Can Slab on Grade Garage be attached to House w/ Foundation?
I need someone familiar with building in New York State and the NYS Building Codes. As I’m reading the codes, it sounds like for a garage, I can build one Slab on Grade. What I cannot figure out is if this garage must be detached, or can it be attached to the house with a traditional 4′ tall foundation?
As those who have read other questions from me, I’m expanding the house, including adding a new garage (to replace the existing one). I was originally going to go with a PWF construction, but after reading more into it, it’s not as simple as installing either a poured concrete and/or precast concrete foundation. So i’m leaning towards not going in that direction, and using precast concrete walls for the room expansion, and I’m hoping to use Slab on Grade for the Garage space, as long as it’s not risky to do so. I do have some concerns that the garage and house would “shift” when the ground freezes and thaws but it looks like the code allows for it, but that doesn’t mean it’s prudent to do so. I don’t want the building splitting and falling apart on me.
The original garage is a concrete slab poured within a 4′ foundation. But if I could go with a Slab on Grade design, tt would make construction so much simpler and easier if all I need to do is pour a footing for the perimeter of the 25’x 40′ garage floor slab, and build the 12.5′ tall ceiling on top of that, than dig for a 4′ foundation. I have been reading about “Frost Protected Shallow Foundations” for unheated spaces, which the garage will be. There will also be a storage room that will be unheated in between the existing house and the garage. So I’m not sure how having one of the four walls having a 4′ foundation wall would impact the viability of using the Frost Protected Shallow Foundation design, as it looks like it requires some form of insulation to be embedded into the ground . So if anyone has actual experience with this type of design, if you could shed some light on that design as well, it would be very helpful. Thanks.
Ken L.
Though I keep reading that the footing needs to be the frost line, so does that mean I need a 4′ deep foundation? Or can I use gravel/crushed stone and dig a 4′ deep trench so the base of the footing is below the frost line?
Any and all help explaining what I can do, I will be very grateful.
Replies
Hi there, I've built a few additions for houses using frost protected shallow foundations. The only reason we did so was because we were in very tight neighborhoods with small houses right on the property lines. Digging for a full basement or even for a crawlspace where our frost depth is 48" would have required a bigger machine than we could maneuver on site, and the necessary "over-digging" risked undermining the adjacent properties rubble foundations. Since these additions were heated with radiant tubing in the slab, they were less risky since the ground below was unlikely to freeze and heave. The exterior insulation was carefully engineered and installed to meet code. These monolithic slabs were also the finished floor, which makes it very tricky. You need good weather, and to be very careful working on top of it. We did pin the edge of the monolithic slabs to the poured concrete and granite walls too.
If you are exploring the idea of a monolithic/thickened edge/frost protected shallow slab as a cost saving measure, think again. It will use less concrete, but still involves quite a bit of excavation, and very critical sites prep and formwork, all of which is usually less familiar to the guys doing the actual dirt work and concrete. A large chunk of the cost of excavation is the mobilization and logistics of gettong machinery and materials on site. The digging itself is usually straight-forward. I would recommend getting footings and frost walls poured for both your garage and addition. The slabs can then be poured anytime really, but typically after the walls and even roof, or floor above, are on so the slab is protected from weather, leaves, debris, etc. As for tying new foundations to existing, there are standard rebar pinning schedules. Just my 2 cents... good luck!
For the Frost Protected Shallow Foundation system, is the Foam used the same kind of rigid insulation used on houses (i.e Formular-250) or is it something more unique and special to buy?
Thanks.
Ken L
Also, looking at this guide:
https://www.homeinnovation.com/~/media/Files/Reports/Revised-Builders-Guide-to-Frost-Protected-Shallow-Foundations.pdf
It provides diagrams and information on several options except the one I have, which is this:
I have decided to go with Precast Concrete for the Crawlspace walls (4' deep) for the new room for the house, BUT for the attached garage that is next to the new room, I was hoping to use a Frost Protected Shallow Foundation (FPSF). This should save immensely on costs as the Precast Concrete is going to cost $20,000+ to include the walls for the garage. The garage is 60% of the walls so i'm hoping to significantly reduce that cost by not using walls for the garage. I am researching as much as I can, but I can't find the answer can I use FPSF attached to a standard foundation wall. Does anyone have a guide or specs on where the FPSF meets the standard concrete foundation wall and what needs to be done? This guide has been helpful, and it shows various options, but not this specific one.
FPSF is not a viable option for the room because there isn't enough room on the two "exposed" sides (i.e. sides that are not either attached to the house or attached to the garage) for the insulation in the ground (it would violate the building and health codes for being too close to a component of the septic system, the garage is past the septic system so there is no issue with the having insulation in the ground. The concrete wall is far enough away, but the insulation violates the "no building" zone, and I rather not take a chance violating the health codes on the one side).
Thanks all!
Ken L