean sewer lines and not replaced. Buy a replacement plug at a hardware store.
- It's less likely, but the water in the toilet trap also could have evaporated. Weeks of disuse could cause this. Of course, simple flushing will restore that water.
- Another culprit could be a bad wax ring seal between the toilet flange and the base of the toilet. This wax ring can occasionally leak, sometimes because of a rocking toilet that has broken the seal. With a leak in this seal, sewer gas will find its way out from under the toilet. If that's the case, you'll have to remove the toilet and replace the wax ring. If the toilet rocks, use plastic shims between the stool and the floor and caulk the joint. This will ensure that a rocking toilet doesn't ruin the new wax ring.
- A more serious prospect would be a broken or cracked sewer line or even a loose connection joint in the ceiling or buried in a wall. If you've addressed the four easier possibilities, use your nose to start investigating, starting with all visible joints. If you can't detect the leak, contact a plumber who specializes in hunting down leaks.