Garage Water Damage and Mold: Drywall, Slabs and Storage

Attached garages take on a lot of water in an Ontario winter. Snow and slush melt off the car, run across the slab and pool against the walls. A worn door seal lets in meltwater and rain. Over time that water wicks into the bottom of the drywall and the framing behind it, and mold grows in the one place most people never look closely.

Where garage water comes from

• Snow and ice melting off vehicles, often carrying road salt.
• A worn or torn bottom seal on the overhead door.
• A slab that slopes toward the house instead of toward the door.
• Downspouts or driveway grading that send water toward the garage.
• Leaks from a laundry sink, water softener or outdoor faucet line that runs through the garage.
• Roof leaks over the garage, which often go unnoticed because the ceiling is not finished.

The wall shared with the house

In an attached garage, the wall and ceiling shared with the house are built as a fire separation. They are usually finished with fire-rated drywall and taped and sealed so that smoke and car exhaust stay out of the house. If water damage or mold means that drywall has to be cut out, it must be replaced with the same type of fire-rated drywall and resealed. Leaving it open, or patching it with regular drywall, weakens a safety feature of your home.

The shared wall is also where water damage spreads into living space. Water that soaks the bottom of the garage side can wick into the bottom plate and reach the finished room on the other side, such as a mudroom, laundry room or basement stairwell.

Signs to look for

• Staining, swelling or crumbling along the bottom 15 to 30 cm of garage drywall.
• White salt lines on the drywall or the concrete at the base of the walls.
• Rust on metal corner bead or on the screws at the bottom of the wall.
• A musty smell when you open the door from the garage into the house.
• Mold or water stains on cardboard boxes stored on the floor.
• Baseboard or flooring damage on the house side of the shared wall.

How to fix it

1. Find and stop the water first: replace the door seal, redirect downspouts, or have a slope or drainage problem looked at.
2. Check the bottom of the walls with a moisture meter to see how far the water went.
3. Cut out wet or moldy drywall at the bottom of the walls, clean and dry the framing, and check the bottom plate.
4. Replace the drywall with the correct type, keeping it a little above the slab so it does not wick water again.
5. Store boxes on shelving or plastic bins off the floor, not in cardboard on the slab.

A small garage job with a strip of drywall along one wall is usually at the low end of the range for basement and garage work, which runs $1,500 to $9,500 depending on the area. If the water has crossed into the house side of the wall, plan for drying on both sides.

Cheap ways to keep it dry

• Knock snow off the car before you pull in, or park on a rubber containment mat that holds meltwater.
• Sweep or squeegee standing water toward the door after a storm.
• Replace the overhead door bottom seal when it is cracked or flattened.
• Keep stored items on metal or plastic shelving at least 10 cm off the floor.

If you are seeing staining at the bottom of your garage walls or a musty smell coming through the door into the house, send us photos. We will tell you whether it looks like surface damage on the garage side or water that has reached the house.

Common questions