Extreme caution required for foundation repairs

Hey there, time traveller!
This article was published 17/06/2023 (811 days ago), so information in it may no longer be current.

Question: I was reading your article from 2021, about Mudjacking and poly foam lifting on concrete slabs. I do have a situation at my house, as I live in a concrete slab-on-grade home in Jameswood on Ness Avenue and I am talking with a contractor about lifting it using poly-fil foam inside the house. Four years ago, water leaked from the bathtub underneath the slab, as there was a small hole in the tub. At the back end of the house there is now a slope downward of about two inches. Your article confirms my concerns with the slab itself. It appears to be in good condition, but again if this company starts drilling holes in the concrete slab will the slab pull apart from this procedure? The house was built in 1948 and restored in 1999, therefore the slab itself is 75 years old. Do you have any experience with these types of homes and if so, can you provide some advice and comments? Much appreciated, Thank you. Tracy M.

Answer: Lifting an entire home, regardless of the type of foundation, is a complex and risky procedure and must be done only by a very experienced contractor, with building permits and plans reviewed by a professional structural engineer. There is more risk of causing untold damage to your plumbing drains, heating ducts, and other systems inside and under your slab foundation than potential benefits.

When doing any type of radical structural work on a home, extreme caution must be taken to prevent damage to various other systems within the home. When that home is built directly on grade, and supported by a concrete slab foundation, those risks are multiplied. Because there will be hidden plumbing drains, heating ducts, possible electrical wiring, and maybe portions of other systems within or under the concrete, lifting it by injecting foam will create changes in that environment. Because the foam expands as it cures, it may have the power to crush heating ducts, which are likely corroded inside the confines of the concrete. They can easily be disconnected at joints, elbows and boots as the slab moves upwards.

Even more significant may be dislodging or damaging older cast iron plumbing drains installed within the sub-slab soil and connected to plumbing fixtures. These may become damaged without warning, until they cause major leakage when water and sewage is run through. If they have gradually shifted, over many years, the soil may have moved with them, preventing serious leakage. A sudden injection of foam between these and the soil, or the underside of the concrete slab, and the brittle old pipes could crack or become dislodged. Either or both of these situations will certainly add significant costs to the overall job, which must be taken into consideration. Repairs would certainly be even more difficult and costly after the foam installation, due to the presence of the new material in any voids.

My other concern with using expansive foam for this type of operation is the lack of control once the material cures. Lifting a concrete driveway or sidewalk at an exterior location with special expansive clay has a very good track record. If the slab is slowly lifted, additional material can be pumped underneath in increments until the desired height and level is reached. With foam that cures into a semi-rigid state after expansion, it may be much more difficult to prevent overfilling voids. Once in place and cured, how can alterations be made should problems arise? It is true that this material will prevent drafts and add a significant layer of insulation beneath the home, but is that worth the cost of damage to other components?

I am sure that there are several experienced contractors that are using this type of product for leveling and lifting various types of residential and commercial concrete slabs, with much success. I doubt that very many of those are willing to assume the liability for an entire house or other building. Regardless, this is major structural work and should not be attempted without obtaining a building permit from the local municipality, ensuring scrutiny for the process. The plan for the project should include details about disconnection and repairs to systems hidden under or within the slab. It should also be reviewed and stamped by a structural engineer, to ensure the plan is sound, from a structural perspective.

The above process will certainly add costs to your overall plans and may not have been taken into consideration by the contractor in question. Upon speaking with you by phone, I was quite surprised by the lowball quote you received and no mention of plans, permits, or engineers. If the contractor was only planning on injecting foam in a small area under the slab where it has settled to the lowest point at the rear, your concerns with concrete cracking and damage may be well founded. They may be able to straighten the floor to some extent, but not necessarily everywhere. You may be much better off spending your money on an experienced Red Seal carpenter to install a subfloor in the uneven areas where you plan to upgrade your flooring.

Lifting a portion, or an entire structure, of an existing building, by injecting any material under a slab-on-grade foundation is an extremely risky and complex job. To consider such an effort, especially with a moderate cost quotation, is not taking all aspects into consideration. The risks certainly outweigh the benefits, especially if the goal is only a smoother and more level surface to allow new flooring installation.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and a Registered Home Inspector (RHI)(cahpi.ca). Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at trainedeye.ca.

trainedeye@iname.com