Stone Foundation

Started by Don_P, December 12, 2021, 10:07:04 AM

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A foundation question this morning made me realize we've been working on another "alternative" or at least kind of non typical foundation. We were called in by a local farm museum to reside the olf farmhouse... oh that it were so simple. There had been a change of use from residential to commercial in making it a museum. The sills and much framing is rotted or termite riddled, and the foundation was collapsing loose rubble stone. We decided it was easier to dig a full basement to get access to the sills and joists without removing the floors if possible. That work will be hidden underground but I needed the above ground portion to look period as much as possible, it also had to meet code. The below ground portion is a typical 12" block foundation on a spread footing all reinforced and grouted for a 7' backfill height, the deepest we were underground.

The above grade portion is where I talked to the building official, did one wall section and let him take a look, and then rolled on. I've done this in some form or fashion on a number of old work jobs but something similar would get approved for a new work crawlspace type of foundation I would expect. Code does approve rubblestone walls (uncoursed masonry) at 16" thick, read chapter 4 of the code for reference there. I was doing this atop 12" thick block walls so asked for a little latitude. We put a plywood back form on the inside face of the wall up to the sill. I worked with 2 mixes. In one wheelbarrow I had a batch of mortar that I laid the stone with, flush to the outer face of the 12" wall. Well bedded and laid ashlar (coursed bond) as much as the rough stone would allow. I then backfilled with concrete bag mix from the other wheelbarrow filling to the form and locking into the backside of the stone. I threw in rebar and welded wire mesh in places when the rocks allowed and we carried the vertical reinforcement from below up through the new sills, that is actually the biggest obstruction to full coursing but new work would give better placement options there.

This is the first trial wall section, you can see a blockout attached to the form that left a pocket for a new midspan girder under the floor (commercial is 100psf floor live load)

This is where we got to last week, happily the weather stayed unseasonably warm for long enough for us to set up winter quarters in the basement, I'll find a 6" diameter rock before spring, collapse the temporary stovepipe and mortar in the rock and patch. Over the winter we'll work on final grade under there, subfloor drains, chimneys (I had to remove 2), etc, then hang the barrel stove from the floor joists and pour the slab. On pretty days we can get to the siding, remember the siding, that's where this all started :D

We brought the skid steer home and welded some cracks it has suffered digging the rock under there and dug for a bad leak deep under the seat. 4 hours later I had the part out, a nicked O ring on a safety solenoid... tomorrow its off to find O rings for every one I disturbed on the way in. I remember something about this from "Zen and the Art of Motorcycle Maintenance" :D

Anyway, if you are equally blessed with rocks and time, this is relatively slow but pretty cheap yet still gives full perimeter bracing and support. It is 4" thinner than true prescriptive but is approaching poured rather than stone masonry so I've had inspector approval several times. I've also just done the full 16" with an interior pour when not constrained to a thinner wall. More options :)


Stone foundations are not as reliable as concrete foundations because concrete can last up to 30 years. I try to use concrete wherever possible, its may be a little more expensive, but you get good quality. I live in a house I inherited from my parents. The parking lot in the garden is made of concrete but is over 35 years old, so I decided to update it. I turned to to remove the old concrete and pour new concrete. These guys did a great job.