What is A Raft Foundation?

Builders employ a wide range of methods to keep their foundation safe. Experienced construction designers know that a dry foundation is necessary for creating a building that will be problem-free for generations.

There are lots of ways to achieve the goal of keeping your foundation dry. A rare but effective method of building a foundation that also helps keep it dry in certain soils is to create a raft foundation.

When should you choose a raft foundation

A raft foundation is not always the best foundation choice. Under most normal soil conditions, a regular poured foundation will be suitable for construction of even the tallest building.

If, however, you are building on weak soil that might give way under the full weight of your building, a raft foundation is the right answer.

It works in these conditions because the raft foundation is one large, flat surface on which you place the entire building. The flat, open surface of the “raft” or mat works to distribute the weight over the entire surface of the foundation. By doing this, the weight on any one given spot is reduced.

By distributing this weight evenly, no one spot carries a great weight. Instead, the building weight is distributed and thus it is less prone to shift, drop, or settle.

What is a raft foundation, exactly?

The raft foundation works as the name implies. Think of a raft of the sort you might use to escape from a desert island. It is a flat surface upon which everything else is built.

It is not like a building constructed on pilings, footings or columns of wood or concrete which only make contact on certain points of the surface below.

The flat surface is its own full surface to build on, but not to drill through or otherwise breach.

It lays on the excavated flat surface at the base of the building site. Once it is waterproofed and poured into place, everything else is constructed on top of it.

A raft foundation doesn’t float, and for this reason it’s best to think of it as a mat foundation, in that it lays there, and everything else happens on top of it.

A raft foundation is not necessarily used for the purpose of waterproofing, though it also comes up in waterproofing conversations. The point of choosing this foundation type is to create the safest, most secure foundation on loose soil.