Concrete patio o.k. as subloor for room?
First time post, although been reading for 20 years, does that count?. I’m a HO, not a contractor, and although I’ve done some remodeling I still only know just enough to be dangerous. Due disclosure 🙂
A second floor bumpout done in 1999 overhangs an identically-dimensioned concrete patio below, both likely done at the same time. Approx. 12 X 15 feet. I want to enclose this patio area, frame up around it, & incorporate into the house as a dining room. Using the existing slab would obviously be a huge saver. But this patio slab runs down from 4 to 6 inches below existing finish floor height at the house. (The rest of the house is a perimeter concrete foundation with crawlspace).
1) Short of excavating around the perimeter I have no idea how to show an inspector how this slab meets code for an addition…or for that matter, how to convince myself. Pairs of 6 X 6 posts on the far outside corners carry the substantial load down from the master bath above, so you’d think it’d be built right…right? Or am I headed towards hiring a guy to do engineering calculations? I plan on keeping the (e) posts and incorporating them into the walls.
2) If I can use this slab, what is the best way to frame the sill, float the floor, and avoid any mosture problems. Using oak t&g floor. Bolt the sill plate down, then cut each stud progressively longer to fit as the patio runs out lower? Guess I’d then be cutting blocking high enough to fit between these studs and act as a nailer for the drywall, and then run p-treated 2Xs on edge in the interior of the patio slab to hold the subfloor up off the slab (and furr/shim the subfloor level). But what about ventilation above the slab once I close it in? Can I just close it up tight?
Sorry for the kludgy first post.
Brian in Oregon
Replies
Unless you know for a fact that the slab was poured suitable for a foundation, don't do it.
"Put your creed in your deed." Emerson
"When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it." T. Roosevelt
I suggest that you drill a few holes 10" or so in from the edge of the slab to determine how thick it is. Note if you find reinforcing. You will need to convince any building inspector that it is thick enough to support any walls you build even if they are non load bearing. For the floor you should be able to put a 2 X 6 down on edge and using a level mark a line to rip it on so that you can get two sleepers from each board. I assume that the slab is sloped for drainage. I would use a pressure or borate treated one and put the cut edge up. Depending how wet your ground is put poly on top of the slab before laying the sleepers to slow moisture movement. Do you get frost heaving? If you do talk to an engineer before you build.
hrider-
Thanks for the post. I haven't heard or seen of frost heave issues here in W. Oregon, just lots of rain.
It looks like a substantilal footing, and the fact that this was a permitted job makes me think they had to build it to properly carry the weight of the second floor bump out. But as they say: "trust, but verify"...so I'll check to see if they got the footing permitted & do the drilling.
I like your suggestion on the poly. Do you think an additional water-resistant benefit might be gained by pressure-washing the patio & then hot-mopping it (before laying down poly as well)? The same idea as I've seen on the outside of some new basement wall foundations before they get backfilled?
Thanks again for the tips.
Brian
The suggestions that I have seen for basement floors regarding water is to tape some poly down say 2 sq feet on all sides and leave it for a couple of days to see if you are getting moisture transfer through the concrete. I don't know if this will work for you as it is outside. I don't think that I would hot mop it as this might just cause spalling of the conctete. I have a damp basement and it blows the paint off it so I have let it stay unpainted as I use it as a workshop and I just use carpet scraps which allow the moisture to escape. The poly that I suggested is to protect the sleepers from rotting.
Concrete patio o.k. as subloor for room?
No. Unless it's around 18" thick at the edges (depending on how deep your frost line is) and it has to have a poly vapor barrier under it,