DIY Network

WEB-EXTRA: Producers' Q&A on Elevated House Structures From "Be Your Own Contractor: Vacation Homes" episode DBCV-104

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Following below are some Q&A's and additional information on building an elevated home from the producers of Be Your Own Contractor: Vacation Homes.

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What should you consider before elevating?

Before you decide to elevate your house to protect against future flooding, you should consider the following:

• What level of flood protection do you want? Do you want to protect against the base flood (commonly called the 100-year flood), the 500-year flood, or some other level?
  
• What kind of flooding would your house be subjected to? Take into account the following factors with respect to flooding:
  1. The depth of flooding. (Even still floodwaters exert pressure on walls and floors.)
     
  2. The velocity of the floodwaters. How fast they move affects the damage potential.
     
  3. How often you might expect flooding in your area (for example, a 100-year flood has a one percent chance of striking in any year.)
     
  4. The rate at which floodwaters rise and fall. (This affects how much warning you may have.)
     
  5. How long the floodwaters will remain before they recede. (The longer a house is exposed to floodwaters, the worse the damage.)
     
  6. The impact of floating debris on your house.

When should you consider elevating your house?

If your house is in a special flood-hazard zone (100-year floodplain) and you have received substantial damage (50 percent of your home's pre-flood market value) or are undertaking substantial improvements (costing in excess of 50 percent of your home's post-flood market value), you must either elevate your house, move it out of the floodplain or wet flood-proof it.

If you live in a low- to moderate-risk area, you may still choose to elevate your home for added protection.

What does it mean to elevate a house?

Elevating a house means raising its lowest floor above the flood protection elevation (FPE), a level at which your chances of flooding are greatly diminished or eliminated. The FPE includes one additional foot of elevation to compensate for the uncertainties that exist in expected flood elevations. When a house has been elevated above that level, it still may be subject to flooding and face some risk of damage, but far less than before elevation.

How is a house elevated?

When a house is elevated, it is separated from its foundation, raised on hydraulic jacks and held up by temporary supports while a new or extended foundation is constructed below. The new foundation can be continuous walls or separate piers, posts, columns or pilings. The method varies if the foundation and the floor of the house are the same.

In areas where there is a low risk of wave action and high-velocity flow, the foundation is saved and existing walls extended, raising the floor level. In areas of high risk, open foundations are preferred because they let the floodwaters flow through, reducing the risk of structural failure. Open foundations are built with piers, posts, columns or pilings.

Masonry houses built on slabs can be elevated by (1) removing the roof, extending the walls up and building a new, elevated floor at the flood protection elevation (FPE), or (2) by abandoning the entire lower floor and moving the living space to an existing or new upper story.

What are the advantages to elevating a house?

• Elevating a substantially damaged or substantially improved house above the required level brings you into compliance with your community's floodplain management plan.
  
• Elevating reduces the flood risk to the house and its contents. Elevating eliminates the need to move vulnerable contents above the water level during flooding.
  
• Elevating can reduce flood insurance premiums.
  
• There are many well-qualified contractors familiar with elevating methods.
  
• Elevating does not require additional land.

What are some disadvantages to elevating a house?

• The cost of elevating may be prohibitive.
  
• The appearance of an elevated house may not be as attractive.
  
• It may be difficult to access the house.
  
• Without special measures, elevating is not appropriate in areas with high-velocity flows, waves, fast-moving debris flow or erosion.

A home built in any flood zone must be elevated above the base flood level, whether it's on pilings or a masonry foundation. For homes built in a Flood Insurance Rate Map flood zone rated for "velocity wave action" (V zone), piles or columns are required. For many other homes in less exposed zones along the coast, piling foundations are recommended but not required. We prefer to use piling foundations on all coastal homes, and even on inland homes where the soils are sandy or the site marshy.

Piling Choices

Although steel and concrete pilings are available, most builders in our area use wood pilings for homes. They come as either round, peeled logs or square-sawn 8x8's and 10x10's. The preference for one shape or the other seems to vary by region. In our area, round poles from tree farms in the Carolinas are most common.

Piling length

Pilings resist loads by surface friction against the soil, not at the base the way a post on a footing does. So the length of a pile is determined by the depth necessary to resist vertical uplift and horizontal loads, as well as the potential for erosion, or scouring, around the pile at grade, the ground elevation, and the base flood elevation. Since soil conditions vary, we try during the design stage to speak to a pile driver who has driven piles at a nearby site to determine the expected bearing capacity. This can range from 10 to 20 tons per piling. In our region, most beachfront homes are designed for 15 tons. Sites with marshy soils along bays are designed for 10 tons.

Embedment

Another criterion that must be satisfied is embedment depth. It's occasionally possible to get the bearing you need without going deep enough to provide the necessary lateral support for the pilings. My engineer usually requires at least 20 to 25 feet of embedment.

Driving

Piles are driven into the ground, tip first, by a crane-mounted diesel- or air-powered hammer. The double-acting hammer, which automatically rebounds from each blow at the piling, is faster and usually more precise than a single-acting hammer, which must be raised by cable after each blow.

Prepping the Site

Before pilings can be driven, the lot must be cleared and leveled. A pile-driver crane runs on tracks, but needs fairly level terrain for maneuvering and stability. Once the lot is cleared, we have a surveyor set a stake for each piling, and for the major corners of the house. If the final tip or butt elevations of the pilings vary, the stakes are color-coded.

RESOURCES :

Resources for Be Your Own Contractor: Vacation Homes episode 104: Mississippi-Delta House

Majesty of the Mississippi Delta
To order this book from Amazon.com, click here.

My Mississippi
by Willie Morris, David Rae Morris
To order this book from Amazon.com, click here.

Square Books
Oxford, Mississippi
Website: www.squarebooks.com

Federal Emergency Management Administration
Website: www.fema.gov