What To Know About Waterproofing a Concrete Basement Floor

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A dry basement is a comfortable basement, and waterproofing the floor helps keep it that way. It's a fairly simple project you can do yourself.

Think of a basement as a concrete box dug nine or 10 feet deep into the ground, where it obstructs the natural flow of groundwater and rain runoff. Concrete seems solid enough, but it’s actually porous. Some of that water is bound to seep through, especially during heavy rains.

Normally, there isn’t enough seepage to form standing water on the floor. But there’s often enough to increase the humidity, and that promotes the growth of mold and rot.

Much of the humid air in a basement rises from the floor. But fortunately, there’s more than one DIY-able way to waterproof a basement floor. It’s a good idea to do it as a preventative measure even if you aren’t experiencing humidity issues. If you are, it can provide some relief.

Why Waterproof a Basement Floor?

A damp basement floor feels uncomfortable and discourages use of the space. That might not matter if you or your kids never plan to go down there. However, the dampness creates related problems that can affect your quality of life:

  • Rot damage: Exposed framing often deteriorates from moisture. Given enough time, rot can undermine the foundation.
  • Wasted storage space: Many of us store furniture and household furnishings in our basements, among other things. If you hesitate doing that because you fear mold damage, you lose this important storage space.
  • Moldy air: The mold that grows in damp basements doesn’t always stay there. If your HVAC system is there, it can circulate mold spores throughout the rest of the house.
  • Pest infestations: Cockroaches, ants, termites and rodents thrive in basement dampness.

Waterproof Basement Floor Options

Assuming there’s no active leakage or standing water, you can apply a waterproofing sealer or an epoxy coating. The best approach depends on how much moisture is in the floor.

You can check that yourself with a calcium chloride test kit. It consists of a calcium chloride disk you place on the floor and cover with sheet plastic. After 24 hours, retrieve the disk, weigh it and compare it to the weight when dry. The difference is the amount of moisture it absorbed.

If the floor is relatively new and dry, a one-part densifying product like Ghostshield Lithi-Tek 9500 simultaneously fills pores and repels water. It can be applied by brush, sprayer or roller.

For older floors with a small amount of moisture detected (less than about 3-lbs./1,000-sq.-ft. as identified by a calcium chloride test), a waterproofing agent like Siloxa-Tek 8510 should be applied along with the densifier mentioned above.

On floors with moisture levels higher than 3-lbs./1,000-sq.-ft., a two-part epoxy coating like Epoxy Zenith is your best bet. Instead of penetrating like a sealer, an epoxy coating stays on the surface and locks moisture into the concrete.

How To Waterproof a Basement Floor

You can apply a sealer or waterproofing agent to your basement floor with a brush, pump-up garden sprayer or a roller. A two-part epoxy coating usually requires a roller.

Before you begin, do some important prep work:

  • Clear out the basement so you don’t have to work around obstructions.
  • Clean the floor with concrete cleaner and water. Remove any grease deposits with a degreasing agent, because grease will prevent the sealer from penetrating or the epoxy from sticking.
  • Remove efflorescence, the yellowish deposits left by groundwater that seeped through the concrete. You can do this with muriatic acid, but it’s safer to use a masonry etch and cleaner.
  • Fill all holes and deep cracks with hydraulic cement. Fix minor cracks with masonry crack filler.

Follow the instructions carefully for whatever products you choose. They may call for two coats, and for some products the second coat must be applied immediately after the first.

With two-part epoxy, note how long it takes for the product to harden, and make sure you can use the amount you mix during that time. Don’t mix too much or you’ll have to throw some away.

Tips for Waterproofing a Basement Floor

  • Cove joint: When pouring the basement floor, the builders intentionally leave a small gap between the floor and the wall. This is called the cove joint, and it should not be sealed. It’s there to allow water to seep when the hydrostatic pressure is high. If you seal it, water could come up through the floor even if you’ve waterproofed it.
  • Radon: Before waterproofing, check your basement radon levels with a home test kit. If the levels are higher than four picocuries per liter, apply a product like RadonSeal. It blocks moisture as well as radon and is the only sealer you need. It’s also a good idea to coat the walls.
  • Leaks: Don’t waterproof the floor if you have active leaking. Call a pro to evaluate the situation and make recommendations.

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Chris Deziel
Chris Deziel has been active in the building trades for more than 30 years. He helped build a small city in the Oregon desert from the ground up and helped establish two landscaping companies. He has worked as a carpenter, plumber and furniture refinisher. Deziel has been writing DIY articles since 2010 and has worked as an online consultant, most recently with Home Depot's Pro Referral service. His work has been published on Landlordology, Apartments.com and Hunker. Deziel has also published science content and is an avid musician.