Sept. 14, 2014

Why are homes built on pilings?



If you have ever visited or lived on the Outer Banks, you may have been wondering why so many homes are built on pilings or “stilts”. You’re not alone, you might be surprised how often this question comes up.

Before moving to the Outer Banks, my wife made me promise that we wouldn’t live in a house on “stilts”. She wanted to live in a “normal” looking house like the one she had grown up in. Sure, this seems like a reasonable request, but the fact is there are very few homes on the Outer Banks that have traditional masonry/concrete foundations. The majority of the homes on the Outer Banks are built on “stilts” or piling foundations.

Why are they built on piling foundations you ask? Most of the reasoning revolves around the Outer Banks topography and flooding. Wood piles are the most economical way to elevate a house above potentially high flood waters. Living in an area with relatively flat topography surrounded by water the risk of flooding is always high. Relatively minor sustained winds can push ocean and sound levels onto normally dry areas. Higher winds can magnify and compound the damage caused by raised water levels. Elevating homes above these waters can reduce damage and ultimately save homes that would otherwise be destroyed.

Piling foundations also provide a deeper, more secure foundation than conventional masonry/concrete foundations. Typical foundations like masonry and concrete tend to be shallower. As a result, they are more susceptible to shifting sands, erosion and undermining. Pilings, on the other hand, are driven deep beneath the sub-grade and are not as affected by surface erosion by wind or flood waters.

How do pilings support a house? Foundation piles support homes in two ways. First, piles support the house load through “skin friction”. Skin friction is the resistance of the soil, typically sand in our area, along the sides of the pile to downward pressure. And secondly, through the piles “end bearing”. The end bearing is the ability of the pile to resistance downward pressure at the “tip” or bottom of the pile. The combination of skin friction and end bearing provide substantial bearing capacities capable of supporting the unique homes on the Outer Banks.

Do you need to build on a pile foundation? This depends on the property location and flood zone. The minimum elevation for a home may vary based on design flood levels and lot location typically designated by FEMA. A qualified engineer or builder can help you decide what foundation type is best for your lot, home and the flood requirements.

In conclusion, homes on the Outer Banks vary widely in terms of construction type and appearance. Many architects and engineers over the years have come up with unique ways to minimize the “stilt look” while maintaining the benefits of a pile supported foundation (as my wife required of our house). Piles also provide a relatively economical way for home owners to achieve extraordinary views including spectacular sunrises and sunsets. So the next time you are visiting the Outer Banks and someone says “I wonder why all the homes are built on those stilts…”, you will have the answer.

Barrett Crook - Kitty Hawk Engineering, PLLC
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