Every time we have damp weather our concrete floor in the garage gets so much moisture on it that it’s dangerous to walk on. What causes this and what can we do about it? -Catherine
There are two possibilities that can cause your garage floor to be damp:
- When warm, humid air comes in contact with a cold concrete slab, water vapor in the air can condense on the surface of the concrete in the same way that a glass with ice sweats in summer. This usually happens more in the spring and early summer, since the ground temperature is still low and the air is warm and humid.
- A vapor barrier may not have been installed under the slab when the concrete was poured. This can allow moisture from the ground to penetrate up through the concrete, resulting in damp conditions. If there are cracks in the slab and the drainage is poor, it may even cause ground water to seep up through the cracks and puddle on the floor.
To find out if the problem is caused by seepage or condensation, tape a 2’ x 2’ piece of plastic to the floor during normal conditions when the slab isn’t noticeably wet. Remove the plastic after 24 hours, and see if the concrete under it is damp. If it is, then a vapor barrier wasn’t installed under the slab when it was poured.
Do not apply a sealer or floor paint to a garage floor that doesn’t have a vapor barrier, since the finish will not adhere well to the surface. There is little you can do to correct this problem other than removing the existing slab and pouring a new one with a vapor barrier under it, or put a vapor barrier on top of the existing slab and pouring additional concrete (minimum of 3” thick) on top of it.
If the concrete under the test plastic was dry, the problem is caused by condensation from the air. To reduce the problem, you could:
- Heat the garage during the times of year you’re experiencing a problem.
- Rough up the surface of the concrete by applying a concrete cleaning/etching solution.
- Apply a garage floor coating with nonslip granules or sand added to provide better traction.
Good luck with your project,