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rtoni 07-18-2007 12:01 PM

question on suitability of monolithic slab
hi all - I'm in the process of planning a foundation (addition for camp / cottage). The original plan was to excavate approx 3 feet (to level ground) then pour footings (24" wide x 8" deep - running approx 30x40 overall) then concrete block wall (4 courses using 8" block), followed by exterior rigid foam for footing / wall insulation (for frost protection - with suitable cover to keep critters and weather from destroying the foam) - and finally approx 2 ' of backfill (with suitable drain tile / sump pit in place as well). basically a frost-protected crawl space.

here's the question - a friend suggested I should do a single pour using the monolithic slab approach - build the concrete wall on top of that - the idea being that I can have a concrete floor in the space without having to do a 2nd pour later. The problem is that this entire slab will be about 2 ' below grade when all is said and done.

after reading some related posts, I get the feeling that this approach (monolith) is only suitable for on-grade applications - and below grade I should stick with the standard approach (footing first, then wall, then floor - in 3 stages).

is this correct - is a below grade monolithic slab a really bad idea in this scenario? if so, why? Assuming I do a decent job with drain tile , will water come in under the wall and along the floor, etc...?.

just trying to get my head around how much trouble I might get into by trying to save a step (the floor pour) later...

thanks in advance


Ron6519 07-18-2007 05:08 PM

The concrete slab you described is what they call a "rat slab" around here. This isn't something you buid on, its 2-3" of concrete poured against the foundation walls to keep critters from digging their way in.
With slab on grade construction you are pouring a 4" slab that has framing members that are attached to the slab.
As long as you keep the footing/foundation below the frost line, install a membrane on the exterior along with the perimeter drainage system you should be good to go.
I'd also fill the block with concrete. I just do not like hollow walls. Just my idosyncrasy.

rtoni 07-19-2007 11:48 AM

critters are a big concern
rat slab - that's a good description - I've seen the damage the critters do to a building once inside (do not want to go there again) so concrete across the floor is a must.

so you're saying that either the rat slab or the slab on grade (which in my case ends up below grade after I backfill the walls) is ok then, provided proper membrane / drainage is in place?

I may still consider the rat slab approach - seems like even though this may take more time (and another pour) I'm wondering if it would be a bit easier for a DIY to handle (do I want to mess with 1000 sq ft of single pour, vs footers only, then a thin floor later - this may be easier for me to manage). If I do the rat slab approach do I need to worry about wire mesh in the floor later (is 2 or 3 inches too thin to even consider embedding a wire mesh - what if I have a 6 mil poly under the concrete?

also - do I need to worry about any finishing work beyond just leveling / screeding the floor - not worried about "pretty" - just want solid and functional (it's a crawl space) - thinking I can just level it out with 2x4 and leave it with that rough finish. if I try to float this out a bit will I likely do more damage than good..?

any thoughts appreciated


Ron6519 07-19-2007 05:49 PM

There's no reason you couldn't pour the footings and the slab all in one pour. Just frame out the box and fill it with concrete. You just need to tie the footing to the block with some sort of rebar and the new footings to the old footings, again with rebar.
I put 2" foam board to minimize cold transfer to the crawl space. I have water pipes for heat and sink running through it. Also put two plastic layers as per insulation manufacturer. No need for wire, the heaviset thing that goes on it is me. The guys just troweled it. But it's smooth enough for the mechanics creeper to slide along.
I would get some help with the pour. Do you know how many wheelbarrows of concrete that is? You should have two guys hauling concrete and somebody to do the leveling and smoothing. Maybe two people. If you do it in the hot weather it will set up pretty quickly.

rtoni 07-20-2007 11:10 AM

will do - thanks
Thanks Ron - appreciate all the feedback and the help

(yes I'll have some help to do the work - but we're all pretty much amateurs beyond the grunt work - so the advice is much appreciated)


GarageGuy09 03-31-2009 05:18 PM

general questions
Hello all, I have a few generalized questions for a garage that I am about to build. Firstly, I am going to be extending my current garage, 18x24, with a new 18x24 garage and am wanting to know if I have to creat a footing where the 2 cement pads will be meeting, and what the best way is to connect the 2 slabs together. I am thinking about drilling holes, adding liquid concrete in the holes and then driving in rebar bars to join the pads. Is this ok, is there another way to do it. Also, is a monolithic slab the way to go. I feel that it is the best way but when will I need to add the block for the perimeter wall, when the concrete is wet, or when it dries, and if so what adhesive is to be used for the blocks. Many thanks for any thoughts that you might have, Cheers.

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