Concrete floor slabs on ground

By The Old House Web

A thickened edge slab
Certain basic requirements should be met in the construction of concrete floor slabs to provide a satisfactory floor.

They are:

  • Establish finish floor level high enough above the natural ground level so that finish grade around the house can be sloped away for good drainage. To of slab should be no less than eight inches above the ground and the siding no less than six inches.
  • Top soil should be removed and sewer and water lines installed, then covered with four to six inches of gravel or crushed rock well-tamped in place.
  • A vapor barrier consisting of a heavy plastic film, such as 6-mil polyethylene or 45-pound or heavier roofing, with minimum of 1/2-perm rating, should be used under the concrete slab. Joints should be lapped at least four inches and sealed. The barrier should be strong enough to resist puncturing during placing of concrete.
  • A permanent, waterproof, nonabsorptive type of rigid insulation should be installed around the permiter of the wall. Insulation may extend down on the inside of the wall vertically or under the slab edge horizontally.
  • The slab should be reinforced with six by six inch No. 10 wire mesh or other effective reinforcing. The concrete slab should be at least four inches thick. A monolithic slab is usually preferred in termite areas.
  • After leveling and screeding, the surface should be floated with wood or metal floats while concrete is still plastic. If a smooth, dense surface is needed for the installation of wood or resilient tile flooring, the surface should be steel troweled.
Types of Slabs

Combined slab and foundation

The combined slab and founation, sometimes referred to as the thickened-edge slab, is useful in warm climates where frost penetration is not a problem and where soil conditions are especially favorable. It consists of a shallow perimeter reinforced footing poured integrally with the slab over a vapor barrier.

The bottom of the footing should be at least one foot below the natural gradeline and supported on a solid, unfilled and well-drained soil.

Independent concrete slab and foundation walls

When ground freezes to any appreciable depth during the winter, the walls of the house must be supported by foundation or piers on extending below the frostline to solid bearings on unfilled soil.

In such construction, the concrete slab and foundation walls are usually separate.

Three typical systems are suitable for such conditions, as shown in the accompanying illustrations.



Reinforced grade beam for concrete slab. Beam spans between concrete piers located below frostline.


Independent concrete floor slab and wall. Concrete block is used over poured footing which is below frostline. Rigid insulation may also be located along the inside of the block wall.


Full foundation wall for cold climates. The perimeter heat duct shown in the illustration is insulated to reduce heat loss.


Adapted from "Wood-Frame House Construction" by L.O. Anderson, originally published as Agriculture Handbook No. 73 in 1970 by the U.S. Government Printing Office and prepared by the Department of Agriculture.


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