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Why Is There Mold Near My Home's Expansion Joints?

Last updated October 25, 2026

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The short answer

What Expansion Joints Do

Expansion joints are intentional gaps built into your home's structure. They allow concrete, brick, or siding to expand and contract with temperature changes without cracking. These gaps are often sealed with a flexible material, but the seal can fail over time.

When the seal breaks, water from rain, sprinklers, or melting snow can get behind it. The joint holds that moisture close to the building materials, creating a damp pocket. Mold spores are everywhere in the air, and they only need a wet spot and a food source like wood or drywall to start growing.

How Moisture Gets In

A common cause is a cracked or worn sealant along the joint. Small gaps can also appear where the joint meets a window frame, door frame, or roofline. If your home sits on a slope, runoff can pool against the foundation and seep into the joint.

Humid air itself can be part of the problem. In areas with cold winters and warm, humid summers, condensation can form on cool surfaces near the joint. Even without a visible leak, the constant moisture from condensation can be enough to support mold growth.

  • Check the sealant for cracks, gaps, or peeling
  • Look for water stains or discoloration near the joint
  • Feel the area for dampness after rain or snowmelt
  • Inspect gutters and downspouts for overflow
  • Note if the joint is in a shaded area that stays wet longer

Is Mold There a Health Risk

Mold in any part of your home can affect indoor air quality. Spores can travel through wall cavities and into living spaces. If you or someone in your home has allergies, asthma, or a weakened immune system, exposure can cause coughing, sneezing, or breathing trouble.

If you feel unwell and suspect mold is the cause, see a doctor for advice. The mold itself is not always toxic, but it is a sign that moisture is where it should not be. Even without health symptoms, you should address the dampness to protect your home's structure.

Steps You Can Take Now

Start by drying the area with towels or a fan. Remove any loose mold you can see using a mild detergent and water, but avoid scrubbing hard, as that can release spores. Wear gloves and a mask to limit exposure.

Check the sealant around the joint and replace it if it is cracked or missing. Make sure the ground slopes away from your foundation so water does not pool. If the mold is inside a wall or under flooring, you may need to open up the area to let it dry fully.

  • Dry the visible area with fans or a dehumidifier
  • Clean small mold spots with soapy water
  • Wear gloves, goggles, and a mask
  • Fix any cracked sealant promptly
  • Redirect downspouts away from the joint
  • Keep the area clear of leaves and debris

When to Call the Pros

If the mold covers a large area, or if you cannot find the water source, local water damage pros can help. They have moisture meters and other tools to trace where the dampness is coming from. They can also dry out wall cavities and treat the mold safely.

For a quick assessment, you can also ask a plumber or roofer to inspect the joint if the leak seems related to pipes or the roof. The cost depends on the size of the problem and what needs to be repaired. Policies vary, and your insurer decides what is covered.

Talk to someone now

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Frequently asked questions

Can I just paint over mold on an expansion joint?

Painting over mold does not kill it. The mold will keep growing underneath and can bleed through the paint. You need to clean the area, dry it fully, and fix the moisture problem first.

How do I know if the mold is inside the wall?

A musty smell near the joint, or bubbling or staining on the wall surface, can indicate hidden mold. The pros use moisture meters to check wall cavities without tearing everything open.

Will replacing the sealant stop mold from coming back?

Replacing the sealant helps only if the sealant was the leak point. If water comes from elsewhere, like a foundation crack or a pipe leak, the mold can return. Find and fix the actual water source.

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