Construction · Lesson 3 of 13Intermediate8 min read

Site and Foundations

A foundation has two jobs. It spreads the weight of the house over enough soil to hold it, and it keeps the wood frame up out of the wet ground. Both start with the site: the soil and, above all, where the water goes.

Start with the site: soil and water

Water is a foundation’s main enemy, and a roof collects a lot of it. In a 1-inch rain, about 1,250 gallons of water fall on the roof of a 2,000-square-foot house. All of it has to go somewhere other than the basement.

So the ground around a house is shaped to slope away from it. Sources give slightly different minimums: University of Minnesota Extension suggests at least 1 inch per foot for at least 6 feet, and the Department of Energy’s building science guidance calls for at least half an inch per foot for 10 feet. Your local code sets the rule where you live. Gutters and downspouts carry roof water away too: Minnesota Extension suggests at least one downspout for every 50 feet of eave, with extensions that discharge the water at least 4 feet from the wall.

Soil also matters. Water can climb up through soil by capillary action, the way it climbs a paper towel. In gravel it rises only a few inches; in sand, 1 to 8 feet; in silt, 12 to 16 feet; and in clay, 12 to 20 feet.

Before anyone digs, the buried utilities have to be located. OSHA’s first rule for preventing electrocution on a job site is to locate and identify utilities before starting work.

Footings and the frost line

A footing is a wide strip or pad of concrete under a wall or post. Since pressure is force divided by area, a wider footing spreads the same load over more soil, so the soil is pressed less.

In cold climates, footings are set deeper than the ground freezes. Water expands as it freezes (ice has a more open crystal structure, which is why it floats), so wet soil that freezes can swell and lift whatever sits on it. That’s called frost heave. The local building department sets how deep footings must go where you live.

In cold places, footings are set below the depth the ground freezes; the local building department says how deep that is. Drawings not to scale. Sources: DOE Building Science Education Solution Center; University of Minnesota Extension; OpenStax (why freezing water heaves soil).

Slab, crawlspace or basement

Slab-on-grade: a concrete floor poured right on the ground, usually with thickened edges or separate footings around the outside. Under the slab goes a capillary break: a layer of gravel, sand or crushed stone about 4 inches thick, covered with polyethylene sheeting at least 6 mil thick, overlapped 6 to 12 inches and sealed at the seams. It keeps ground moisture from wicking up into the concrete.
Crawlspace: short foundation walls lift the floor framing off the ground, leaving a low space underneath. The soil inside is covered with polyethylene at least 6 mil thick, sealed at the seams and run at least 6 inches up the walls and piers, whether the crawlspace is vented or not. Without it, moisture from the soil can lead to mold and rot on the joists.
Basement: full-height foundation walls with a slab floor. The outside of the walls below ground is damp-proofed, the backfill is gravel so water can drain down, and a perforated drain pipe at the footing carries the water away.

Parts of a foundation and what they do
Part	What it does	Signs of trouble
Footing	Spreads the load of a wall or post over the soil.	Settlement: cracks, sticking doors, sloping floors.
Foundation wall	Carries the house down to the footing and holds back the soil.	Cracks that grow, bowing or leaning, leaks.
Slab	A concrete floor on a gravel base.	Cracks, dampness, floor coverings that lift.
Capillary break	Gravel and plastic sheeting under a slab or over crawlspace soil.	Damp slab, musty crawlspace, mold on joists.
Damp-proofing	A coating, such as asphalt emulsion, on the outside of below-ground walls.	Damp walls after rain.
Drain tile	A perforated pipe in gravel at the footing that carries water away.	Water seeping in at the floor-wall joint.
Sump pump	Pumps collected water out of a pit in the basement floor.	Running nonstop, not running, a flooded basement.
Sill plate and anchor bolts	The wood plate on top of the foundation, bolted or strapped to it.	Rot, termite damage, gaps that leak air.

Concrete hardens by hydration, not by drying

Concrete is a mix of portland cement, water and aggregates (sand and gravel or crushed stone), and the aggregate makes up roughly 70 to 80 percent of its volume. The cement and water form a paste that binds the aggregate together.

Here’s the surprising part: concrete doesn’t harden by drying out. It hardens through a chemical reaction between the cement and the water, called hydration. As long as water is present, hydration keeps going, for years. Concrete that is allowed to dry in air gains only about half the strength of concrete kept continuously moist.

That’s why fresh concrete is cured: kept moist and warm while it gains strength. The first 7 to 10 days are critical; the concrete shouldn’t freeze or dry out. Builders keep it wet by ponding, spraying or fogging, or wet coverings, or seal the water in with plastic sheeting, curing compounds, or by leaving the forms in place. About 90 percent of its strength comes in the first 28 days, so 28-day strength is the standard reference. Most structural concrete reaches 3,000 to 5,000 psi in compression.

The mix matters too. The ratio of water to cement is the single most important factor in strength: less water makes stronger concrete. About 0.3 is the minimum needed for full hydration, and practical mixes run about 0.4 to 0.6. Too much water makes weak concrete; too little makes it unworkable.

water–cement ratio = water ÷ cement lower ratio → stronger concrete (practical range about 0.4 to 0.6)

Rebar: steel for the tension

Concrete is strong when squeezed but weak when stretched: its tensile strength is only about 10 to 15 percent of its compressive strength. So it’s almost never used without reinforcement. Steel reinforcing bars, or rebar, carry the tension and limit how wide cracks can open.

Concrete also shrinks as it loses water, and that shrinking creates tension inside it. That’s one reason concrete and concrete-block foundations usually develop some cracks, and why a hairline crack is not automatically a problem (Lesson 11).

Keeping water and soil gas out

Gutters, downspout extensions, grading, damp-proofing and drain tile all work to keep outside water away from the foundation. A sump pump is a backup that can fail, and its pit should have an airtight, childproof cover. A dehumidifier helps with dampness and odor but doesn’t fix the cause.

Soil gas matters too. Radon is a naturally occurring radioactive gas, made by the decay of uranium in soil and water. You can’t see it or smell it, and it can enter homes from the ground and build up to dangerous levels. EPA says radon exposure is the second leading cause of lung cancer after smoking, that testing is the only way to know your level, and that all homes should be tested.

You canYou can

Keep gutters clean, and extend downspouts so water lands well away from the house.
Keep soil and mulch sloping away from the foundation.
Test your home for radon with a test kit, as EPA recommends.
Learn where your main water shutoff is (Being an Adult 101).

StopStop: foundations and trenches are for pros

Foundation repairs, underpinning and anything that changes how the house is supported need a permit and a structural engineer or experienced foundation contractor. If a crack keeps growing, or a wall bows or leans, have it looked at.

Digging is dangerous too. OSHA says never to enter an unprotected trench or excavation 5 feet or deeper without an adequate protective system, and some shallower trenches need one too.

Check yourself

Tap an answer to check it, or open “Show the answer.”

How does concrete harden?
Show the answer
By a chemical reaction with water called hydration. Hydration is a chemical reaction between cement and water. That’s why concrete is kept moist while it cures.
Concrete allowed to dry in air gains about what share of the strength of concrete kept moist?
Show the answer
Half. About 50 percent, according to Penn State’s civil engineering notes. Curing matters.
Why do footings in cold places go below the frost depth?
Show the answer
Freezing water expands, and frozen soil can heave. Water expands as it freezes. Wet soil that freezes can lift a shallow footing.
According to University of Minnesota Extension, downspout extensions should discharge water at least how far from the wall?
Show the answer
4 feet. At least 4 feet, with at least one downspout per 50 feet of eave.
EPA says the only way to know your home’s radon level is to…
Show the answer
test for it. Radon is invisible and has no smell. EPA says testing is the only way to know, and all homes should be tested.

Go deeper

University of Minnesota Extension: Moisture in Basements: Causes and Solutions Moisture in basements: causes and solutions, in detail.
Penn State Civil and Environmental Engineering: Curing Concrete How curing works, from Penn State’s civil engineering program.
U.S. EPA: Radon Radon basics, testing, and finding a test kit or a pro.
OSHA Fact Sheet: Trenching and Excavation Safety OSHA’s trenching and excavation fact sheet.
Lesson 10: Moisture, Mold and Building Science Lesson 10: where water comes from and how it moves.

Sources

I finished this lessonSaved on this device only.

A free course from Teddy Rook Book. It explains how houses are built in general and follows U.S. sources; the local building code, the building department and the manufacturer’s instructions are the final word.