Pro Guide to Basement Waterproofing

You can waterproof your clients’ basements from the inside or the outside, but the most thorough jobs will use a combination of both. Always apply sealants on both sides of a basement and foundation to prevent water from getting trapped in the concrete and damaging it over time. While only one type of drainage system is necessary for each basement, a sump pump can help push water out of interior or exterior drains.

The most common basement waterproofing methods include:

• Exterior waterproofing coating
• Interior sealants
• Interior water drainage
• French drains (interior and exterior)
• Sump pump installation

Sometimes, a client will only want to seal or coat their foundation instead of undertaking a more expensive drain installation. However, this will not protect the foundation from contact with water on the outside.

Advise clients that interior sealants alone are only a temporary measure. Add an exterior sealant or membrane to prevent long-term damage to their basement walls.

Basement waterproofing jobs require the following tools and materials:

• Excavator
• Breaker hammers or sledgehammers
• Shovels
• Compactor
• Hydraulic cement
• Waterproofing paint
• Concrete sealer
• Perforated corrugated pipes
• Landscape fabric (also known as filter fabric)
• Gravel

Waterproofing methods that involve paints or sealants usually do not require special equipment or training and can cost less. Waterproofing jobs that need excavators or other equipment may require permits or licenses and can cost more.

In general, interior basement waterproofing has lower costs while many exterior waterproofing methods cost more. Apart from the waterproofing method, factors that can influence costs are the basement size and the home’s age and geographic location.

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Exterior basement waterproofing methods are typically the most complex. The process begins with excavation around the perimeter down to the foundation. Once the bottom of the footings is visible, the foundation should be cleaned and dried before applying any coatings or membranes.

There are several exterior waterproofing coating options, and each has their own advantages. The following list explores the different types of coating and their materials:

• Liquid synthetic membranes like polyurethane and rubberized asphalt
• Synthetic sheet membranes like thermoplastic, bituminous and PVC
• Solid bentonite sheets
• Cementitious coatings

Liquid synthetic membranes are typically the easiest coatings to apply to a foundation. They come pre-mixed and ready to be painted along excavated and cleaned foundation walls. Materials like polyurethane can deeply infuse the concrete to protect it for up to 25 years.

Bentonite sheet is a rolled membrane that typically comes with a clay-based concrete adhesive. The material expands as it dries, allowing it to fill any pockets of empty space left behind during application. Bentonite waterproofing typically lasts the lifetime of the concrete structure it is attached to, especially in below-grade work where it is only exposed to water and soil.

Synthetic sheet membranes like thermoplastic require some extra steps. Apply thermoplastic to the foundation walls and then melted in place, forming a polymer against the surface of the concrete. This is particularly useful when using flexible ductwork, or when protecting pipe plugs, sump pits or tanks.

Pair waterproof coatings with a drainage system that guides high volumes of water away from the home. French drains are the most common type of basement drainage system. Install them in the perimeter trenches before backfilling. Surround the drain with gravel before adding any soil for backfilling. For more information on French drains, see our guide “How to Install a French Drain.”

Once the waterproofing coating is set and the drainage system is installed, backfill the trench with soil in 6-inch layers. Use a yard stick to measure the soil as you fill it in. At the 6-inch mark compact the layer with a compactor before adding more soil. Continue until the trench is backfilled and densely packed. Grit sand is also a common compacting agent, and can be helpful for soil with high clay content.

Interior waterproofing begins with digging a trench around the edges of the basement floor and a sump pit when installing a sump pump. Once the drainage system and pump are installed, seal the basement walls and replace any wall material that was removed such as wood paneling.

Apply interior sealants to the interior walls and floors of the basement to prevent moisture from seeping through. They are typically used as a temporary solution or in conjunction with other waterproofing methods. Remember to fill any cracks in the basement walls before applying sealant to prevent larger amounts of water from collecting in the wall and damaging it further.

Dig interior trenches by breaking up concrete with a jackhammer or other breaking hammer. Create a continuous channel along the perimeter of the inside of the basement. This is where you can install a French drain and connect it to a sump pump or other drain pumping system.

Interior French drains are designed to collect water and direct it away from the basement. However, they do not push water away and rely on pumps to carry the water farther away into deeper soil.

It is best to install a sump pump at the same time as the French drain, so that you do not have to excavate concrete twice. In a corner of the basement, excavate a pit that is a bit larger than the tank for the sump pump. Ensure it connects to the French drain channel on the perimeter. Install the sump pump and tank in the pit, connecting your perforated pipe to the tank.

Finally, connect the sump pump to a discharge pipe positioned away from the home. When the sump pump is turned on, any water in the tank will be pumped away from the foundation and allow the French drains to fill without flooding.

Backfill interior French drains with gravel before relaying cement to fix the basement floor. Once the cement is set and dry, the sump pump and drainage system should operate as intended.

Waterproof paint costs $1-$6 per square foot and is the least expensive way to waterproof a basement. The paint prevents water from seeping through concrete walls. However, it will not plug existing cracks on its own, and is best applied after all cracks are filled and dried.

Another way to mitigate waterproofing costs is in the choice of concrete sealant. Acrylic sealants are useful for long term projects as they do not need to be reapplied often. These sealants typically cost $3-$4 per square foot, whereas silicate and epoxy sealants can cost up to $5-$7 per square foot.

For exterior jobs, sodium bentonite can be cheaper than other coatings or membranes and is easier to apply. Sodium bentonite is a natural liquid sealant that dries inside cracks and absorbs water before it reaches the concrete. This typically costs $2 per square foot but may be more expensive in conjunction with interior sealants.

Advise your client to consider the utility of upgrading their rain gutter system as well. If their basement only needs damp-proofing and isn’t prone to major floods, their issues may be solved by making their gutters more efficient. Gutter jobs are typically much cheaper than exterior waterproofing, which requires more labor and materials.

Options include the moving the gutters farther out from the roof edges, extending downspouts away from the home or connecting a downspout to an existing drain. All of these will prevent rain from dampening the soil around the foundation, which causes water to build up and seep into the basement.

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