A Lesson in Basics: Foundation Inspections, Part 1
What do you tell clients about foundation defects you find during an inspection? Is the defect severe? Is it stable? Does it require further evaluation? When I give a presentation on this topic to home inspectors, I always start with the basics — a description of common defects and related conditions. Depending on where you work, you may inspect one type of foundation more than others, for instance poured concrete in clay soil. Or the foundations you inspect may vary greatly in materials, conditions and age. But even with regional differences, common defects can be categorized as settlement; cracking, bulging and bowing; slippage; and material degradation. A basic description of defects in the first three categories is presented here. Material degradation will be covered in the second part of the article next month.
Settlement
- Sandy soils contain large granular particles with a relatively small volume of voids. The settlement on sandy soils is usually quick (one to two years), and slight.
- Clay soils contain smaller particles and relatively more voids than sand. The settlement on clay soils tends to be greater and to occur over a longer period of time than other soils. Clay soils also tend to be somewhat unstable when subjected to periodic drying (shrinking) and wetting (expanding). Some clay soils can change by as much as 50 percent in volume between wet and dry conditions.
- Organic soils containing a large amount of organic matter often behave in a manner similar to clay soils. The organic matter shrinks as it dries and expands (like a sponge) when it gets wet.
Differential settlement occurs when the soil under one portion of a foundation settles at a rate greater than at other foundation locations. This can cause cracking, shifting of the supported building structural members, out of level floors, out of plumb walls, and sticking doors and windows.
Cracking, bulging and bowing
Settlement can cause cracks. The force that caused a crack was at a right angle (90˚) to it. Knowing this may help you determine the cause of the cracks you find.
If the foundation wall is plumb vertically and straight horizontally, and there is a vertical or uniform angle crack in a concrete wall or a stair-step crack in a block wall, it’s probably the result of differential settlement. If the crack is somewhat wider at the top than at the bottom, this is also an indication of differential settlement. If a house built on sandy soil is more that a few years old, differential settlement is likely finished. On clay or organic soils, the motion may be continuing.
An example of a differential settlement crack.
Monitors can be installed across a crack to determine if the motion is seasonal or expanding. The monitor can be as simple as a glass microscope slide fixed with epoxy on either side of a crack. Or, it can be more sophisticated, such as a sliding grid system with a grid element glued on one side of the crack and a cross-hair element glued to the other side. The glass slide determines there is motion (if the slide breaks). The cross-hair system can be monitored for up to a year (or more) to determine if any motion is seasonal. If the motion is seasonal, it can be said to be stable. If the motion is not seasonal, corrective action may be required. Although this type of testing is beyond the scope of a house inspection, clients may appreciate the information.
If the wall is plumb vertically, but bulging inward horizontally, a vertical crack may indicate there is hydraulic pressure along the outer surface of the foundation wall.
When this is present, check the following:
- How is the soil graded adjacent to the foundation? Re-grading may be needed at the rate of about an inch per foot for about 3′.
- Are gutters and downspouts installed? If so, do the gutters drain well away from the foundation wall?
- Is there evidence of water seeping through the foundation wall – through the crack(s) or low along the wall?
- Is there a sump pit? Is it full?
- Is there damage to one wall or all walls?
- What is the major drainage path outside the house? Is the foundation wall acting like a dam for ground water?
- Do you know if the soil conditions are sandy, clay, or organic?
Correcting a problem with hydraulic pressure depends on the actual cause. It may be as simple as re-grading and/or installing gutters and downspouts, or somewhat more expensive when installing added vertical bracing is judged to be in order – steel bracing and/or installation of pilasters. It becomes even more expensive when the wall damage is severe, requiring excavating and installing an engineered drainage system and/or the replacement of the wall. These judgments are best left to foundation repair experts, engineers and architects.
Some home inspectors identify defects, cover what may have caused the problem and suggest what can be done to correct it. (See my article on “forensic inspections” in the January 2001 Reporter.) Those who are not licensed engineers may prefer to report the defects and recommend that clients call in other professionals with the experience, expertise, and credentials to suggest corrections.
If the wall is generally straight horizontally and out of plumb or bulging vertically and the crack is horizontal, the likely cause is also hydraulic pressure. But the source of the pressure may be slightly different than noted previously. If the crack is generally about at the frost line (the soil depth below which freezing generally does not occur during winter months), it may have been caused by water freezing and expanding. Check for the conditions previously listed. Assuming the motion is not so severe as to be a structural concern, controlling water ponding or collecting near the foundation wall may be all that is required to correct the problem. If the motion is severe, call an engineer.
If the wall is plumb vertically and straight horizontally, small random cracks are usually shrinkage cracks. Shrinkage cracking generally occurs in concrete walls or floors. It occurs when the concrete mix is wetter than needed. As the concrete dries, it tends to shrink. A small crack, generally originating at a point of local stress, such as at a basement window corner, is as a rule due to shrinkage. Small shrinkage cracks can also be observed in block walls when the mortar used to join the blocks was too wet. This type of cracking usually appears as very thin cracks along horizontal mortar joints between the blocks. Shrinkage cracking is generally not a structural concern.
This photo shows a local stress crack. In this case, the problem was exacerbated by water collecting in the basement window well. The former owners had attempted to seal the crack, but water seepage is clearly evident beneath the window. This crack was not considered to be structurally significant. A well cover plus gutters and downspouts were recommended to ease the problem.
O
Slippage
I’ve only scratched the surface of what there is to know about settlement; cracking, bulging and bowing, and slippage, but if you’re more interested in learning about foundations than when you started reading, I’ve accomplished my purpose. Next month, the article will continue with an overview of the different types of materials used for foundations and the common deterioration defects to watch for with each.
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