Mold Remediation Guides · Mold Remediation Guides

Leaks That Lead to Mold: Pipes, Roofs and Appliances

A leak is the most common way that mold starts in a home, and it is often the quietest. A pipe can drip behind a wall for weeks, a roof can let rain in for a whole season, and an appliance hose can weep slowly onto a floor, and in each case the water soaks into the materials around it long before any growth shows on the surface. The leaks that cause mold questions that homeowners ask most often are about pipes, roofs, and appliances, and this guide explains each of them in general terms.

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This guide describes the leaks, the hidden places where they do their work, and what to do first in general terms. It makes no claim about any specific home's pipes, roof, or appliances, and it makes no health claims. The practical message is simple: a leak that is found early is a small job, and a leak that is found late is a larger one, so the checks that find leaks are worth building into a routine.

Pipe, roof and appliance leaks in general terms

Pipe leaks are the most common source, and they can be large or small. A burst pipe releases a large volume of water in a short time, and it is found quickly, but a slow drip from a joint, a fitting, or a valve can run for weeks without showing, wetting the wall or the floor around it a little at a time. Pipes under sinks, behind toilets, and in walls are the most common places, and a pipe in a floor above can drip into the ceiling below for a long time before a stain appears.

Roof leaks are the second source, and they are often seasonal, appearing after rain and drying out between storms, which can make them easy to miss. A single slipped tile, a failed flashing around a chimney or a vent, or a gutter that sends water back under the roof edge can each let rain into an attic, where it soaks the insulation and the sheathing. Appliance leaks are the third source: a washing machine hose, a dishwasher connection, a refrigerator water line, or a water heater can each weep slowly onto a floor, and the water can run under flooring and into a wall before it is noticed.

Hidden leaks behind walls and under floors

Hidden leaks are the ones that cause the most trouble, because they wet materials that cannot be seen. A pipe joint behind drywall can weep into the wall cavity, a roof leak can drip into a ceiling void, and an appliance hose can leak under a floor, and in each case the water spreads behind a surface before it shows on it. The growth that follows often starts behind the surface too, and it can be well established before a stain or a smell gives it away.

There are a few indirect clues. A water meter that moves when every tap is off and no appliance is running points to a leak somewhere in the system. A water bill that rises without a change in use points to the same thing. A wall that feels cool and damp, a floor that sounds hollow in one spot, or a smell that is stronger near one wall than another can all point to a hidden leak. The mold inspection and testing page explains what an inspection covers in general terms, including the check for moisture behind walls and under floors.

Leaks That Lead to Mold: Pipes, Roofs and Appliances — California mold remediation guide

What to do first in general terms

When you find a leak, the first step is to stop the water if you can do it safely from a dry place. The main shut-off is usually near where the supply line enters the house, often in a garage, a utility cupboard, or outside near the front of the property. Turning it off stops the flow from a burst pipe or a failed fitting. Do not wade through standing water to reach the shut-off, and do not reach through water to get to a valve under a sink or behind a washing machine. If the valve is on the other side of a flooded room, leave it alone.

Once the water is stopped, move belongings away from the water where you can do so safely and without wading, and take a photo of the leak and the area it has wet. Note when you first noticed it, and whether it is still running. Then call a professional and describe what you found, because the inspection that follows will check how far the water has spread and what it has touched, and that is what decides the plan.

Call a professional

Call a professional when a leak has wet a wall, a ceiling, or a floor, and the area has not been fully dried, or when a sign of growth appears in the weeks after a leak. Call too when the leak was hidden, when it went unnoticed for a while, or when the water reached insulation, drywall, or flooring that cannot be dried in place. An inspection checks the moisture in the materials, looks for the growth that may be hidden behind a surface, and settles what needs to happen next.

When you call, describe the leak in plain terms: where it was, what caused it, how long it ran, and what was done to fix it and to dry the area. Homeowners in San Diego and across the state face the same judgement after a leak, and the mold removal page explains what the removal work covers in general terms, once the moisture source has been found and the growth has been dealt with.

Leaks, hidden water and the checks that find them

The checks that find leaks are simple ones, and they are worth building into a routine, because a leak that is found early is a small job, and a leak that is found late is a larger one, and the difference between the two is usually measured in weeks of inattention rather than in the size of the leak itself. Look under every sink, behind every toilet, and around every appliance that uses water, look at the ceiling below every bathroom and below the roof, and look at the walls in the corners where air does not move, and listen for running water when the house is quiet, and look at the water meter when everything is off to see whether it moves, because a meter that moves when every tap is off and no appliance is running points to a leak somewhere in the system, even if you cannot see a single wet patch, and a water bill that rises without a change in use points to the same thing.

It also helps to know where your shut-off valve is, and to check that it turns, so that if you do find a leak you can stop the water at once, from a dry place and without stepping into standing water, and to take a photo of the leak and the area it has wet, and to note when you first noticed it, and whether it is still running, because those notes are useful when you call a professional, and because the inspection that follows will check how far the water has spread and what it has touched, and that is what decides the plan. A slow drip, repeated day after day, is enough to soak the materials around it, and the damage it causes is often worse than a single larger leak that is noticed at once, because the small leak has had time to work its way deeper, and that is why the checks that find a slow leak are the ones that matter most.

Key Takeaways

Pipe leaks are the most common source, and they can be large or small.
Hidden leaks are the ones that cause the most trouble, because they wet materials that cannot be seen.
When you find a leak, the first step is to stop the water if you can do it safely from a dry place.
Call a professional when a leak has wet a wall, a ceiling, or a floor, and the area has not been fully dried, or when a sign of growth appears in the weeks after a leak.
The checks that find leaks are simple ones, and they are worth building into a routine, because a leak that is found early is a small job, and a leak that is found late is a larger one, and the difference between the two is usually measured in weeks of inattention rather than in the size of the leak itself.

Your Next Step

If this guide matches what you found at home, the next step is a visit — not a guess. Call and describe what you found, and the visit is arranged from what you describe. The Mold Removal page explains what that service covers, and the Mold Remediation page explains what that service covers, and the services hub lists every service in one place.

While you wait, do not disturb a large area of visible mold, and keep the room aired where you can.

Keep the guides on this site bookmarked: the first guide, the signs guide, and the preparation guide cover the checks worth repeating — after moving in, and after bringing used fixtures home.

Mold remediation help is available across California. See mold remediation in Sonoma, CA for one local example, or find your town on the service areas page.