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Residential Slab on Grade

Residential Slab on Grade

Residential Slab on Grade

I'm being asked to check a slab on grade for structural adequacy. This is for a realtor friend. It's a slab on grade home and the slab has settled in the kitchen. It was noted by the home inspector and the buyer wants it to be looked at by a structural engineer. My friend called me. To be perfectly honest, I wouldn't even know where to start with this. Is there anything that be set up for a couple weeks to see if it's still moving?

It's an old home and my gut is telling me it's a little settlement that is nothing to worry about, but there is nothing at all known about the slab or foundations (if there are any).

Is there any literature or guidelines out there for something like this?

RE: Residential Slab on Grade

You could run a level loop through the house to establish a base line and do it again in a couple weeks. Settlement probably happening slower than that though.

If you want to learn more about the foundation I would grab a shovel or a mini excavator and see if it does have footings, or a frost wall or even thickened edges. All at seller's expense...

What are the soils? Probably more of a geotechnical issue than a structural one.

RE: Residential Slab on Grade

It's a 50+ year old house. I would think that most settlement has happened by now. I wouldn't expect much more at this point. It's my understanding that their concern is of a structural nature, so slab on grade settlement just doesn't rise to that level, at least in my mind.

If I dig up some of the exterior grade to look at the footings and there happens to not be any, would you simply note there is no footing and that this should be checked or would you check it?

A bigger question I'm coming up against now is where do you draw the line. When you start digging into stuff you always run into things that just don't work on paper. Do you note these deficiencies in the report or investigate them and make conclusions/recommendations?

There really is nothing structural to even check for a residential slab on grade, unless it's a slab on grade foundation.

RE: Residential Slab on Grade

If it's clay soil and recent development has resulted in more wells, underdrains, or stormwater systems that could draw down the water table the settlement may not be over. If that's not the case, you're probably right and it probably won't keep moving.

Maybe learning more about the foundation won't help. We already know it doesn't work very well. Even if you can run some calculations and it passes or fails on paper, it failed in real life and that is what matters - at least from a serviceability stand point.

If I was buying the house two things would matter to me. How bad the settlement is and what kind of soils there are. If the cabinets are level, door reveals are uniform, sheetrock isn't cracked etc. I wouldn't find it objectionable. If there is good soil I wouldn't expect to do anything more. If its clay soil and there is reason to believe that movement is recent I'd stay away and I would advise your clients to do the same. Maybe not as a structural engineer but as someone with common sense.

RE: Residential Slab on Grade

If it's lasted 50 years it's probably not going anywhere anytime soon. Now, if they recently added a storey or changed something major that's a different situation...

If it were me and my friend I'd think about putting marbles on the floor, watching them roll towards the corner, and say something like, "Yeah, it appears to have settled at some point in the past."

It sounds like you're doing your realtor friend a favor to facilitate a sale. That's nice of you and you might even get paid a little bit for it. But just be careful how you write the report because YOUR name will then be associated with that house and that condition, effectively guaranteeing it not to be a problem. The realtor is your friend but is the buyer? And what about the next buyer? They might be bat-boogers crazy and at some point try to come after you for some type of damage. For example, what if there's water intrusion into the building at some point? They might claim it's due to settlement and they'll claim you told them, "that wouldn't be an issue..." You then might have to hire a lawyer to defend yourself...

Hopefully that's the worst case scenario and as Hokie66 rightfully pointed out, who better to conduct a structural investigation than a structural engineer? But, just remember that you could be sailing into dangerous waters so be careful and take every step necessary to protect yourself. Good luck.

RE: Residential Slab on Grade


That's exactly right; I'm doing a favor for a friend. I'm not charging him, but I'm sure he'll take me out for dinner.

I will consider wording to make it clear that the opinion is based on conditions observed at the time of the inspection and is made without the benefit of a geotechnical investigation. I will also separate the structural aspect from the serviceability aspect.

You bring up a good point about being tied to this indefinitely. That is certainly a concern and a good reason to separate the structural from serviceability considerations here. Probably a good reason to check if there are footings. If there are footings, then I can state with good conscience that the slab on grade is not a structural concern regardless of the settlement. Then I can talk about the settlement/serviceability without feeling the need to parse words as much.

RE: Residential Slab on Grade

Lion06....there's actually a lot of info you can impart to the owner and realtor.

First, you need to determine if, in fact, there has been settlement and what is affected. Check cracks in the slab to see if they are faulted and if so, the direction of the faulting. Check the construction to see how the slab interfaces with the footing. For a 50-year old house, the most likely construction is a footing with stemwall and an in-filled slab. If the slab is the only portion that has settled and the footing and cladding show no distress, then look for isolated issues that would have only affected the slab. This could be soils issues or something as simple as a broken or leaking pipe or infiltration into an existing or an old abandoned pipe.

As others noted, clayey soils can be problematic with moisture changes. Also, trees and their roots get bigger and broader with time and can significantly affect the slab and foundation, both by taking more moisture out of the soil and by physically pushing on the structure with root growth.

Good luck, keep your eyes open and put plenty of caveats in your report!

RE: Residential Slab on Grade

Also forgot to add....try to determine the age of the cracks. They might have been there for years. Could be just shrinkage and could be shrinkage exacerbated by other issues.

RE: Residential Slab on Grade

Great points, Ron!

At the risk of sounding way more green than I am, how do you go about determining the age of concrete cracks other than looking for dirt/debris in the cracks vs. clean cracks?

RE: Residential Slab on Grade

Another thought. Would it be worthwhile documenting the degree of settlement from a reference plane in case there are any issues later? That way I would have something showing the degree of any additional settlement that occurred after the report was written. As I'm thinking about that, I would need to survey to determine a reference elevation as any reference in the home could potentially move as a result of additional settlement.

RE: Residential Slab on Grade

Older cracks tend to have a "weathered" edge I've found. The edges of the crack are super sharp when they first happen but over time I find the sharpness starts to round out. But that's just my experience.

RE: Residential Slab on Grade

Most likely, it settled years ago. Any backfill for the slab was probably NOT compacted, therefore it could have settled.

On the other hand, it is possible there is/was a broken or leaking sewer that could have caused a washout and thus settlement.

I would caution NOT to make any ABSOLUTE statement about what has caused it. You can list possibilities but AVOID making any definite statement unless it can be confirmed.


RE: Residential Slab on Grade


One thing you may want to research in the process of this is what is the statue of repose in your state. As I understand it that's how long a client has to bring a claim against you. It can range from 5 to 10 years to beyond but it's good to know, whatever it may be.

As for your investigation, perversely, the more you disrupt existing conditions the more vulnerable you become. That is, the more you dig around the foundation the more reason the buyer will have to claim that you should have been aware of problems if he later decide there is one, and, he might even go so far as to claim that your digging caused the problem! Now again, that's assuming there's no reason to believe that there *really* is a problem. If you suspect there is then actual engineering takes over and you should do what the situation dictates.

RE: Residential Slab on Grade

I would not go below any footing. At most I will dig locally against the wall to the top of footing just to make sure something is there.

I think once I verify there is a footing (I will be sure to state in the report that the adequacy of the footing is outside the scope of the report) I will be comfortable with the judgment that the slab on grade is not a structural issue.

RE: Residential Slab on Grade

If unusual settlement, check to see if any broken pipes washing out subbase?


RE: Residential Slab on Grade

A couple thoughts:

1) after this, you may want to block calls from your realtor friend. I avoid real estate transactions because they always need it asap, it's not clear who the client is (you are generally not working for the owner of a property), and I don't like billing for hundreds of dollars for a wishy washy report that is filled with caveats.

2) One thing a prospective buyer generally wants to know is if it's safe to be in there. If it's a problem isolated to the slab on grade, you can probably make a judgement about its capacity with some sounding and recommendations for repair.

3) The other thing they want to know is if they'll be able to sell the house later. Nobody wants to buy someone else's problem. Your documentation will give them the way to verify the condition of the slab to future buyers. A seller may say, "that crack was there when we bought the house 20 years ago and hasn't changed," but it's rare for anyone to have actual measurements from 20 years ago to prove it. That's part of what a survey would provide and may be all you can do because it's probably not going to move while you're watching it.

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