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―ifanything―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 commonsense helps, too. Does water collect and stand on your property? Doyour neighbors have leaky basements? If it seems water might be aproblem, assume it will be a problem, and design in one or both ofthese drain systems.

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

• Stop rising flood waters.
Groundwater rising with the water table during rainy seasonsand spring thaws can threaten basements from below. To preventthis, you need a drainage system below the basement slab. We usethe same perforated pipe as on the outside, only this time withoutthe fabric cover. That's because the concrete floor sits on a12-inch-thick layer of filtering gravel that blocks dirt fromdrifting upward and clogging our pipe. We run the pipes to a sumpbox beneath the floor and install a pump to carry away thewater.

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

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