Our slab assessment process

Our process is based on how slab foundations are designed and distort over time

What do we mean by this? Simply that the performance of a slab foundation should be assessed in terms of what it is designed to do. One would think that this approach would be completely non-controversial, but that is not the case at all.

Understanding what slab foundations are designed to do from a structural perspective

Slab on ground foundations serve a very specific purpose: they mitigate the damage that swelling and shrinking soils cause to the house as the supporting soils experience dry out and wet up.

One implication of this resides in the word “mitigate.” Slab-on-ground foundations are effectively incapable of being designed to not have any damage due to foundation movement over the life of the house. That is reality. I emphasize do this because many people who move here from areas that are not plagued by expansive soils are alarmed when signs of structural distress appear.

Slab foundation performance parameter #1: damage to the house

The purpose of a slab-on-ground foundation is two-fold: it transmits the weight of the house to the soil in a way that keeps settlement to a minimum and to mitigate the distortion that the house would undergo as the supporting soil shrinks and swells. The success or faliure of the foundation in performing as we want it to is shown in the damage and dsitress that the house undergoes.

Slab foundation performance parameter #2: foundation bending

Much of the distress and damage that is caused by slab foundation bending can also be caused by other things. If foundation elevation measurements are consistent with a high degree of bending, then it may be more likely than not that the distress and damage is in fact caused by foundation movement.

Many people and, unfortunately, some engineers  believe that elevation measurements alone can condemn a slab foundation. This is simply not true. If a foundation is constructed with a high degree of curvature, but if it is stable, there is no structural reason to condemn the foundation.

Let me put it this way: all the required engineering design and construction rules and guidelines are clearly intended to make the foundation perform so that there is no or little damage to the house caused by foundation movement. There is nothing in the engineering literature, including the design manuals, that even remotely are concerned with levelness. The over-riding concern is to make the foundation

The over-riding concern is to make the foundation stiff at a reasonable cost. Ironically, the stiffer a slab foundation is, the more likely that the foundation will be less level over time.

The slab foundation repair decision

For the large majority of the foundations we look at, the key question to be answered is the question of whether repair is necessary or not. We call this the repair question. When we evaluate the performance of a foundation, we do it in a way that gives us a good basis for whatever answer we give.

A mistake that many people make is to assume that if there are any irregularities, distress or damage that are judged to be due to foundation movement, that fact alone means that the foundation should be repaired.

Our slab foundation assessment process

Assessing the damage from the outside

One key part of our assessment process is to document how much damage foundation movement has caused to the exterior of the house, especially in the brick veneer. As with every part of the assessment process, we are looking for damage that is consistent with one of the two distortion modes that are characteristic of slab foundations on expansive soils: center lift and edge lift.

center lift damage

In a center lift distortion mode, it is a the central area of the foundation that experiences most of the movement. Distress and damage to the house that results from center lift will be mostly likely seen in the forms of interior door issues, drywall cracks and cracks in the floor tile.

There may also be cracking in the brick veneer. Brick veneer cracks will typically be stair-stepped. They will have a specific pattern in that the cracks will be wider at the top than at the bottom. There may be separations between the brick veneer and window and door frames. If so, the separations will also be wider at the top than the bottom.

Since the long-term distortion mode for the large majority of slab foundations in Southeast Texas is center lift, this pattern is distress and damage is what we normally see.

edge lift damage

In an edge lift distortion mode, the perimeter of the foundation will experience most of the movement. Distress and damage to the house that results from edge lift is usually in the forms of exterior door issues, diagonal stress marks, and tight cracks in the floor tile.

Brick veneer cracks will typically be stair-stepped and will be narrower at the top than at the bottom. Separations may develop between the brick veneer and window and door frames. Those separations will also be wider at the top than the bottom.

If the brick veneer has movement joints filled will a pliable sealant, the may be a visible separation at the bottom of the joint with the sealant in the upper portion of the joint compressed.

Since the edge lift distortion mode is a short-term distortion mode, this pattern of distress and damage is not typical.

mixed damage

Normally one for any specific foundation, one of the two distortion modes will be dominate. That does not preclude a house showing distress and damage patterns that are a mixture of edge lift and center lift modes. It is possible for a slab-on-ground foundation to move from one distortion mode to the other mode as the weather changes and as the foundation ages.

Assessing the damage from the inside

sheetrock cracks

Sheetrock, or more accurately, drywall can crack for numerous reasons. In fact, it is quite possible that more irregularities, distress, and damage in drywall is not caused by foundation movement. I freely admit that I cannot prove that statement; it is largely based on experience.

So how can you tell if a specific drywall crack is due foundation movement? Diagonal cracks at the corners of window and door opening are the most reliable indication of foundation movement, especially if the cracks are severe. What is “severe?” I would say any crack 1/8th inch or wider is severe. Some engineers might say wider than 1/16th inch.

Here is a laundry list of other causes of drywall cracks besides foundation movement:

cracks in ceilings: normally caused by a framing issue, moisture, or age

vertical and horizontal cracks in the walls: normally caused by framing irregularities, poor drywall application or age

door frame distortion

In our assessment process, we pay close attention to doors, especially those on the ground floor. Typically doors are installed so there is an open gap between the door and the jamb or around 1/8th inch. It does not take much distortion to close that gap. Typically, the first thing a homeowner notices is that the door will not latch, or the door sticks when it is opened or closed. In more extreme cases, the door might not be able to be opened or closed.

floor slopes

It should go without saying that as a home gets older, the floors typically slope more and more. Typically the ground floor will not have any noticeable floor slopes when the house is new.


Making and using finished floor elevation measurements

A common mistake made by many people is to focus solely on elevation measurements when reading a foundation performance report. Elevation measurements are not very meaningful in the absence of corresponding damage.

Here is a mistake that is made far too ofter: report reader will focus his whole undivided attention on the largest elevation difference between any two points on the floor surface. This number tells you nothing about how the foundation is performing.

why we make floor measurements

We make floor measurements for two reasons. One is to estimate how out of level the foundation is. Specifically, such measurements often tell us that, so far as the overall levelness is a consequence of expansive soils swelling and shrinking, if the levelness is outside the normal construction tolerance. This estimate almost always requires at least four measurements, one at each outside corner if the foundation is basically rectangular.

The second reason is to estimate how much the foundation has bent from the front to the rear and side to side going across  the middle area of the foundation.

understanding construction tolerances


the effect of floor coverings

why all measurements should be viewed with scepticism

measuring for levelness

measuring for tilt

measuring for bending

Is slab foundation repair optional or necessary?

There are always two questions regarding foundation repair. One is whether foundation repair is necessary. If there is a structural safety issue, if correcting the safety issues requires that the foundation underpinned, and if the cost of the repair is reasonable in relation to the value of the property, then a case can be made that foundation repair is necessary. Fortunately, it is unusual for expansive soil movement to create a structural issue.

If you have a situation where there is damage to the house that is likely to be due to foundation movement, then foundation repair is an option that might correct the situation. Of course, just because foundation repair is an option that should be considered, does not mean that it is a good option, much less the best option.

Putting it all together – the importance of judgement

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