The basement or garage floor drain. Why it matters: it's the lowest opening in the property, which makes it the place sewage arrives first if a main line blocks. It's also the drain most likely to have a dry trap, because nothing runs into it for months at a time. What goes wrong: the water sealing the trap evaporates, and sewer gas comes up through it — which is where a great many mystery basement smells come from. It can also silt up, so when it's genuinely needed it doesn't take water away. The free check: pour a couple of litres down it monthly. That's the entire maintenance requirement, and it resolves the smell instantly if that's the cause.
The laundry standpipe. Why it matters: a washing machine discharges a large volume very quickly, and the standpipe has to take all of it. A partially blocked one doesn't stop working — it overflows, usually behind the machine where nobody sees it until the floor is wet. What goes wrong: lint and detergent residue accumulate steadily. It's gradual, invisible, and predictable. The free check: watch a discharge cycle occasionally. If the water level rises noticeably in the standpipe before draining away, it's already restricted.
The spare bathroom, or any fixture nobody uses. Why it matters: the same evaporation problem as the floor drain, and it's the single most common cause of "there's a smell in that room and I can't find it." What goes wrong: the trap dries out over weeks, the seal is gone, and sewer gas has an open path into the room. Nothing is blocked. Nothing is broken. The free check: run every unused tap and flush every unused toilet monthly. Two litres of water and the problem disappears.
Outdoor drains — yard, driveway, patio, and downspout connections. Why it matters: these are what stop surface water reaching your building. They fail seasonally and they fail exactly when they're needed — during heavy rain, when leaves have just fallen, or when ice has formed. What goes wrong: leaves, silt, and debris. Then water backs up against a wall, a doorway, or a light well. The free check: clear the grates before the wet season and after heavy leaf fall. Ten minutes, twice a year, and it prevents the kind of water ingress that isn't even a plumbing problem once it's inside.
The HVAC condensate line. Why it matters: air conditioning and high-efficiency heating equipment produce condensate, and it drains away through a small line. When that line blocks, the water goes somewhere — frequently into a ceiling, an attic, or down a wall. What goes wrong: algae and sludge build up inside a narrow pipe that nobody has ever looked at. It's routinely misdiagnosed as a roof leak or a plumbing leak, because water appears in a ceiling with no plumbing above it. The free check: if you see water near your air handler or a stain appearing during cooling season, check the condensate drain before assuming it's a pipe.
The main cleanout — which isn't a drain at all. Why it matters: it's the capped access point into your sewer lateral, and it decides the cost of every future drain call at the property. With one, cabling, jetting, and camera work all start at ground level in minutes. Without one, access frequently means pulling a toilet. What goes wrong: it gets paved over, buried by landscaping, or simply forgotten — and then somebody spends the first part of an emergency callout looking for it. The free check: find it and mark it. If you don't have one, adding it during any other drain or sewer work is inexpensive and it pays back on every visit afterward.