Is your house on a solid footing? If not, you may experience problems with building movement. All buildings move, and houses are no exception. Ensuring a new house is grounded on solid footings is the first and most important step in minimizing building movement caused by settlement over time.
Footings receive loads from foundation walls or columns and transfer these loads to the earth below. The type and size of footings required depends on soil conditions and the amount of weight to be supported. For stable soils, the 1 to 2 rule is usually sufficient. If the foundation wall is 200 mm thick, the footings should be at least 400 mm wide, or twice the width of the foundation wall. The footing thickness should be at least equal to the foundation wall thickness, which in this case is 200 mm. In any case, footings should project a minimum of 100 mm past either side of the foundation wall.
Where unstable soils or a high water table are encountered, footings must be wider and specially designed to suit the site conditions. Steel reinforcing may be necessary in some situations. The loadbearing capacity of soil depends on its type and the amount of water present. Digging a minimum of three test pits is recommended to check soil quality before buying land or building.
Footings must rest on undisturbed soil, rock or well compacted granular fill, and be protected against frost action. To prevent frost heave, which can lift part or all of a house, the footings must be located below the frost line where the soil temperature is relatively stable, and above the freezing point, all year around.
The frost line varies with latitude and region, and a check with the local municipality will confirm the depth of frost penetration in your area. In most areas of southern Canada, the frost line averages 1,200 mm below grade. In northern areas of Canada, the frost line can exceed 2,400 mm below grade.
Frost action can also be prevented by providing good subsurface drainage around the perimeter of the foundation and by shedding surface water away from the house. A weeping tile system at the base of the footings will remove subsurface water. The finished grade around the house should slope 5%, or 1 in 20, away from the house.
Column footings are required for wood or steel posts supporting floors. The posts should be centered on the footings and spaced a maximum of 3 meters apart. On stable soils, column footings for one story houses are required to be 650 mm X 650 mm X 200 mm thick. Column footings for two story houses carry more weight, and must be larger, usually 870 mm X 870 mm X 200 mm thick.
Column footings should be poured at the same time as wall footings, and never with the basement floor slab. The basement floor slab carries much less weight than a column footing. If a column footing is cast in one piece with the basement floor slab, the slab around the footing will deflect and crack. Basement floor slabs are often as little as 75 mm thick and cannot be relied on to support the loads imposed by the weight of the house above. This will lead to considerable settlement, and possibly failure, of the posts, with resulting damage to the supported floor structure.
On sloping sites a stepped footing is required. The vertical portion of a stepped footing should be placed at the same time as the horizontal portion, and be the same width and thickness. The vertical steps should not exceed 600 mm in height, and be not less than 600 mm away from the next vertical step.
Concrete strength for footings and foundation walls should be a minimum of 15 MPa. For exterior footings supporting deck posts, fence posts or carports use 20 MPa concrete.
Footings are hopefully long forgotten after a house is completed. How well the footings are designed, placed, cured and drained affects a house over its lifetime. Don’t take chances when installing footings. Make sure your house is well grounded and moves as little as possible.
