Is slab on grade sufficient for 20x24 1.5 story?

[willie1280]

Is a 20'x24' slab on grade with wire mesh, 4" thick with vapor barrier, 12"x12" grade beams with (2) #4 rods, 1/2" j-bolts and 2" foam sufficient to support the 1.5 story plan?

[mgramann]

It depends on what it is built upon.  If it is a well drained, stable area, then I would suspect it is adequate.  I can't speak for the exact structural dimensions of the monolithic slab on my current cabin, since I bought it already built, but my 22x26 built on a sandy base has had no issues in the 12+ years it's existed.

[rick91351]

Where are you looking to build?  Is engineering required?  Will it be permitted? 

By grade beams are you referring to footings or footers?  They are the thick perimeter mass that walls or slabs sit on.

Looks like you are sort of proposing a monolithic slab

http://www.youtube.com/watch?v=g9haYlEvrHE

If working in and area that requires engineering no matter what we say here will not really apply.   

   

[MountainDon]

Concrete slabs are my favorite foundation to build on. Easy to insulate with rigid sheet foam (special high density for under concrete). Radiant heat is great if you want to go that direction. Maybe even place the tubing even if you might not use it right away. Once the concrete is placed you have a complete solid level floor to begin building on. Make it high enough off the ground so any siding materials you may decide on has proper ground to siding clearance.

Yes, there can be downsides with buried plumbing but thousands of homes in the SW are built on slabs with few issues. 28 years and counting on one here.  Section 4 of the IRC has all about footings and foundations. IMO, an experienced concrete contractor is worth the expenditure.

[Squirl]

Where?  This just came up in another thread.  Sufficient for load bearing?  Sure.  A 12" wide base should be wide enough for most soil types for up to 2 stories of light frame construction. 

But what about frost protection?  12" isn't deep enough for most locations.  I believe even FPSF require 18" (6" above grade and 12" below, I haven't perused it in a while though).

[MountainDon]

Here in my part of the SW it's 18 inches, not at all hard to do. 12" is min I believe even if there is no freezing. If I was in a more northern clime I would probably rethink the slab. Not sure on that as we have absolutely no plans to move, especially to move farther north. 

[Squirl]

 

I scrolled through Willie's prior posts, which is what concerned me.

Wisconsin.

http://countryplans.com/smf/index.php?topic=12894.msg168057#msg168057

Even for a FPSF you would probably need so much wing insulation that you would move less dirt installing a full basement.

[Don_P]

backing up just a second, if you check your loads, very often regular density foam is adequate under a slab, remember there's 144 square inches in a square foot. I'm trying to remember if regular density is 10psi or 15.

[mgramann]

Monolithic slabs are actually used quite frequently here.  As I mentioned, my house is built on one.  The biggest factor is the soil conditions.

[MountainDon]

the regular I have is 15 psi and the below grade stuff was 25    maybe the 25 was more than needed it's a number of years ago we used it and I took the bldg dept word on what to use.

[fcpnorman]

I thought about a slab on grade but was concerned about snow sitting up against it all winter. Are you extending your eave lines to deal with this?? Where I am going to build can have up to 6-8 feet of snow all winter....

[MountainDon]

Not sure I understand... if it was not built on a slab what would it be built on and how do you perceive that being better?  Would this be full time or seasonal use?

[Don_P]

the regular I have is 15 psi and the below grade stuff was 25    maybe the 25 was more than needed it's a number of years ago we used it and I took the bldg dept word on what to use.

There we go, 15psix144si/sf=2160lbs per square foot crush resistance in the foam. We typically design the footings to be able to bear on 2000psf soils. The regular foam provides slightly more crush resistance than typically assumed soil bearing capacity.

[fcpnorman]

I was wondering the opinions on slab vs raised foundation. With a raised foundation you can cut down on on the amount of snow that is resting up against the siding, and perhaps eliminate mositure issues. For me it would be seasonal use. I am thinking that good enclosed soffit with a good steep pitch would keep the snow away, but probably not eliminate it.

Does anyone with slab foundations have issues with water from snowmelt? I know I have seen many home in Lake Tahoe that have standing water in the crawlspace much of the spring from snowmelt.

[MountainDon]

The cabin in the mtns has approximately a 2 ft horizontal overhang at the eves. The snow ends up several feet away from the building. A typical raised foundation as done by most DIY offers much less lateral movement resistance. It is hard to beat a home bolted down to a concrete slab for stability.  A bonus to the overhang on the cabin is that it completely shades the windows in summer.

The issue with standing water means somebody did not properly prepare the ground before building. Insufficient or wrong way slope. Our home on a slab drains away from the building all around and then around one end from back yard to front and eventually out to the street if the amount of water is greater than the couple of small ponding areas can handle.

[MountainDon]

There we go, 15psix144si/sf=2160lbs per square foot crush resistance in the foam. We typically design the footings to be able to bear on 2000psf soils. The regular foam provides slightly more crush resistance than typically assumed soil bearing capacity.

Well its under the slab now and staying there 

[fcpnorman]

I like the idea of the slab because it is both easier to do and harder to screw up........! I have though a lot about it and that may be what I end up doing. Many of the houses in Tahoe were really cabins turned into year round homes- so they were not always built properly. My ground where I am going to build has a gentle slope, so the slab would work.
Thanks for the info.

[Don_P]

Quote from: Don_P on Today at 05:59:47 PM
There we go, 15psix144si/sf=2160lbs per square foot crush resistance in the foam. We typically design the footings to be able to bear on 2000psf soils. The regular foam provides slightly more crush resistance than typically assumed soil bearing capacity.

Well its under the slab now and staying there 

Probably more than a weekend job  .
It's a little bit of a racket or it just seems like a good idea at the time, and there's nothing wrong with using high density. I was hoping to keep that nickle in others pockets. That little bit of math above should help an inspector see it. An engineer saw high density on my specs on a basement garage job and took me through the math.

A way off topic aside that popped to mind,
There are large commercial buildings on rafts of foam over poor soils. They remove the weight of the building in muck, or whatever portion the soil cannot bear, and replace that with lightweight foam. The building on top, weighing no more than the soil removed doesn't sink in the weak soils.

[Patrick]

Not sure if this will be helpful but I am in a suburb of the twin cities(minnesota) and how we did my shop was removed all the top soil then brought in washed coarse sand to float the slab on we made 16"x16" perimeter footings with a 6" field 2 rows of #4 in the footings and #4 rebar every 6 feet tied to the footing rebar then 6" mesh over the whole thing 20x40' there is no vapor barrier or insulation there is one coarse of 8" block the roof has 16" overhang with no gutters the top of the slab is about 3" above grade Its around 7 years old now and has been problem free not even a crack. There are alot of slab houses in this area but  most are 1 level.