Thread: New slab connection to old pier-and-beam

Are there any guidelines or advice in regard to proper construction of an interface between a new portion of house with a slab attached to an old, renovated, original house with a pier-and-beam foundation? I would imagine the two would move independently and so would require some sort of expansion (flexible) joint through the house. Or could they be attached rigidly some way to avoid future problems? (This is for a house I may be inspecting and haven't seen yet.)

If there is a stem wall up against the P&B, it could be done without much movement here. Texas has a lot more soil movement than we do, so I would expect some different conditions. Some type of preparation at the seam would be a great idea, to expect some settlement issues ahead of time.

There would almost always be some movement around here but you have much more rock around Austin so maybe movement would not be as much of a problem.
Not that you could see it in a finished house, but I would be drilling holes and using rebar dowels to tie in to the existing foundation with any type of expansion.

Originally Posted by Tim Kallmer

Are there any guidelines or advice in regard to proper construction of an interface between a new portion of house with a slab attached to an old, renovated, original house with a pier-and-beam foundation? I would imagine the two would move independently and so would require some sort of expansion (flexible) joint through the house. Or could they be attached rigidly some way to avoid future problems? (This is for a house I may be inspecting and haven't seen yet.)

Hello Tim,

The short answer is it depends on what the Engineer designed for the new slab and any connection to the remainder of the existing foundation. I would expect that the Austin area is no different than DFW and I have yet to encounter an AHJ that does not require a foundation design for a major structural addition. There are many to many variables at any one site to know what would have been best for that addition. As Inspectors about all we can do is to check for signs of issues displaying a potential issue with the new and old foundation sections. Obviously if significant issues exist they should be reviewed by a Professional Engineer well versed and experienced in residential foundation design.

You can attempt a permit search if the local AHJ has them online and see if a proper permit was ever obtained. Advise the client if you find one and the information on it which might include the Engineer of Record for the design. If nothing else it should list the company performing the work. The client can then inquire with the Engineer for more information and/or inspections if they are still concerned about the design and construction and/or speak with the company that performed the addition and obtain the Engineer's information.

At the minimum I would have installed piers under that stem wall to keep the movement or stop the movement altogether before I poured a slab to it. You would want a solid footing and an attachment to the slab or there would be constant up and down movement were the slab and srem wall abut. at the walls I would certainly have a fairly wide control joint so when that pier and beam is moving around the outside would not be constantly cracking. The attachment would be the piers in the ground that the stemn wall and pier are to be attached.

A proper treatment of the soil at the stem wall before the slab is poured. The treatment would be Termidor for termites. You don't treat it and you will more than likely get termites coming up from the ground between the two. Talk to your local Pest control company for treatment.

Of course the engineer is the best man to listen to, sometimes. You dry the soil out completely on the side of the home the slab is going to sit on simply by the slab being poured. As the soil dries the stem wall will want to settle down. Thus the piers at that stem wall and slab juncture.

I am doing an inspection Wednesday morning and he actually had similar questions about the home he is about to buy. Go figure.

You need to be asking this question of an engineer in your area. Soils can vary greatly between locations, even in one area. If you do not know for sure what you are doing, you could be creating a lot of future angst for yourself. I am not saying this because I am an engineer. I am saying it because I do a lot of structural inspections where people are in a great deal of angst.

Thanks, everybody! I did find out the new foundation and interface has been engineered with plans including the detail of the interface. He has the edge of old floor above the new slab with a step-down, so that the original joists bear on top of the new beam, sort of using it as a stem wall. This seems to make sense to me, as opposed to having both floors at the same elevation. I think this will allow for some differential movement.

Around here, central California, even with expansive soils, all additions are doweled to the existing foundations. If the slab and existing move differentially, what happens at the wall connectins above? Drywall joints don't take movement real well.