Regardless of the type of foundation you decide upon, you need to make sure that it's a good quality foundation. It isn't as simple as throwing up a few forms and then pouring cement to create a slab or walls. So what makes a "good" foundation?
Building Foundations and Their Features
To get a good foundation, you have to take several factors into consideration, including the frost line, the type of soil, the water table, site prep, and the quality of the backfill. And that's just at the house site. A good foundation will also have several specific features including steel reinforcements. Builders often use rebar to strengthen basement walls and to help keep it from cracking. The quality of the concrete and the method used to pour it also matter.
Additional features of a good basement include:
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3,000 psi concrete, 10 inches thick for basement walls.
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Anchor bolts that anchor the mudsill to the foundation, placed every 2 to 3 feet.
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Half-inch rebar placed horizontally in the upper and lower thirds of the walls.
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If the basement walls are over 8 feet tall, vertical rebar placed on 16- or 18-inch centers.
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Waterproof membrane - either spray-on latex or rubberized asphalt.
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Form ties.
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In area with non-porous soils, such as red clay, or in areas with high water tables, the basement should have filter fabric and a drain membrane on the outside of the basement walls.
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Two-inch extruded foam insulation between the filler fabric and the waterproofing.
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Half-inch expansion joints.
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A 4- to 6-inch thick slab of 3,000 psi concrete.
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Wire mesh in the slab.
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A 2-inch insulating layer of extruded foam below the slab.
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Two inches of sand below the insulation, then a vapor barrier below the sand.
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Four to six inches of compacted gravel below the vapor barrier.
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On sites with a high water table, 4-inch Schedule 40 PVC with holes drilled into it should be placed 6 inches below the top of the footing, holes down. This pipe removes excess water away from the foundation.
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Twelve-inch deep footers of 3,000 psi concrete. The footers should not be more than 12 inches wider than the wall.
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Three-quarter-inch crushed stone on top of the drain pipe and against the exterior of the basement wall.
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Porous backfill under 1 to 2 feet of top fill against the basement walls. The topfill should be sloped away from the basement walls pursuant to your area's building codes, but at least ½-inch per foot.
Slab Foundations
A slab foundation should have the same features of the slab in a basement, including the layers of compacted crushed stone, a vapor barrier, sand and insulation. The slab should be protected against frost heaves if you are using a slab foundation where you have a medium to high frost line. When the frost line is deep, a slab foundation is not recommended.
Newer Technology for Foundations
Newer technology, meaning technology that has been around for less than 50 or so years, includes using insulated concrete forms to build an insulated basement, precast foundations, and adding PEX tubing near the top of the foundation walls to heat the walls enough to repel moisture. The PEX tubing is routed through the boiler and is the same type of tubing that you find in heated floors or heated walkways.
You can also use self-leveling concrete. It contains a chemical in it that makes the concrete mix flow like water, but it retains the concrete's structural integrity. Usually, if the mix is too thin, the aggregate sinks to the bottom before the concrete can dry. With self-leveling concrete, the chemical keeps the aggregate from sinking. You can pour footers from one corner and the concrete will fill in the entire footer.
Builders also have fabric-formed footings they can use now. The footing forms are lightweight and conform to sites that have slopes or are otherwise uneven. The forms stay in place after the concrete is poured and act as an insulating layer.
Ask your builder about some of these newer technologies when building your home.
