Basement Foundations, Materials, Waterproofing and Defects

Basement foundation

Description

A basement is an area entirely or partially below grade, with soil against one or more sides, in part or in whole. When the whole basement is below grade, it is sometimes called a cellar. The name walk-out or daylight basement is sometimes given to a basement with a full-height exterior door. A bulkhead door does not qualify.

Construction

Modern construction begins with an excavation about 2 feet wider than the basement walls on all sides. The extra width leaves room to build the walls, apply waterproofing or dampproofing, and lay the foundation drains. The sequence is:

1. The footing forms are set and leveled, and the footings are excavated and poured.
2. The basement walls go up.
3. Plumbing and electrical parts that belong under the slab are put in, along with any radon mitigation system parts.
4. Dampproofing or waterproofing goes onto the walls, and the foundation drain is laid.
5. Backfill goes against the foundation and should be compacted.
6. A vapor retarder and gravel may be placed under the basement floor.
7. Reinforcing steel and crack control wire go in, if any.
8. The basement floor is poured, with the garage floor if there is one.

Concrete basement walls

Concrete is often the material of a modern basement wall, which is either cast-in-place or precast. A precast wall is also called a panel.

Cast-in-place walls

A cast-in-place wall is concrete poured into wood or metal forms that stand on footings poured earlier. Two details resist the wall sliding off its footing. The footing should receive vertical dowels of reinforcing steel, and a keyway, a U-shaped groove, should be cut into the wet concrete at the top of the footing. The wall should also have horizontal reinforcing steel, which helps it resist the soil's lateral load.

The thickness of the wall and the size and amount of its reinforcing steel depend on wall height and on how much soil is outside it. Each form is set, braced, squared, and plumbed so the walls do not move during the pour. Vibrating the concrete consolidates it, so it has no honeycombing, voids, or aggregate pockets, which are areas with less concrete.

The forms are usually removed. Insulating concrete forms include insulation and are left in place.

Precast walls

Precast panels are factory built, trucked to the site, and set in place by crane. They usually arrive with foam insulation sheets attached. Standard panels are 8 to 10 feet tall and up to about 16 feet long. An engineer should determine wall thickness, reinforcing steel, and how walls are secured to the footings and panels to each other.

Concrete masonry unit (CMU) basement walls

Many older basement walls and some modern ones are built of CMUs. A CMU is sometimes called a cinder block. A CMU wall is built in steps similar to a cast-in-place wall.

In a modern CMU wall, the hollow cores are often grouted or insulated and reinforced with steel. Openings sometimes visible at the bottom of the wall allow inspection to confirm it was fully grouted. In an older CMU wall, the cores are often hollow and the wall is not reinforced.

Permanent wood foundations

Some areas use permanent wood foundations, and they are found more commonly in cold climates. Compared with other foundations, they can offer better energy efficiency. Their disadvantages are that the wood can deteriorate, so grade and drainage are very important, and that wall height is limited relative to backfill depth.

The footing is crushed stone or gravel, 16 inches or more wide and 8 inches or more deep. A preservative treated footing plate 2 x 8 or larger is required, as is a preservative treated bottom plate. Gravel should also go under the basement floor, with a vapor retarder above it. Drainage through the stone or gravel is important to preserve the foundation. An active sump pump system should drain the entire foundation system. As with other foundations, a home inspector can rarely determine whether these components were properly installed.

The walls are preservative treated plywood. They should be dampproofed at least, and walls below grade should have a moisture barrier of 6 mil polyethylene or thicker. Caulk or another suitable sealant should seal the panel joints. Stone or gravel fill should go against the walls.

Stone foundations

Stone foundation walls are found frequently in older houses, usually one hundred years old or more. Modern construction still allows them. The stones were dry-stacked, meaning laid without mortar, or laid in mortar. The stones were either cut to regular shapes and coursed like masonry, or set as they came out of a field or river. A separate footing may or may not be present, and in very old houses there often is none.

Current requirements set a stone foundation wall at 16 inches or more thick, with soil against it 8 feet high or less and soil pressure at or below 30 pounds per square foot. A new stone foundation wall may not be built in the D seismic zones, where seismic activity is likely.

Water issues are frequent in basements with stone foundations. Liquid water can leak through or between the stones, and water vapor can migrate through or between them. The mortar can deteriorate, and occasionally the stones can too. Old stone foundations were usually laid with a soft lime-based mortar, prone to deterioration if exposed to moisture. A repair with modern or other cement-based mortar can make matters worse. It holds water against the old mortar, which deteriorates further.

Brick foundations

In some markets, older houses may have brick foundation walls. Most stone foundation water issues also apply to brick foundations. Because brick is hygroscopic, meaning it absorbs water, those issues can be more serious. Grade and drainage around a brick foundation are extremely important.

Waterproofing, dampproofing, foundation drains

Foundation drains should be installed at a basement with usable or habitable space below grade, except on sandy soil and gravel. Every basement should be waterproofed or dampproofed. These parts are usually concealed and not available for inspection, and older homes often lack them.

Waterproofing

Waterproofing is meant to stop liquid water under hydrostatic pressure. Hydrostatic pressure is water pressure. One method for a concrete or masonry wall is a tar-like bituminous coating, covered from the footing top to finish grade with 6-mil polyethylene. Modern construction especially uses other materials and methods, among them proprietary systems that may contain insulation. Waterproofing that is properly installed should last as long as the structure.

Dampproofing

Dampproofing is meant to stop water vapor, and liquid water in small amounts, that is not under hydrostatic pressure. A masonry wall should first get parging on its exterior side, a coat of 3/8 inch or more of Portland cement. A bituminous coating may then be applied to a masonry or concrete wall. Other approved methods, materials, and proprietary systems for dampproofing may be used. Its service life is ten to twenty years, depending on the system and on soil dampness at the foundation.

Foundation drains

A modern foundation drain is perforated plastic pipe in a fabric sock similar to cheesecloth, laid on gravel and covered with gravel. Older houses with foundation drains commonly have clay drain tiles, which are also allowed. Proprietary systems and rigid plastic pipe with holes are also used. Where possible, a foundation drain may discharge to daylight; where not, it should discharge to a sump pump.

Two drain types are the sock pipe drain and the gravel drain. In a sock pipe drain, the pipe in its sock has 6 inches or more of gravel over it and 2 inches or more under it. In a gravel drain, gravel extends 6 inches or more above the footing top and 12 inches or more out from the footing base, with a filter membrane over the gravel. Waterproofing or dampproofing material covers the wall in both.

Typical potentially significant basement defects

Analyzing a foundation defect depends on the foundation material, such as concrete or CMU. It also depends on the defect's type, location, and extent, and on whether it may still be active. The causes listed are only the most common ones. Earthquake and storm damage are other possible causes in areas prone to them.

Defects of a foundation wall are not mutually exclusive. One wall can have several defects, each with more than one cause. They often appear as cracking in interior wall coverings and brick veneer. Foundation wall problems also present at doors and windows, as cracks and as operating problems such as sticking or out of square.

No agreed upon standard defines a reportable foundation defect. The possible measurement guidelines below come from home builders. A home inspector may wish to report a foundation defect that measures smaller than the guideline.

1. Wall rotating (usually leaning in from top): the possible causes are a wall built out of plumb, a wall not secured laterally at the top, backfill too high for the wall's thickness, and vehicles driven too close to the wall. Others are a wall backfilled before it cured, expansive soil, frost heave, and hydrostatic (water) pressure. A possible measurement guideline is rotation exceeding 1 inch in 8 feet.
2. Long horizontal cracks in walls, and wall bulging (part of the wall protrudes inward from the exterior side): the possible causes are tree roots, backfill too high for the wall's thickness, and inadequate steel reinforcement. Others are a wall backfilled before it cured, expansive soil, frost heave, and hydrostatic (water) pressure. Possible measurement guidelines are a bulge exceeding 1 inch in 8 feet and a crack exceeding 1/4 inch wide.
3. Stair step, diagonal, or vertical cracks that are wider at one end: the possible causes are expansive soil, frost heave, and hydrostatic (water) pressure. Footing causes are a footing with no reinforcing steel, a footing too narrow or too shallow, and settlement of soil under the footing. A possible measurement guideline is a crack exceeding 1/4 inch wide at its narrowest point.
4. Out of plane cracking (part of the wall moved horizontally relative to the rest of the wall): the possible causes are tree roots, backfill too high for the wall's thickness, inadequate steel reinforcement, expansive soil, frost heave, and hydrostatic (water) pressure.
5. Out of plane cracking (part of the wall moved vertically relative to the rest of the wall): the possible causes are expansive soil, frost heave, and hydrostatic (water) pressure. Footing causes are a footing too narrow or too shallow, a footing with no reinforcing steel, and settlement of soil under the footing.
6. Wall sliding (the entire wall moves relative to the footing; the wall may not be cracked): the possible causes are a wall not secured to the footing, expansive soil, frost heave, and hydrostatic (water) pressure.
7. Foundation settlement or rotation (the entire foundation settles or rotates; this can be uniform, or only one side or corner can settle or rotate): the possible causes are frost heave, expansive soil, and footing settlement.
8. Building movement off of the foundation: walls that are not attached can move off the foundation. Possible causes are foundation anchors absent or improperly installed, wind storms, expansive soil, frost heave, hydrostatic (water) pressure, snow load from the roof, and earthquake.
9. Water stains on the wall, dampness, and liquid water; also owner belongings raised above the floor: the possible causes are a high water table, normal leaking through stone foundations, flooding, and a plumbing drain backup or a significant water supply leak. Others are irrigation system problems, downspouts discharging near the foundation, blocked gutters, lack of gutters, and improper grading at the foundation.

Typical less significant basement defects

Structurally, these defects may not be significant. Water can still enter through cracks, voids, and honeycombing. Home inspectors should check below each defect for brown stains, which indicate possible water infiltration.

1. Uniform width cracks less than 1/8 inch wide: the possible causes are concrete or mortar that dried too quickly and an improper mix of concrete or mortar components. Shrinkage of the concrete or mortar is another, usually from too much water in it.
2. Voids, aggregate pockets, honeycombing in concrete walls: the possible causes are aggregate too large for the concrete mix and inadequate vibration of the concrete during the pour.
3. Spalling (disintegrating material, which could be structural if large amounts of it are unsound): the possible causes are old or poor quality foundation materials and water in the wall from gutter and grading problems.
4. Efflorescence (white powder on walls): the possible cause is moisture moving through the wall because of irrigation system problems, downspouts discharging near the foundation, blocked gutters, lack of gutters, or improper grading at the foundation.
5. Cold joint: the cause is the lower part of the wall setting before the concrete above it is poured.

Numbers and Rules to Memorize