Although most common residential foundations are made of poured concrete, many homes in the United States, especially in areas where transportation of concrete is an issue, sit on foundations made of concrete block, sometimes called cinder blocks or CMUs (concrete masonry units.)
Concrete block foundation walls sit on a footing, which is a pad of poured concrete wider than the foundation wall that helps distribute the weight load of the house. The construction of the foundation begins with pouring the footing in a trench containing wood or metal forms. The footing must be located below the freeze line and may be deeper when a full-height basement is desired.
The width and thickness of the footing will be determined by the size and construction of the house and the load-bearing capacity of the soil.
After the footing has cured, masons lay the first row, or course, of concrete block on top of it in the basement excavation to describe the perimeter of the structure to be built. Typical concrete block for foundation walls is 8 inches wide but blocks up to 12 inches can be used for higher walls and heavier loads.
Additional courses of block are laid in a running pattern (like brick) with masonry mortar between blocks until the desired height is reached; windows and other openings are created during this construction. The wall is then left for the mortar joints to cure.
Sometimes the hollow cores of the block walls are completely filled with mortar but it is more common to fill only sections of the wall such as corners and edges of openings. Steel rebar may also be added for extra strength.
When the walls are completed, the concrete basement floor is poured and the floor framing for the ground floor is completed. With the foundation supported at top and bottom, soil is backfilled against the foundation and compacted and aboveground construction begins.
Fixing a Leaking Basement with a Concrete Block Foundation
A concrete block foundation has the same load-bearing strength as a poured one but is more susceptible to lateral pressures that create seepage problems. Concrete block walls can admit water to the basement through cracked mortar joints and through the porous block itself.
One of the best ways to waterproof a block foundation is to install an exterior waterproofing membrane. This can be done during construction or by excavating around an existing home.
An exterior waterproofing membrane is a thick coating of asphalt-modified polyurethane that is applied to the outside of foundation walls with a trowel to create a barrier that is impervious to water. The membrane should not be confused with “damp-proofing” spray, which is a thin liquid sprayed onto foundation walls during construction to prevent condensation on the interior.
When the water table is high, the membrane can be augmented with exterior drain tile to carry off ground water and by installing drainage board on the walls to protect the membrane and channel water downward.
If exterior waterproofing isn’t an option, interior drain tile can be installed to manage seepage from a block wall. A vapor barrier is usually installed on the interior walls to transport wall seepage to the drain tile and conceal it from view. The vapor barrier can be covered by a finish wall.
Regardless of the source of seepage or the waterproofing method, a homeowner that wishes to waterproof a concrete block foundation will require the advice and services of a basement waterproofing contractor that has experience working with concrete block walls. At U.S. Waterproofing, thousands of our 300,000 satisfied customers have block foundation walls, especially those in southeastern Wisconsin and northwest Indiana and we have been keeping them dry since our founding in 1957. Why not ask for our free advice?
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