New foundation under exising house?
I need to replace the foundation under a wing (addition) of an existing house. The existing foundation was hollow 6″ CMU and fell into the crawlspace due to earth pressure outside (approximately a 12″ differential in height). The bottom of the existing strip footing is about 3′ below grade which is not enough in this part of VT, so that needs to come out and be replaced at 5′ down. The addition (approx. 20’x20′) is currently supported on 3 rows of jacks in the crawlspace and has been for several years. This existing block wall is not doing anything and has fallen in, in several places. So far, things are easy…I can rip out the existing and get a new footing in without moving the building, with a bit of care not to undermine the outer row of jacking. By the way, there was never an original center girder; 2×8’s going 20′. I intend to leave some posts and the temporary beam forever. Now what? I can’t jack up much higher without compromising the roofing/siding/framing between this wing and the main house. I was thinking of using to courses of ICF forms to make up the 4′ between top of new footing and sill. That way I would not need to worry about pulling forms out of the crawlspace (There is a small access hole maybe 2’x20″). If I set the first course on the footing I can backfill tight to both sides to keep things in place. I think the glue would be sufficient for one course above, but I may be able to reach in and do zip ties. The difficulty I see is placing the concrete right to the top of the forms. If I pull off the existing sill plate, rim joist and some siding I could place the ‘crete. I would have liked to leave the rim to help keep things aligned; the jacks are about 3’ in so I would have a decent cantilever to the bearing walls, but the loads work out fine. I would need to be jacked about an inch high to slide a new sill in. Any ideas for bolting to the foundation? Does this scheme seem workable? There are no building permits or inspections.
Regards, Chad
Replies
If the addition has been sitting on those jacks for years without problems, and it's just a crawl space you'll rarely if ever enter, why do you need a new foundation?
Your rows of jacks are currently functioning as a 'pier' foundation system. Let them hold the addition up. (Or, if you don't like leaving steel jacks there permanently, build a concrete block pier next to each jack, raise a tad and shim tight, then lower onto the pier and remove the jack.) Build yourself a non-load-bearing skirt wall around the perimeter, and brace/backfill it from the inside so it won't get shoved in like the original block wall did.
K.I.S.S.
Dinosaur
How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....
Structurally, that approach would be fine. However, that would not keep out the vermin or Mr. Noreaster. The floor is lightly insulated, but keeping out the wind will be a huge help. ICF will eliminate the need for seasonal banking with haybales.
I don't see any reason you couldn't insulate a non-structural curtain wall. Rigid foam, spray urethane, whatever you like can be applied to the inside of it. And just because it isn't actually supporting anything other than itself doesn't mean you couldn't carry the sheathing and siding down onto the face of it from the building wall above it. That should keep rodents and wind out.
None of this is to say your idea of a full-blown ICF foundation is a bad one, but you were looking for suggestions on how to do the job without separating the addition from the house so you could jack it up high enough to do that.
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....