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increased inward lateral pressure caused by the new soil to be installed for the patio.

Raised patios using paver blocks are supported on soil that is placed on top of existing ground (grade) along the foundation wall. This additional soil can cause large increase in lateral pressure that can severely damage the wall.

All too often, contractors persuade homeowners that installation of a raised patio along the back wall of a house, with full basement, can be accomplished without obtaining a building permit and without involvement of an engineer. Many contractors that install patios do not understand the effect of a new raised patio on the adjacent foundation wall.

In addition to supporting weight of the house framing, any foundation wall along a full basement must also act as a retaining wall to resist inward pressure from soil, known as "lateral" soil pressure. Weight of soil is one of the primary factors that determine how much lateral pressure is caused by soil against the foundation wall. Other factors include type of soil (sand, or clay) and water content of the soil.

Groundwater (hydrostatic pressure) above the basement floor slab causes large additional lateral pressure against a foundation wall. If there is evidence of high groundwater, such as a wet basement, remedial work to correct eliminate high groundwater should be performed first.

Lateral pressure from soil "backfill" against a foundation wall varies with the square of the backfill height (above basement floor slab). Therefore, even what may be considered relatively small increases in backfill can cause large increases in lateral pressure against the wall.
Additional weight (of anything) placed on top of soil backfill causes additional lateral pressure against the foundation wall.

For example, if original height of soil backfill (above basement floor slab) is 6-feet, and 2-feet of new soil is placed on top, total lateral soil pressure against the adjacent foundation wall is increased by 78 percent (not just 33 percent). Weight of patio pavers (on top of soil) increases lateral soil pressure further.

Damage to foundation walls due to increased lateral soil pressure is most frequent with concrete block walls, which have much less capacity to resist lateral pressure compared to solid concrete walls. Excessive lateral soil pressure generally causes long horizontal cracks in mortar joints, near mid-height of the wall. Short vertical and horizontal "step" cracks are is also seen near the end of the wall (at corners of basement). Inward vertical curvature (bulging) can also occur, usually away from corners.

A foundation wall resists inward lateral pressure by acting as a beam (or slab) in the vertical direction. Picture the foundation wall horizontal, with soil on top, to understand the behavior. Just as a floor joist would crack at the bottom due to excessive weight, the foundation wall cracks on the inside face due to excessive inward pressure.

If a foundation wall is not strong enough to resist lateral pressure from the proposed patio, the wall can be reinforced. However, the cost of reinforcement, and effect inside the basement, may not be acceptable.

Learn more about this author, John F Mann.
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