Common concrete slab foundation questions

Author John Cahill

I've been inspecting concrete foundations since 1987. Here are some of the most common questions I hear.

What is concrete slab movement?
Typically, foundation movement is comprised of settlement, heave and tilt. Settlement goes down; heave goes up; tilt is an overall movement. Deflection can be settlement and or heave. Many factors contribute to the causes.

How much is too much movement?
The common starting point for deflection (heave and settlement) in homes is 1/360 or 1 inch of elevation variation per 30 feet. Deflection is generally comprised of an average over a stated distance.
Tilt is typically perceived at 1%. The American with Disabilities Act considers 2% unacceptable. Let's consider that:
- 1% of tilt over 360 inches is 3.6 inches. 2% tilt over 30 feet is 7.2 inches (wow).
- 1/360 deflection 1 inch over 30 feet or 2 inches over 60 feet. 
- These guidelines are considered with many other variables to form an opinion.
- Too much movement is usually related to personal preference and expectation. I have not seen many homes condemned by the city because the foundation moved. Its often a matter of value and not habitability.

What standards are there?
There are guidelines but not strict rules for acceptance criteria regarding foundations. A common starting point is 1/360 (see above).

What does the Texas Real Estate Commissioner (TREC) consider acceptable?
The people at TREC are dedicated people talented in many ways however they do not have a clue about foundation acceptance criteria. They provide no real guidance to home inspectors on the matter. Your much better off talking to an engineer or the guy who dug the hole for the pier. A former Inspector Committee appointee said "If you put your coffee cup on the table and it slide off the foundation may need repair." That's pretty accurate.

Is the guideline accurate?
It all depends on who is judging the condition. You can get 5 different answers on one foundation.

Are foundations the same?
No. They started to be used around 1955 and their design criteria has changed several times. A new foundation is usually stronger than an old foundation however the soil it sits on has significant impact on future performance.

How does soil cause movement?
Much of North Texas has expansive clay soil. Soil types vary. When clay gets dry it shrinks. When clay gets wet it swells. The wet dry cycle can move part of a home, all of a home, the sidewalks and the city streets. Much of North Texas is a sea of slowly moving clay. Resistance is futile.

How to reduce soil movement?
Keep the soil consistently moist. Not too wet. Not too dry. Trees and landscaping remove water from the soil causing it to dry. Poor drainage allows the soils to become too wet. Seasonal droughts can cause soil shrinkage over an entire region. In the last North Texas drought I saw entire streets sink 3 to 4 inches. Prolonged rainfall with good drainage can result in movement. Your goal is to maintain consistency and eventually Mother Nature will probably win.

What the best foundation pier or piling?
Concrete piers, pressed piling, pressed steel and other methods all have benefits and problems. It depends on the soils. A main consideration is having an experienced engineer and repair company do the work.

What is a lifetime warranty?
Mostly marketing. Lets simplify. Someone digs 6 holes and installs a support system (6 piers). If the foundation moves they come back out, dig the hole and install or remove a shim. It takes a shovel, jack and an experienced repair person. It's not rocket science.

I forgot to get the warranty transferred to me?
It takes a shovel, jack and experienced repair person to adjust a pier. Keep in mind the warranty only covers the piers that were installed. All other areas of the home, including any prior repair done by someone else, are not included in a foundation warranty.

Will the walls crack again?
Better to assume yes rather than no.

Can I expect my home to remain perfectly stable?
You could, but you are likely to be disappointed.

What is the most important thing to understand about future risk to a foundation?
The risk of sudden future movement on any concrete slab involves a plumbing leak under the slab. Poor drainage and landscaping tend to have a slower affect.

What do floor elevations determine?
They determine the up / down movements in the foundation. They are good for a first impression however reliable judgment requires measuring over time.

What is stability?
I saw a foundation with 3 inches of slope on one side. The engineer recommended 15 piers. Ten years later I returned to the same home for a new buyer. My prior buyer (the seller now) told me he never put the piers in. The home had not changed at all. It was stable. In another home there were no movement indicators. The soil outside was dry. It rained all night long. The house cracked in half the next day. Stability is relative to observation over different periods of time and predicting the future accurately is a guess.

How much should I water?
Water enough to the keep the soil damp but not wet. Avoid dehydration. Automatic drip irrigation systems can make it easier but they too need to be maintained and monitored.

Are soaker hoses OK?
Sure, if they are monitored. I find most people forget to water after the first week or two. It's just something most people do not think about.