How to attach, if necessary, a 4″ slab on tamped gravel with a perimeter foundation to the existing house. The job is in Kentucky. The existing house is brick veneer with a basement and the new slab’s height is even with the existing house’s wood floor (on 10″ joists). The length of the addition along the house is 31 feet. The footer and concrete block stemwall of the addition, ( on the three exterior sides ) will be pinned to the existing foundation, but what about the 8″ thickened slab edge along the existing house? This edge carries only its own weight and a 2×4 partition wall. Extend down piers and pin them to the existing concrete?
Edited 4/25/2007 3:55 pm ET by boonedoc
Edited 4/25/2007 3:56 pm ET by boonedoc
Replies
Greetings boonedoc,
Somehow your post got lost in the cyberworld and arrived here way late.
Regardless, perhaps the subject can be salvaged so
this post, in response to your question, will bump the thread through the 'recent discussion' listing again which will increase it's viewing.
Perhaps it will catch someone's attention that can help you with advice.
Cheers
-Thoreau's Walden
From the description, is this what you're talking about:
View Image
The short answer is likely, if you're aligning the new slab to the existing subfloor, then, generally, yes, you will want the turndown deeper so it cant be doweled to the existing foundation. Unless you can expect less than 1/4" of movement (or your engineer can). Could you use piers & a beam? Dunno, back to needing an engineer's opinion on that.
This is why I generally prefer to match floor to floor, so that slabs are not abutting veneer brick, or rim joists, or the reverse.
Thanks for the feedback. The sketch you sent is right on.
Yeah, extending the slab edge downward to abut the existing foundation and then pinning with rebar dowels in epoxy....keeps the slab at the edge immobile, a very good thing as the new floor would be always at the level of the existing at the door (3'). Also the other three sides are fixed as they are resting on foundation as you have sketched. My concern: I read slabs rely on the tamped gravel bed and soil beneath for support, so if the support in the middle settles and the edges can't move I'm guessing the stress would cause major type cracks where one side is higher than the other? The soils around here are stable, but even a little settling would stress the slab?
Another thought....will removing some brick veneer , say 2 rows of about 3 bricks wide directly on top of the ex. foundation, allowing the new thickened edge into the space thus created, be another way to allow tabs of support bearing directly on the ex. foundation?
I appreciate your ideas.
will removing some brick veneer , say 2 rows of about 3 bricks wide directly on top of the ex. foundation, allowing the new thickened edge into the space thus created, be another way to allow tabs of support bearing directly on the ex. foundation?
Could, but, then you're committed to removing all the courses above that, or find a way to repoint them back in after the slab goes in, and then bearing on the new slab.
Begs a question, that. How much of the existing wall will be demolished to get into the new addition? Are you abutting a gable wall or the like, and the addition "bumps" into that? See, thinking about the bottom courses of brick started me thinking of the top courses--how that airspace behind the masonry is flashed/sealed/etc. turns into an issue (having the wall "weep" into one's addition can affect the decor a bit <g>).
Your questions are part-and-parcel of why my first "reflex" would have been to specify a perimeter wall foundation with a crawlspace and a wood floor to match the existing floor structure. (All while being of two minds about "hanging" a ledger, deck-style, for the floor from the veneer brick vice making a partial foundation wall against the existing.)
The subgrade for the slab is critical. The least-best compaction, by necessity, will be up against the existing structure. You "get" some "back" in pouring to a perimeter beam (you are isolating the subgrade from moisture changes/swings). But, you still have to compact either around formwork or a drying perimeter wall--which is not a machinery-friendly situation.
Now, having noodled on this a bit, I'd recommend putting in a footer right around the four sides of the addition. Then, "frame" the perimeter wall in ICF, also four-sides. Within that perimeter, you could then use carton forms, and go with a stronger slab, say a 6" 4000psi with #4 or #5 at 12-15" OCEW (obviously, you'd need a locally-qualified engineer to sign that off, not some hatchet-maimer on the internet <g>). Your plan for the addition would best determine whether to brick ledge from the ICF or from a let-in brick ledge in the slab (or at all, the addition might not use brick).
But, that's me and my experience, education and/or belief (to use the split-tongue weaselwords); others differ.Occupational hazard of my occupation not being around (sorry Bubba)