Water bill climbing for no reason, a damp spot on a ceiling below, or the meter creeping with everything shut off? Send a few details and one vetted, licensed San Francisco plumber gives you a straight answer. Not five companies fighting over your phone number.
Tell us what you’re seeing. A stain, a sound, a high bill, a spinning meter. And which floor it’s on. That sharpens the first read before anything gets opened up.
A hidden leak in a San Francisco home has a signature, and it starts with the water. Hetch Hetchy runs about 2 to 3 grains per gallon, very soft, and soft water is mildly aggressive to copper. Thin-wall Type M copper from a mid-century remodel is the usual victim. It corrodes from the inside and weeps pinholes behind lath and plaster. Then the fog-damp climate keeps everything from drying, so a slow leak hides for months. The job is to find the source without opening a wall on a guess.
The meter test comes first
Before any tool comes out, a plumber confirms the leak is real. Shut every fixture. Watch the meter. If the little dial keeps turning with nothing running, water is escaping somewhere under pressure. That one check saves you from paying to chase a leak that turned out to be a running toilet flapper. Once it’s confirmed, detection narrows it down.
| What you notice | What it often points to | How it’s found |
|---|---|---|
| Meter moves, all off | Pressurized supply leak | Isolation test, then acoustic listening |
| Warm spot on a floor | Hot-water slab leak | Thermal imaging |
| Stain on a downstairs ceiling | Leak in the unit above | Moisture mapping between floors |
| Only wet when a fixture runs | Drain leak, not supply | Dye test, camera |
| Musty smell, no visible water | Slow weep behind plaster | Moisture meter, single access point |
Never let someone start cutting drywall on a hunch. If a plumber wants to open a wall before running acoustic or thermal, that’s the wrong plumber. Precise detection is exactly what keeps the repair to one hole instead of a dozen.
Which unit, and which line
This is the San Francisco wrinkle. In a stacked flat or a Victorian split into units, a leak shows up a floor below where it started. That ceiling stain in your unit may be the neighbor’s supply line. Sorting out whose line and whose water is half the work, and doing it before demolition saves an argument and a wrecked ceiling. Detection also separates a supply leak under pressure from a drain leak that only shows when a fixture runs, and both from roof or window intrusion that has nothing to do with plumbing. Different problems. Different fixes. Different people who pay.
What the number at the end tells you
Once the source is pinned, you get an honest read on scope. A single failed fitting is a quick fix. One pinhole in a long run of 1960s Type M copper is a warning: where there’s one, more are coming, and a section repipe may be the smarter money than chasing leaks one at a time.
Related San Francisco plumbing services
San Francisco leak detection questions
How do I know I have a hidden leak?
A bill that climbs without a change in use, a meter that moves with every fixture off, a warm patch on a floor, a musty smell, or a stain that spreads. The moving meter is the clearest tell. If the dial turns with everything shut, water is getting out somewhere.
Why does older San Francisco copper spring pinholes?
Soft water. At 2 to 3 grains per gallon it’s mildly aggressive to copper, and thin-wall Type M pipe from mid-century remodels corrodes from the inside out. Add a damp climate that hides the moisture and you get the slow, hidden pinhole leaks plumbers trace in older homes here.
Can you find it without tearing up my walls?
That’s the entire point. Acoustic gear, thermal imaging, and moisture meters locate the source so the repair opens one spot, not a whole wall or slab.
Send your San Francisco leak details
Tell us the symptom and the floor it’s on. You’ll get an honest read on how the leak gets found and how big the repair is likely to be. No pressure to book.
No cost, no obligation. Just a clear answer from the one vetted, licensed San Francisco plumber who’d actually do the work.