How to build a house » Foundation » How to build a raft foundation and its features

How to build a raft foundation and its features

A raft foundation is not often used for private home construction. However, throughout the world it is used quite extensively for a wide variety of buildings. The properties of this basis type make it suitable for many climatic zones. Of course, in addition to substantial advantages, there are some downsides. Let’s examine the process how to build a raft foundation and its features!

What is a raft foundation?

Raft foundation constructionIt is made of reinforced concrete slab and set on the whole building area on a ground level or dug inside. Raft foundation purpose is the load distribution on a volume area, which prevents deformation of constructed facilities at ground movements.

The concrete slab thickness can vary from 8 to 12 inches. Slabs laying involves soil preparation and arrangement of sand “cushion”.

Raft foundation types

There are two types of this foundation kind: classic and floating.

  1. Classical type is located below the normal level of soil freezing. This way allows to supplement your construction with a ground floor or basement.
  2. Floating type is laid at a depth of 25-35 inches, which eliminates the possibility of building a ground floor or basement.

Raft foundation detailAlso, there are following types of a raft foundation:

  1. Ordinary. Its laying is possible with conventional digging a pit, which is then rammed and leveled. The minimum thickness of the “cushion” should be at least 6 inches.
  2. Solid (monolithic) or insulated, which is most often used in cold climates.
  3. Whole lattice slab. It is used mainly for huge facilities.

Scope of use

A raft foundation is considered to be the best option in the construction of houses made of various materials, both easy and heavy. It is allowed on any soil variety, from sandy to peat. This slab perfectly aligns vertical and horizontal ground displacements. It does not afraid of ground waters, which provide a pretty big impact. In private construction the foundation with slabs can be combined with any materials.

Technology laying process

Initially, it is necessary to examine the soil of a future construction location for determining the sand “cushion” thickness. Only then make a final plot.

The raft foundation installation includes following steps:

  1. Remove the top ground layer and dig a pit. Its depth should exceed the soil freezing depth. Level and ram a pit bottom.
  2. Place a sand “cushion”, also level and ram it.
  3. Install waterproofing and drainage systems.
  4. Reinforcement of ribbed raft foundationPlace formwork on the entire perimeter of a future building.
  5. Further, also on the perimeter, install fittings and make reinforcements.
  6. Starts pouring concrete. Concrete should be poured in layers of 6 inches each. If you use ready-made rafts, make of concrete screed on the top.
  7. Implement vibrating of cured concrete to provide additional strength.

A raft foundation is different in types of used slabs:

  • For small constructions builders usually use monolithic slabs, characterized by ease of molding technology and little laying depth;
  • For industrial and high-rise buildings they use baseplates with stiffeners, assembled from individual details fastened together, or use a fully monolithic construction;
  • For superstable structures, especially in areas with high seismic activity, they use box structures assembled from modular or monolithic concrete boxes, rigidly fixed to each other.

Advantages and disadvantages

The major advantage of a raft foundation is its large bearing area, which significantly reduces the pressure on the ground. As a consequence, it is excluded differential settlement and walls deformation.

Raft foundation ready for pouringAmong other advantages are the following:

  • Ease of installation;
  • Safety and long-term strength guarantee;
  • Confrontation of both ground and groundwater;
  • Prevention of ground motion;
  • Basement creation;
  • Use of rafts as base of the first floor.

The significant disadvantage is its value, higher in comparison with other embodiments. However, they are justified by a guaranteed reliability to adverse natural conditions.


Construction and repair of the foundation slab detailed instructions.

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