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August 4, 2010

What is a helical screw foundation?

Filed under: Glossary,Sunrooms,Tips — Sunrooms by Brady @ 4:37 pm

Brady-Built Sunrooms often recommends a helical screw foundation for the installation of
sunrooms as an alternative to a concrete pier (often called a Sonotube) foundation. What is it and why use it?

First; it is best to understand what a concrete pier foundation is. A concrete pier foundation requires the excavation of a hole large enough to accept the pier form (usually a cardboard tube).The hole must extend to below the frost line, which in most of New England is set at 48” by building code. Depending upon soil conditions and the weight of the structure that the pier will support, a flared bottom is sometimes required on the pier. This is achieved by using a plastic form under the cardboard tube form that gives the pier an upside down ice cream cone shape.The hole for this type of pier is significantly larger than a straight tube pier. Once the hole is dug, the forms are put in place and back-filled. Before concrete can be poured, the local building inspector must confirm that the foundation depth is adequate and that the soil conditions are suitable for the load that the pier will support. Following inspection, the forms can be filled with concrete. Once the concrete has set for 2 to 3 days, construction on the new foundation can begin. The whole process, from digging through inspection, pour and cure can take a week or more. Besides the time it takes to complete the process, the large holes required for each pier (most sunroom require at least three piers) cause a great deal of damage to landscaping and other yard features.

A helical screw foundation, as the name implies, is a screw that is fabricated from a very heavy gauge steel tube with a thick steel plate welded to the bottom. The steel plate is set at a down-ward angle and wraps around the steel tube like the thread on a screw. A small wheeled machine, about the size of a large snow blower, is brought to the installation site. The helical screw foundation tube (I’ll call “the screw”) is connected to the machine. Once secured, the machine positions the screw and applies pressure while turning it. The steel plate at the bottom of the screw bites into the soil and begins pulling the screw into the ground. Once the bottom of the screw reaches the desired depth, the post is tested to be sure it exceeds the specified bearing capacity. This process is repeated for each support location.

The helical screw foundation offers a number of advantages that are especially important on some jobs. It causes very little damage to the yard since there is no excavation required. Additionally, the machine that installs the screws was designed to be able to drive over lawns without causing damage. The screws can be installed in places where excavation may not be possible. The entire installation process takes only a few minutes per screw and does not require inspection by the local building official. This reduces the time between start of project and commencement of framing. The helical screw foundation is especially good on construction sites near wetlands, because the process does not result in loose soil that could cause silt runoff.

The helical screw foundation has been tested by independent authorities and has been approved for use on any building by all US building codes.


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