Cold Climate Slab on grade with stem wall above grade

[NathanS]

Editing to clarify my question: How do I put the floor system (think the height of the bottom of the doors) about 6" above ground level, but protect the framing of the house from the conditions that occur in the first 24" inches above ground level.

This is something I have seen done in garages, but I can't find much on doing it for a house. Where we live, we have a 4 foot frost depth, and a 50 lb snow load. Most houses around here have full basements because by the time you get 16" above grade, you already have 5+ feet of block or concrete.

I would like to build on a slab with radiant floor heat, but I really can't get behind the idea of melting snow sitting up against the stick frame and gradually rotting the walls. Ideally I'd like to have the stem walls be 2 feet above grade. With a platform framed house, now we're talking about the floor being close to 3 feet off the ground. From a design standpoint, I really don't like being way up off the ground. I'd rather one step down and I'm walking on the grass.

I'm not sure of two things here..

1) not having the edge of the slab rest on the foundation wall. Not too worried about this one... if the soil beneath the slab is drained correctly, I don't think we should have any movement. If there is movement, the whole slab moves (this could push the upstairs floor around..?). If the slab is tied into the foundation and moves, it will crack.

2) the masonry stem wall to stud framing connection. Normally in stick framing there would be a floor here that would structurally hold this joint together. Is the masonry stem wall strong enough to keep the stick framed portion from buckling?

Anyone think there is anything structurally wrong here? The engineer that is designing our septic will probably do the final foundation design for us too. He has already really pushed for a full basement (because he likes having one), but said he will design anything we want.

[NathanS]

I want to add on to this...

I posted this on green building adviser and, surprisingly, the response has been that 8" above grade in an area with heavy snow fall is plenty.

I know some of you are builders... do you agree with that? I have seen a lot of sill rot in older buildings in my area. I even saw a beautiful 1970s timber framed house with ~8" clearance above grade where the timber sill had completely rotted away to the point that the building was beginning to fall over.

Maybe there is some way to detail your wall to prevent this, but it seems wiser to just put snow/slush/water sensitive materials ABOVE the snow/slush/water, when an extra two courses of block costs virtually nothing. All of the old well built buildings in this area are at least 16" above grade, and that's when they were laying rubble.. I don't believe they were doing that to have more head room in their dirt basements.

[Triathlete]

If you are considering a slab and want it on-grade, have a look at Frost Protected Shallow Foundations (FPSF).  It's widely practiced here in Canada where frost levels go down below 6'.  And rather than use concrete, you might consider building crawl space walls using only pressure treated wood.  The building process is fast, cheap, and proven.  Crawl space walls are insulated and the crawl space is reasonably warm.

http://www.publications.gc.ca/collections/collection_2011/schl-cmhc/NH15-457-1998-eng.pdf

http://www.albertasoils.com/docs/revisedfpsfguide_sept2004.pdf

[MountainDon]

Seems to me if the frost depth is 4 feet I'd give a lot of thought to a full basement. I grew up in Winterpeg, MB and at times I do miss having a basement. Basements are rare in NM where frost depth may be only 12 to 18". 

You can still do radiant heat in the floor. In fact I would think you could do the basement slab floor and the main floor on independent loops.

And I'd have enough height above grade for the main floor so I could have basement windows.

Hmmm... that sort of describes our old house....       

[azgreg]

Seems to me if the frost depth is 4 feet I'd give a lot of thought to a full basement. I grew up in Winterpeg, MB and at times I do miss having a basement. Basements are rare in NM where frost depth may be only 12 to 18". 

You can still do radiant heat in the floor. In fact I would think you could do the basement slab floor and the main floor on independent loops.

And I'd have enough height above grade for the main floor so I could have basement windows.

Hmmm... that sort of describes our old house....       

Don't forget the stairs Don.

[Triathlete]

Don't forget the stairs Don.

Basements add a lot of cheap space but as we age, stairs become a real chore and a potential danger.  Life is full of compromises and I've learned that we have to choose what compromises are "best" for us.

[azgreg]

Basements add a lot of cheap space but as we age, stairs become a real chore and a potential danger.  Life is full of compromises and I've learned that we have to choose what compromises are "best" for us.

Oh, I know. Don has mentioned how much he hates stairs in the past here. Just reminding him. 

[MountainDon]

True. But if you have to dig for a 4 foot deep foundation footing a basement can be worth it.

[NathanS]

MtnDon, all your points are completely valid. Doing it this way is mostly a design choice (with some $ savings, smaller heat system, more energy efficient, using the slab as a finished floor). At the same time, I do not want to sacrifice having a masonry stem wall to have a slab on grade. It really doesn't make sense to me start the framing 8" off the ground in a cold-humid climate.

Triathelete, I am very familiar with FPSF. I was leaning that way for a long time, but I actually think the home site we selected has a little bit too much grade for it to be cost effective. Also, the prescriptive method accepted by the IRC does not have a provision for anything over 12" above grade. I am way more comfortable laying block than doing form work for a monolithic pour. Lastly, I would prefer not to have any insulation on the outside of the foundation below ground level. I think that battle will be lost to pests/ants over the years.

[MountainDon]

You need to dig the trenches for the footing and foundation wall. Where does the removed earth go?  Pile it inside and tamp / pack down with a plate tamper?  That would elevate the slab some.... Make any sense?

[NathanS]

Not sure if there might be some miscommunication here... We want the slab to be a few inches above grade. Roughly 6" from the top of the slab to taking a step down onto the grass. That being said, I think it's a terrible idea (in this climate) to start framing 6" off the ground. That's why I am thinking a stem wall that rises another 8" or 16" for the sill plate to sit on would be a pretty bulletproof solution.

I am feeling a little more comfortable with the idea now... I had only ever seen it done in garages, but I found a few pictures that show what I'm thinking of doing:

Notice the stem wall raises 6-12" above the rest of the slab.


Outside completed view, stem wall with stone veneer.. notice the french doors sit below the stem wall. This is exactly what I was thinking of doing. If the doors need to be replaced in 20 years, that's fine.

[Don_P]

I've wanted to build one with stone to the windowsills and timber above that. I prefer to have wood well above the water if at all possible.

[NathanS]

Don P, glad to hear that you think something like this could be a good idea.

[azgreg]

You're thinking something like this?

[MountainDon]

I think he means something like that with a couple of courses of CMU on top so none of the wall wood is close to the ground.

[NathanS]

azgreg.. it looks like the top of that slab will be below the top of the stem wall, so yes, similar to that. I think I want the stem wall to be closer to 2' above grade though...

I am thinking CMU because it is more DIY friendly than doing formwork.

[azgreg]

OK, I get what the Rock is cooking now. 

[NathanS]

I edited the first post to try to clarify what I'm asking... I also added Slab..."on Grade" to the post title. the real question is -

How do I put the floor system (think the height of the bottom of the doors) about 6" above ground level, but protect the framing of the house from the conditions that occur in the first 24" inches above ground level.

The safest (and simplest) solution to me seems to use rocks (CMU/Concrete) for the first 24" on the exterior, and then start framing. There may be some other solutions out there, though.

[azgreg]

For some reason I'm thinking of Matthewpeace's build here: http://countryplans.com/smf/index.php?topic=13742.25 post #33. Just set the level of fill where you want it.

[Don_P]

Or another way to do it... why start framing? A superior precast insulated wall with window and door rough openings cast in. Applied finish can be whatever.

[NathanS]

Don... $$$

[Don_P]

Put pencil to paper, I'm not sure. The erected wall is insulated, furred out and ready for interior finish, sleeves are in the studs ready to pull wire. The exterior is not degrading while you get round tuit. Swing in trusses and it's framed.

ICF's might be another thought. A friend built his out of them and buried 3 sides about 4' deep then a walkout at grade onto a covered patio. A very efficient home.

[NathanS]

You might be right about materials costs. I have never done any concrete formwork though. If I have to hire people to do the work for me, then the price of the house could double.

[Don_P]

Superior walls, if available there, is a precast foundation wall. You have a gravel footing trench ready and they come in with a crane and a truck full of insulated concrete panels, swing them in and bolt them together and the walls are done in a day. From there put in the foam, plumbing, slab wire and radiant tubing and call the flatwork guys to pour the slab.

For ICF you stack and brace the foam legos, brace, and call the concrete and pumper trucks, then later the slab as above.

[akwoodchuck]

Hmmmmm....DIY, cost considerations.....treated wood foundation sounds like a shoe-in. Inexpensive, easy to insulate and finish, need little more than a hammer and skilsaw.