Attached garage next to building on posts?
Our house has a 22′ x 22′ addition that was built (not by us) using posts/sonotube footings. No frostwall. It is, what it is, as they say.
We’d like to build an attached garage next to the addition. So far one contractor has said he won’t take the job because he fears excavating for the garage foundation will disturb the posts and without additional support for the addition, we’ll have a problem.
I presume that if we throw enough money at this, we could arrange for a temporary support of the addition while we do the work. But that’s going to add significantly to the price.
Are there other options we should consider? Could we use the same post/sonotube footings for the garage and then float a slab between them?
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John,
The easiest solution is to separate the buildings by 8 feet and install a breezeway between them.
I suppose that you could build a post-frame building for your garage, and then pour a slab between the posts.
Thanks Martin. We thought about the breezeway idea -- I'm assuming the breezeway would be supported on post footings -- but unfortunately we're right up against our property setback as it is. If we did the post frame/floating slab, are there techniques for sealing the wall/slab connection so we don't get a lot of water intrusion?
One reason to use sonotube (Pier) footings could be that your house is built on expansive soils and/or have high PVR. If that is the case, you are required by code to consult with a Geotech Engineer before any type of addition to your house. A soil test will determine the best solution for your foundation.
I'd bite the bullet and replace the sono-tubes on the wall adjacent to the garage with a poured concrete foundation on proper footing. Since you'll need to excavate for the garage foundation anyway, maybe just make the footing big enough to support a concrete foundation wall thick enough to carry both the existing addition and the new garage. Shouldn't end up too costly, since you'll need to shore up the existing building anyway, as the excavator pointed out.
There are ways to excavate and pour the wall in phases, but wouldn't this be an ideal candidate for a shallow frost protected foundation? I.E gradebeam type foundation. The thickest gradebeam a typically don't exceed 24", and for a garage a 12-18" thick gradebeam would be the norm. By going that route, you wouldn't be fully exposing the sonotubes.
Thanks Rick -- I appreciate that. I have no idea about foundation options other than the standard frostwall, so any new ideas I can throw at my contractor/excavator are very helpful.
John,
For more information, see Frost-Protected Shallow Foundations.
Thank you very much.
I actually see no issues at all. Just do a post and beam garage next to the post and beam addition.
Don't get set in doing a project in just one way. One of the best foundations I know of was used by Frank Lloyd Wirght that uses gravel past the frost line with a supporting beam on the surface of that gravel. Perfect drainage and last forever. The idea comes from train tracks.
The post and beam foundation is a good foundation: greener, cheaper and last a long time done right.