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RE: Flatness Tolerances for Residential Slab

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First, I don’t think the AEC-residential(--nospam--at) has many subscribers left. Most have moved over to the light-framing(--nospam--at) (go to to subscribe to the list on the right side of the home page) and I will foreword your message to.

I don’t know of a written specification in the UBC / IBC although I have also sent your message to our local CVSEA group of local engineers – one of whom is a geotechnical (Shelton Stringer of Earth Systems Consulting). I think the question needs some clarification.

1. Is this a new slab?

2. Over what distance is the change noticed (the 3” canyons).

3. Do you know if the slab was floated level when it was placed?

4. Was there a Geotechnical study done to indicate potential problems with the soil conditions?

5. Was the slab to be designed as post-tension or simply as a slab on grade?


I think if some of these questions are known that we might be able to identify who was responsible and what can be done. The last is the most important. Assuming that there is no geotechnical problem that caused the low spots, you might try to us a self-leveling material to get the floor even if you intend to use a hard surface floor such as marble or tile. The same is true for carpet if the changes in elevation are noticeable by someone walking over it. Assuming it is a quality control issue, you should be able to get the contractor to pay for the leveling materials. Check out the Sonneborn Sonoflow™ floor leveling cement based underlayment. You can download the technical manual using this link: . I believe that this is the same material that we recommended to a local club-house for a private golf course. The materials are not that expensive and should solve the problem.


What about cracks in the slab? If the contractor had his concrete sub place the slab and walk away from it then there may be temperature cracking that is common in all slabs around my neck of the woods. You need to be care of these if you intend to install a hard surface floor as the cracks will radiate through the flooring. Sealing the crack will not solve the problem in most cases and older contractors had used builder’s paper as a slip-sheet to stop the crack. I have been called out on a number of these for insurance companies and homebuyers. The builder’s paper ultimately adheres to the thinset used to secure the tile to the slab. There is a fabric and elastomeric membrane that, although expensive, covers the crack about 9 to 12 inches on each side and should be set over the crack before the thinset is used. A good tile shop will know where to obtain the membrane.

Speaking from experience, I set the membrane over cracks in my slab eight years ago and not one crack radiated through the soft pavers I have throughout the home. I made this part of my specifications so that contractors will be compelled to install the membrane before laying any hard surface flooring. Our local source for the membrane is D’Mundo Tile Company in Palm Desert California and they will fax me the specifications on the membrane they sell. I will e-mail this to you later today.


Hope this helps.






-----Original Message-----
From: Bill Polhemus [mailto:bill(--nospam--at)]
Thursday, October 16, 2003 7:15 PM
To: aec-residential(--nospam--at); seaint(--nospam--at)
Subject: Q: Flatness Tolerances for Residential Slab


Although I’m willing to do my own research, I thought I’d try the lazy man’s way, and ask here:


What are the governing standards one can cite for the flatness of a residential slab on grade foundation?


Is there anything specific in IBC/IRC? Or would ACI be the place to go?


I’ve got a residential slab foundation that varies more than 3” in elevation between high and low, has “canyons” and saddleback ridges, etc. I need to know how to ding the contractor for this, chapter and verse.



William L. Polhemus, Jr. P.E.

Polhemus Engineering Company

Katy, Texas USA