Build New House on Old Foundation

[Simba13]

Ok, got a call from an existing client that our firm does some civil work for. He wants to tear down an existing house (house is a generous term based on what I'm seeing in the pictures, so let's go with shack), the shack is a small masonry house built in the 70's. To be honest this call kind of threw me, I'll talk to my boss before actually coming to any kind of conclusion but is tearing down houses and rebuilding on old foundations an acceptable practice? Seems a little jank (pardon the uneducated term but it's the first word that comes to mind here) to me to be honest, if you're going to go through the effort to tear down a building is it really that much more cost/effort to just pour new foundations? I mean you're tearing down the whole house and rebuilding it, so under the rehabilitation code you'll have to bring the entire structure into compliance with IRC (VA residential in this case) and I'm not sure if the existing foundations would be anyway. I've rambled a bit, but basically I'm asking if this is something you all have seen/done before?

 

[jayrod12]

I've seen it done. But, the two times I've had it proposed on my projects, I've given them the requirements for upgrades and had them price that, versus tearing out the old foundation and providing new, and the cost of upgrading the existing foundation has come out significantly higher in both cases. Therefore we went to all new both times.

 

[Simba13]

Fair enough, generally speaking what kind of upgrades would be needed to bring 50 yr old block foundations into current day code compliance?

 

[jayrod12]

In my most egregious case, the owner wanted to add a foot of headroom and a structural basement slab, so the upgrades were substantial. In the other, they wanted to pout a heavier structure (heavier wall cladding) above as well as waterproof and insulate the outside of the foundation wall. Therefore there was a bunch of prep work required and once they were excavating out the foundation to do the insulation and waterproofing it just seemed easier to everyone involved to just put a new foundation in.

 

[Simba13]

Gotcha, it doesn't seem like the client plans to do anything complex (need to speak to him a bit more) but if we can somehow verify the strength of the footers, and they're sized to code (which if it's just a one story with a crawl space then they are) then I guess it's possible. Thanks for the help!

 

[IRstuff]

This is routinely done in California, since remodeling incurs a different property tax than a new build. By retaining a (tiny) portion of the original structure and using/extending the existing foundation, the infamous "California Remodel" is born. Oddly, well after construction is completed and everything is signed off, the remaining piece of the original structure gets "damaged" and is replaced, resulting in a new structure on the old foundation.

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[r13]

I agree with you on that a brand new car set on the used tires do not make sense, if you can pay for the car at first place. The owner has already saved a bundle by not requiring extensive excavation, if the old foot print can be used. Also, the code requirement on bring it up to today's code...

 

[phamENG]

Simba13 - what part of Virginia are you in? In my area (Hampton Roads), there's actually a contractor/developer that specializes in this. They buy a house, tear down everything but the foundation and a single load bearing wall, and build a new house leaving those few pieces in place. Yes, everything has to be to code, but there's some quirk in the code (not building - I think it's a zoning thing) that gives them a big advantage. I didn't do any work with them (another engineer at my last firm handled their work), so I'm not sure about all the ins and outs.

It's doable, but I'd be very careful and unless you're going back with something of a comparable size I'd insist on exposing the entire footing and verifying it's thickness in several places before signing off on the idea.

 

[r13]

Most practical way to make call is to do a quick simplified analysis using current code specified loads, and compare to the existing.
If too many locations require strengthening, or need to make sure there is adequate reinforcing, then, scrap it.

 

[BrianPetersen]

https://www.google.ca/maps/@43.5372...3jcbIk8NQC6y7JDEDQrQ!2e0!7i16384!8i8192?hl=en

That nice new-looking building is waaaayyy too close to the roadway. How did that happen?

I remember it as the Arkell General Store, a building that was probably 100 years old when I grew up not far from here. (My sister was married in the church across the street. The age of the original General Store was probably around the same as the church.)

Then they renovated the front half of the building ... then they renovated the back half ... then they gutted the interior ... then they re-did the exterior cladding and the roof. All separate projects. At some point after I moved away, it looks like they tore down the building that was next door and made a parking lot out of it.

edit: Of course, writing this prompted some more digging.

https://www.guelphmercury.com/commu...carry-on-family-business-tradition-in-arkell/

And this is what was there before ...

https://lh3.googleusercontent.com/p...NcixFMlSY8lBUrCU-BjzhC3FD5xO7E2p4nB4UVeA8xxPg

 

[Simba13]

Thanks for the advice guys. I guess I can see the advantages for the builder, he did actually dig down to the bottom of the footers. They're 6"W x 12"D, so at least in size alone they'd be okay to build a one story with a crawl space as per R403.1(1) of the Virginia Residential Code. I'm not sure how deep in the ground they are/were for frost protection though. I guess at this point it kind of comes down to strength... I'll have to mull this over, because you need at least 2,500 psi. In any case, I appreciate everyone's help, at the very least now I know that this guy isn't completely crazy for considering this.

P.S. phamENG I'm in the Fredericksburg Area.

 

[phamENG]

If you're really concerned about it being less than 2500psi, you could either have a core taken or see if anyone up there does concrete sounding. Sounding is usually a comparative test, but for something as minor as this a good operator could probably tell you whether or not you're better than 2500psi without having to do a test specimen.