Leak detection is the work of finding where water is entering a building before anyone opens a wall. It combines moisture mapping, thermal scanning, cavity inspection and, for pressurized lines, acoustic or pressure testing. It comes first, because a remediation that starts before the source is found gets done twice.
Open a wall without knowing where the water comes from and you are running an experiment, billed by the square foot. This page covers the six sources, what each tool does, and how to confirm a repair held.
Almost every building water problem traces to one of six categories, and behavior tells them apart.
One system, two failure patterns, and confusing them costs people months.
Supply lines are under pressure every second of the day. A supply leak does not wait for anyone to turn a tap. It runs overnight, it runs while the house is empty, and it worsens fast. The tell is damage that grows with no relation to weather or fixture use. Shut every fixture off and watch the water meter: if it still moves, water is going somewhere.
Drain lines only carry water when something above them runs. A cracked drain, a failed wax ring or a leaking shower pan wets the cavity for twenty seconds at a time, then stops and dries at the surface. That intermittency is why drain leaks hide, and why they are often a year old when found. The tell is a stain that darkens after a shower.
On any two story house: a brown ring on a first floor ceiling is more often the bathroom above it than the roof above that.
The open field of shingles almost never fails first. Failures happen at the interruptions, and plumbing vent boots lead the list, because the rubber collar splits long before the shingles wear out. Then chimney counterflashing, valley metal, skylight curbs and step flashing. The tell is timing: roof leaks follow rain, often rain driven from one direction, which is why a house leaks in a March storm and stays dry in a heavier one in July.
Water gets into the rough opening around the unit, not through the unit. When the sill pan, head flashing or perimeter sealant fails, wind-driven rain works into the framed opening and runs down inside the wall. Look for staining at the lower corners rather than the center, rust bleeding through paint at the corner bead, a soft jamb bottom. These are directional: the windward window leaks while the identical unit across the house never does.
Most basement and crawlspace water is not a foundation defect. It is water delivered there by a downspout discharging at the wall, soil graded back toward the house, or a clogged footing drain. The signs: efflorescence on masonry, a damp band across the bottom foot of wall rather than a defined stain, and timing that follows heavy rain by a day or two rather than during it. That lag is the giveaway, because soil takes time to saturate.
Air conditioning makes water on purpose: a cooling coil pulls gallons a day out of humid air, and all of it leaves through one small drain line. Clog that line with biological slime and the pan overflows; rust through the secondary pan and nothing stops it. The tell is seasonal: water in cooling season, staining beside an air handler, or a wet attic patch tracking a duct. Read more: mold in HVAC systems
People miss this one because they are looking for a leak. Hold indoor relative humidity above 60% and any surface slightly cooler than the room air will grow mold with no liquid water involved. Growth appears in several places at once: outside wall corners, behind furniture, in north-side closets. No stain has a shape, because there is no entry point. A hygrometer settles it. Read more: dehumidifiers and indoor humidity
Water obeys gravity and travels along whatever solid surface it lands on, which gives the most useful rule in the field: the wet spot is usually not below the leak. Water entering at a roof penetration runs down a rafter until something stops it, maybe a joist eight feet away. Timing narrows it back down.
One habit separates the people who solve these from the people who argue about them. Draw a pencil line around the edge of the stain and write the date beside it. Two weeks later you know whether it is growing.
Believing one past its limits is how confident people reach wrong conclusions.
| Tool | The question it answers | What it cannot do |
|---|---|---|
| Moisture meter | Is this wet, and how far | Say where the water came from |
| Thermal camera | Where should I look closer | See moisture. It sees temperature |
| Borescope | What is inside this cavity | See past that cavity |
| Hygrometer or logger | Is the building running humid | Find a leak. It is not looking |
| Acoustic listener | Where is the escape on this pressurized line | Hear a drain leak, which has no pressure |
| Pressure or dye test | Does this line, pan or window leak | Test what you have not isolated |
| Tracer gas | Where is a leak nothing else found | Be cheap or quick |
A moisture meter reading means nothing without a dry reference on the same material in the same building. Thermal imaging only narrows the search, because a missing insulation batt and an air leak both read like wet drywall, and when the meter disagrees with the picture the meter wins. A borescope settles it: a quarter inch hole shows dry framing or standing water.
A slab leak is a failure in a water line under or within a concrete slab, and it is misread more often than any other source. The signs: a warm spot on the floor, which means the hot line, since hot leaks announce themselves and cold ones do not, running water audible with every fixture off, and a meter that keeps turning.
What gets them misdiagnosed is appearance. Water tracks along the slab and wicks up into the bottom of the drywall, so the first visible damage is baseboard and wall board, which looks exactly like a wall leak. Crews open the wall, dry the framing, close it, and the floor is wet again a month later.
ANSI/IICRC S520, the governing standard for professional mold remediation, is explicit about who owns this. Responsibility for identifying and correcting the moisture source rests with the property owner, not the remediator.
A contractor can clean a space to Condition 1 and his obligation ends there. With the leak still running it will not stay at Condition 1, and the contract will say the work was performed correctly. It was. So the sequence is not negotiable: find the water, stop it, dry the structure, remediate, rebuild. Read more: the mold remediation process
A source is not corrected because a plumber says so. It is corrected when the materials dry and stay dry.
Take readings at marked points before the repair, immediately after, then on a schedule over the following days. Mark them in pencil so you read the same spots each time, and log the dry reference too, because ambient conditions move and the reference moves with them. Give it days rather than hours: framing dries slowly.
You want a curve trending down toward the reference, then holding. Readings that plateau above it mean the material is still connected to water. Readings that reach it and climb again after the next rain mean one path is still open.
Plenty of leaks are findable by a homeowner: a dripping connection under a sink, a downspout dumping at the foundation, a condensate line you can clear with a wet vacuum. Bring in a professional when any of this is true.
By narrowing rather than searching. Timing comes first, because whether damage tracks rain, fixture use or nothing at all points to a category of source. Thermal imaging then flags suspect areas, a moisture meter confirms them, and a borescope shows what is inside the cavity.
Usually, yes. Thermal imaging, moisture meters, hygrometers and acoustic equipment all work through finishes without damage, and a borescope needs a hole about a quarter inch across. Some cases still need a dye test or a small opening, and the instruments put that opening in the right place.
Price follows what the job requires. A visual and moisture survey of an accessible area sits at the low end. Cost rises with thermal scanning, with the number of areas checked, with access difficulty in crawlspaces and attics, and sharply with slab leak location or tracer gas.
A failure in a water line under or within a concrete slab foundation. Warning signs: a warm spot on the floor, running water audible with all fixtures off, a meter that keeps turning, and flooring that lifts. They are commonly misread as wall leaks, because water wicks up the drywall first.
Because the source was never corrected. Repainting a ceiling treats the evidence rather than the cause. Water also travels along framing before it becomes visible, so the leak is frequently several feet from the stain, and a repair aimed directly above it misses.
Yes. ANSI/IICRC S520 places responsibility for identifying and correcting the moisture source on the property owner, not the remediator. A space cleaned to Condition 1 with water still entering will not stay there, so the remediation gets done twice, including the materials installed the first time.
It can find where to look. A thermal camera reads surface temperature, not moisture, and wet material often reads cooler because evaporation carries heat away. Insulation gaps, air leaks and cold water lines produce the same signature, so a thermal image is a reason to take a reading.
MI&T is an inspection and testing company. We do not perform mold remediation, we do not sell equipment, and we take no referral fees, so we have no interest in how large a scope turns out to be.
What we do is assess: establish whether a building has a moisture problem, find how far the damage extends, document it with moisture readings, thermal scans and cavity inspection, and put it in a written report for whoever does the repair. Where the moisture is coming from is the first question that assessment answers, because everything downstream is scoped from it.
If you have a stain that keeps returning, or a claim that turns on whether water was sudden or long-term, an assessment by someone with no stake in the repair is the cheapest step available.
Find your nearest MI&T location to schedule an assessment.
