Keeping Water Out

Terry Pommett
In the second installment of our yearlong Nantucket house project, we attack a common problem in coastal construction: how to protect against damage from underground water.

With our footing in, it's time to think about what―if anything―we need in the way of underground drainage systems. Here are some tips:

• Know your site.
Soil tests can predict flooding around your home. But common sense helps, too. Does water collect and stand on your property? Do your neighbors have leaky basements? If it seems water might be a problem, assume it will be a problem, and design in one or both of these drain systems.

• Catch water before it gets to the foundation.
Our outside perimeter drain runs alongside the footing, just below the level of the basement floor. The system's core is a bottom-perforated PVC pipe, 4 inches in diameter, surrounded by gravel to aid water flow. The pipe and gravel are wrapped in a durable synthetic fabric designed to keep dirt from getting to the pipe and clogging the perforations. Water headed toward the foundation filters through the fabric and gravel and is captured in the PVC, which carries it to an on-site dry well and pump-out system, a storm sewer, or (in our case) a natural drainage area.

• Stop rising flood waters.
Groundwater rising with the water table during rainy seasons and spring thaws can threaten basements from below. To prevent this, you need a drainage system below the basement slab. We use the same perforated pipe as on the outside, only this time without the fabric cover. That's because the concrete floor sits on a 12-inch-thick layer of filtering gravel that blocks dirt from drifting upward and clogging our pipe. We run the pipes to a sump box beneath the floor and install a pump to carry away the water.

Does every house on the coast need these precautions? No. Depending on your lot's elevation and soil mixture, you may get away with no built-in drainage systems. Or you may need only the exterior perimeter drain. But the point is to make a decision when your alternatives are cost-effective. If we had to retrofit a system to fix basement flooding after the fact, the price tag would triple. Better an ounce of prevention now than gallons of trouble in the future.

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