Garage Foundation Options in the Northeast

[frankd]

New member here. I came across this site while doing some research for a garage and I'm hooked. I was hoping you guys could help me out with an upcoming project.
Me and my father will be building a garage at our cabin in upstate NY. We're currently fighting over the foundation. The structure will be 20x30. Walls will probably be 10ft. My father initially wanted to build this thing right on the dirt which I think is ridiculous. He initially thought that we needed a true foundation with a 36"-48" footer which would be very expensive. I called around and was told that we could do a monolith slab/alaskan slab. I was quoted anywhere from $4-8k if we prepped the site. Of course my dad being the guy that he is, he wants to tackle this himself in an effort to save money. I offered to just pay for it to save us the headache since I'll ultmiately inherit this place anyway but he refused. We had a neighbor prepare the size with his dozer a few weeks ago. Right now the tentative plan is to put down 6" of gravel, and on top of that we would put down 1" XPS (Foamular 250) and then pour a 6" slap that's 12" around the perimiter. Reinforced with rebar of course.
I'm not sure the XPS is necessary and have heard opposing arguments but for the extra $400 I'm thinking it's probably worth it.

Does anyone in colder climates have experience with monolith slabs? I know there's another thread out there on this topic but I was hoping to get more info. Is the XPS really necessary? Also, the property tends to get pretty wet in the spring so I am concerned about the amount of water in the soil. I guess all that I can do about that is try to make sure I have proper drainage around the building.
I appreciate the help!

 

[jwhcars]

Since the ground gets pretty wet perhaps a pier build would be another choice.
You can insulate the floor and not have worry about water.

 

[frankd]

When you say "pier build" are you referring to framing the floor out of wood and putting the whole structure on cement piers that are buried below the frost line? Or building more like a pole barn and using a dirt floor?

 

[Chuck]

In that climate you'd never regret the XPS. Just make sure it's the under-slab rated variety and not wall insulation, which is too low strength. If you have soil moisture and ever plan to have any sort of tile, epoxy, or sealer on that slab, a layer of 10 mil plastic directly under the concrete and over the XPS will also be very much worth the few bucks.

Make sure the XPS runs all the way out to the edge of the slab, and up the side.

Otherwise, for soil moisture problems, a lot of it is drainage just like you said. A perimeter drain around the base of the wall to get water out of the crushed stone base is your main line of defense. Ideally it should drain downhill to daylight. Beyond that, make sure the pad is built up high enough that you can get a reliable slope away from the wall for several feet on all sides.

 

[p_mori7]

Mine is built on a monolithic slab, and I am even further north than you are.

No problems.

Photos in my build thread link in my sig below.

 

[frankd]

Thanks Chuck! The "foamular" XPS is rated for under slab use. I figured I'd go with the 1" thick version rather than the thinner ones. I'll also add the 10mil plastic barrier. It can't hurt and it's cheap. Since the site prep was done fairly recently we're keeping an eye on the area to see how wet the surrounding area is. Might have to dig some channels or french drains to divert water away from the area.

 

[frankd]

Mine is built on a monolithic slab, and I am even further north than you are.

No problems.

Photos in my build thread link in my sig below.

Great build thread, thanks!

 

[Chuck]

...Since the site prep was done fairly recently we're keeping an eye on the area to see how wet the surrounding area is. Might have to dig some channels or french drains to divert water away from the area.

Glad to help! It also never hurts to add more stone. If it's not staying dry, you can build the pad up higher with it. Gives you a little bit more "reservoir" to hold water while your perimeter drain carries it out. As long as the stone is well compacted, you aren't going to have any strength problems, and its primary job is to serve as a "capillary water barrier" to keep the water from wicking up to the bottom of the slab.

Once put a 3/4 million pound concrete structure in the middle of a mud hole you couldn't stand in without sinking to your waist (literally, coworker had to be lifted out with an excavator from a nearby dike). Excavated the mud and put in twelve feet of compacted #57 stone. It's still there and level seven years later - there's really no such thing as too much stone as long as it's properly compacted and there's enough in the wallet to get the stuff.

 

[jwhcars]

When you say "pier build" are you referring to framing the floor out of wood and putting the whole structure on cement piers that are buried below the frost line? Or building more like a pole barn and using a dirt floor?

Cement piers below the frost line. You can make the cabin as high off the ground as you want.

 

[p_mori7]

there's really no such thing as too much stone as long as it's properly compacted and there's enough in the wallet to get the stuff.

 

[Teikas Dad]

I added a 32X24 addition on to my barn a few years ago for a workshop. I used a Frost Protected Shallow Foundation on mine. All the info is here, start at the bottom of page 5: http://www.nahb.org/assets/docs/pub...ected-shallow-foundations_1211200244041PM.pdf

I'm in NW CT and it gets pretty cold and frosty here. They use FPSF foundations in Alaska so I think that it would work anywhere in the NE. When I had the mason come in to build the foundation for me he didn't know what I was talking about. I printed out the booklet in the link above and gave it to him. He really like the process and did a great job with it.

 

[Stuart in MN]

Mine is built on a monolithic slab, and I am even further north than you are. No problems.

They're very common in Minnesota, so you know they work in cold climates. My garage was built on one and it's been fine for 20 years now (and the slab in the garage it replaced was fine for 40 years before that.)

 

[600SL]

A monolithic slab can be built anywhere. How deep the sides of the monolithic slab are is according to local codes. My slab is 18" below the soil level code here in NC requires 12"

 

[ConCretin]

You basically have three choices;

Frost walls, which extend below the frost line and protect the structure from movement. It's the most expensive but it's a tried a true system. Frost walls are a necessity if you're connecting to an existing frost protected structure. You should still protect the slab sub grade from freezing by placing insulation around the perimeter.

A shallow frost protected foundation consists of shallow foundations or a mono slab that uses frost resistant materials and insulation to keep the frost from getting under the structure.

The third option is the floating slab. It's the cheapest but involves an element of risk because it moves with the frost. It's very important to have a free draining, granular base that will be naturally frost resistant. It's also important to properly reinforce the slab and perimeter haunch to ensure everything stays together if it starts moving around.

I built a 28 x 32 garage on a floating slab up here in sunny Maine and wouldn't hesitate to do so again. I haven't seen a single crack or noticed any differential movement at all.

You just need to understand the characteristics of the system you choose and build accordingly. Good luck with your build and don't forget to put up some pics.

 

[frankd]

Thanks again for all the great info! Will probably get the gravel delivered and put down in a few weeks. But the actual cement pour may have to wait until spring. Either way, once we get started I'll take plenty of pictures.
I appreciate all the help and feedback!

 

[Radix2]

I added a 32X24 addition on to my barn a few years ago for a workshop. I used a Frost Protected Shallow Foundation on mine. All the info is here, start at the bottom of page 5: http://www.nahb.org/assets/docs/pub...ected-shallow-foundations_1211200244041PM.pdf

I'm in NW CT and it gets pretty cold and frosty here. They use FPSF foundations in Alaska so I think that it would work anywhere in the NE. When I had the mason come in to build the foundation for me he didn't know what I was talking about. I printed out the booklet in the link above and gave it to him. He really like the process and did a great job with it.

that is a good publication, but I note that it says that the FPSF designs in it must only be used in buildings that are maintained over 65 degrees - and should never be used under porches,garages or any other unheated space (use a deep foundation in those areas)

It also says that there are alternative designs for unheated spaces, but they may be more expensive than conventional deep foundations....and are not code approved in the US..


So what gives ?

Are you guys committing to keep these garages 65+ all the time or are you doing something more complicated than the shallow perimeter and insulation method in that document?

It may be that with heated floors, one would only have to maintain a slab temp of 65 or perhaps lower considering that a 65 forced air building would have a colder slab.... So maybe a 50 or 55 degree slab would be enough to make the system work ...?