Slab/no slab in frost-protected foundation?
Robert Swinburne
| Posted in Green Building Techniques on
A frost protected shallow foundations is basically a tub of foam with a concrete slab in it. And very carefully laid out plumbing and utilities. I am wondering about leaving some of the slab out in places such as under a bathroom and simply framing that section in so the finished floor is level with the adjacent concrete floor (at doors) The framing would serve as a stop during the slab pour and prevent movement.
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Replies
Robert,
can you expand on why you want to do this and where else, except for the bathroom, you think it would be a good idea?
Robert, I would be in favor of an intact slab. If you think you might move major things like plumbing and utilities, and where the straight pipe for your passive (for now) radon tube goes, etc, you probably need to spend some more time planning. IMO. No reason to put a hole in a perfectly good slab.
Further, as one builder posted on here some time ago, people make changes to slab houses all the time. Concrete saw, blah, blah, changes done in a day. It's not the big deal that people who aren't used to building slabs think it is...that's my impression. So changes can be done later if required.
...reading your post again, it's not clear why you may want to leave part of your slab unfinished. Is it because you think you'll make changes?
My experience as an architect is that even with years of planning and models and discussions, the client will move things around and make changes during construction or one of the subs will make a mistake resulting in some "improvisation" Also trying to get clients to make exact choices of tubs, toilets, showers (lots of decisions there) is darn near impossible in a timely fashion. I couldn't think of any ramifications so I thought I'd put it out there to see if others could. I was thinking of leaving the bathroom area out and hiding the joint under walls. Part of being an architect is knowing when you are dealing with clients that will make changes.
Robert,
I don't see why not. You leave a hole for the drains under the tub and shower anyway, why not make it bigger? I fall on the opposite side of the slabs are easy to renovate debate than Andrew. Anything that makes both the initial construction and future changes easier has my vote.
How would you detail it? I'd be inclined to have the rim joists of the floor set just inside the room so that the subfloor could be installed after the walls were all framed, leaving the maximum time for late changes.Structural screws half sunk into those perimeter joists so they tied into the slab when it was poured might work to keep everything level.
Robert,
The type of assembly you are proposing -- wood floor joists that either rest directly on rigid foam, or float a few inches above a layer of rigid foam -- makes me nervous. If you really can't get the homeowners to decide where to put the toilet before the slab is placed, I would leave a void for the bathroom when the slab is placed; finish the plumbing rough-ins after the homeowners got their act together; and then I would bring in the Ready-Mix truck to fill in the rectangular void in the slab with concrete.
There are code issues here: Does your proposed framing create a crawl space? If so, some code officials might rule that the height of the crawl space must allow for human access, and so a 3-inch-high crawl space won't pass muster.
Although the framing you propose will probably stay dry enough to avoid rot, it's a little worrisome. Pressure-treated lumber could be used, of course, but still -- a slab is a slab, and a crawl space is a crawl space, and this is more of a mule than a horse or a donkey.
Robert,
One last point -- a minor one of vocabulary: I think that "a tub of foam with a concrete slab in it" is an insulated raft foundation, not a frost-protected shallow foundation.
The framing would sit directly on the foam with no crawl space. I saw the issue being protecting the vapor barrier during plumbing. It would solve a problem also with doing a sloped no barrier shower drain (I wouldn't have to rely on concrete contractors to form it.)
Instinctively, like Martin, I was worried about the durability of the floor framing. But the joists are at the same elevation as the slab, and in the same relationship with the vapour control layers as the wood framing on top of the slab would be - or for that matter the framing of a PT wood foundation. To meet code the joists have to be PT, and it's probably a good idea to use ground contact approved lumber, but I don't think there is any more chance of moisture being trapped under the subfloor than there is under the slab.
I would look at Frank Loyd Wright's foundation. Uses the same principal as railroad tracks. Instead of foam it uses gravel below the frost line on the perimeter so water is drained around the cement pad. The main footing is on the surface not below the frost line. Saves cement and is built to last. You could still add foam, but the foundation isn't depending on foam. This techinique solves drainage around the house while the frost (foam) protection shallow foundation doesn't address this issue. The future home owner doesn't destroy the integrity of the shallow slab in future remodels.
We had to cut a commercial slab to add another dentist chair not too long ago. my job was to do the under slab Plumbing. When I arrived Friday evening as we started the job the guy that cut the slab already had the slab cut and removed and we still had three days left to finish all the work so the slabs not a big deal in the commercial world.
True, but cutting into the thicker edge of cement pad that supports the building could be.