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Waterproof the Basement in New Home Construction

While the cost of waterproofing a basement during construction can cost several thousand dollars, it's important that homeowners understand this is a critical measure that can save them thousands more in the long run, according to Merlin Taylor, the principal and an owner of MST Builders in Salt Lake City.

Flooding, termites and mold do more damage to homes in the United States than fire, and builders must convey the importance of a sound waterproofing plan to clients.

“In new construction, it’s imperative to plan a water management system that includes the proper foundation sealants,” says Taylor, whose construction company specializes in custom residential construction.

So how do you persuade more clients to take extra precautionary measures? Start by explaining that these extra measures will protect the value of their home. If they choose not to, and the foundation is damaged, they might have no choice but to cut into the concrete and install a drain cavity — a labor-intensive process that could easily cost them twice as much as waterproofing during construction.

Waterproofing Options

Especially in the Midwest and Gulf states where moisture and flooding are most common, builders should take extra precautions to keep water from building up outside foundation walls of homes. Encourage your clients to consider a combination of techniques:

  • Damp proofing: Around $300, damp proofing involves spraying a tar-based emulsion on the external basement walls to prevent moisture from seeping through the concrete. “It’s the most widely used, the cheapest, and probably least effective method, but it does meet code [requirements],” Taylor says. He adds, however, that although code requirements are a minimum standard, you can always exceed the code specification with a better product.
  • External waterproofing: In any region, Taylor recommends a water barrier — with a warranty of at least 10 years — that prevents water from coming into the foundation. In rainy climates, builders often hire subcontractors to adhere a thick, rubbery membrane to the outside of basement walls and floors to prevent water seepage through basement walls.
  • Drainage system: In most regions you also should install a footing drain, often called a “French drain,” to draw below-ground water away from the walls. Dale Shadbegian, co-owner of Worcester, Mass.-based GoodFellas Construction, usually puts drainage pipes in the foundation footings that carry the water to the outside perimeter of the home, to either a storm drain or other tie-in. Companies like MST Builders charge an extra $2,500 to $3,000 to install a typical home’s waterproofing system in combination with the outer membrane, because both are necessary to keep water away from the home. Without the system, water could stagnate against the walls below ground, which would eventually corrode the home’s foundation. “The cost of trying to repair the area could be tens of thousands of dollars,” Shadbegian says. Taylor also recommends encapsulating drain pipes in a mesh or fabric barrier to prevent clogging by debris and sediment.
  • Internal waterproofing: In arid climates, clients might opt out of external waterproofing but should still apply sealants to any cracks or irregularities in the inner walls of the basement. This won’t protect the home’s foundation, but it will prevent mold. Certain building methods also might help keep basements dry. When framing basement walls, for example, Shadbegian often leaves a space between the frame and the foundation wall to prevent rot and warping. This technique requires connecting pressure-treated wooden blocks to the floor, so budget in time for any extra work. Builders also should ensure basement floors are sloped toward the sump pump so that any water can be pumped out automatically.

Do Proper Site Work

“The most important part of waterproofing is the site work,” Shadbegian says. Make sure your workers always use a level. The International Residential Code requires a minimum grade of six inches within 10 feet around the perimeter of the house. If the ground is too flat, water will gather at the foot of the foundation and eventually leak in.

During the sub-rough phase, Taylor also recommends filling the backfill with gravel rather than soil, which adds stability to the concrete floor. “In the unfortunate event that water gets underneath the footing, you’ll have a gravel reservoir where water can gather” instead of seeping in, he says.

It’s easy for homeowners to ignore a process that costs more and is largely out of sight, but the extra investment will protect the value of their home for years to come. Educate them about the benefits of waterproofing before breaking ground.


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