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vtjpm 12-30-2005 08:57 AM

slab thickness in the northeast usa
I plan on building my oven in the northeast, usa (vermont). I'm concerned about frost heaves, etc. Is the standard slab, 6" or so deep enough? Has anyone had any problems? Thanks. Jim

james 03-11-2006 01:01 PM

We have earthquakes (we have a huge number of $ in our house on earthquake regualtions), but no freezing.

I have read various comments from builders in colder places on what you need to do to make a slab secure. Anyone have experience with that?


CanuckJim 03-14-2006 06:26 AM

Slab thickness

I live in a somewhat cold climate, though it is considered balmy by Canadian standards. Building code here calls for 4 feet of depth, because, very, very occasionally, it might go that low. About 30 inches is more common. Also, I have worked in stone masonry and house construction in my time, so I'm quite conversant and I have an irrational drive to overbuild everything.

Slab construction methods and thicknesses have a lot to do with soil conditions. Where I am, there's about 18 inches of topsoil, then right to very stable hard clay studded with glacial rocks. For my AS oven, I excavated to the clay level, drilled ten 10 inch diameter sono tube holes evenly spaced inside the excavation and built a 2 x 10 form, staked into the ground along the outside perimeter. The reason for so many sonos is because the pad was built large enough to accommodate a 7 foot long portico in front of the oven. If I was building only for the oven itself, only 5 sono tubes (one dead centre) would be needed. Overbuilding again; 8 inch diameter would no doubt be fine, even 6 inch, perhaps? My feeling was I'm only going to do this once.

Next, I filled the hole up to the bottom of the form boards with clear A gravel, called drain stone here, slightly dished in the centre. Over that I built a 1/2 inch rebar grid, starting with a wired together outline around the perimter about 4-5 inches in from the finished slab edge. Then I laid rebar pieces both across the pad and lenthwise, wiring everythind together. With pieces of brick, I raised this grid to the middle level of the 10 inch thick slab. Then, I bent 10 U shaped pieces of rebar to connect the grid to the tubes by sticking them over the bars and into the tubes (legs of the Us, about a foot on each side). Over that, I laid 10 gauge reinforcing mesh to the size of the grid, also wired on.

The finished slab is 10 inches thick, well beyond code, which is 8 inches. In all likelihood it would survive Armageddon. Even so, I was well aware that there is considerable ground water on my land and that we get severe frost-thaw cycles, where the temperature might raise 20 C one day and drop 30 C below that the next, particularly in January. A 40 degree swing would not be uncommon.

All in all, I'd say knowing as much as possible about the substrate you're building on, realizing that you'll only do it once, understanding that taking every precaution against cracking (of the slab AND the oven) caused by frost heave all add up to building thicker, heavier and more reinforced than you think you need. Building codes are merely minimum guidelines; treat them that way. Okay, I do overbuild, but labour, rebar, gravel and cement are cheap compared to what's coming next.

Hope this helps.


jengineer 03-15-2006 12:49 PM

CJ - thanks for the write-up. A few of us are attempting to repackage the Pompeii instruction string that is found on this site. Essentially it is putting the information into a PDF format and updating it with inovations that the members have posted, such as the island hearth, reducing the concrete thermal subfloor and increasing the insulating subfloor. A request was made of us to include ground preparation and the support slab for those in colder climates. Unfortunately until your post I/we have been somewhat stumped. I know from my tenure in Colorado that a 6 inch concrete driveway would heave each fall/winter/spring as the whole thing floated on the exapansive ground. It would have been less than average to build on it as each ear it settled more in one corner than anywhere else - this as a 40 year old slab. It probabaly did not have the sono-tubes or pilings to anchor it in place

if you don't mind we will edit your instructions and add them to the project

how deep did you drill for the sono tubes? - 4 feet?

thank you


jc3tech 03-20-2006 09:15 AM

Slab recommendations
We are in the early phases of planning our outdoor kitchen which will include some type of masonry oven (still deciding between kit or Pompeii design). I have been working with a friend who is also a concrete contractor to make sure that the foundation will support the weight of everything. He is recommending a 6" slab with 36" (12" wide) footers. I live in the Cincinnati, OH area. I am sure we do not get as cold as the northeast but since I have been here (8 years) we have had 3 winters of 20 consecutive days under 20 degrees F. I can share more details of the pad construction if you want.

CanuckJim 03-20-2006 01:21 PM

Slab Thickness
I live in Ontario, but in the southern part, and our weather here would be a bit colder than yours, but not a lot. (Rochester, south across Lake Ontario, gets much colder.) Anyway, I went for an 8 inch slab, heavily reinforced, with sono tubes down four feet below that. Then again, at times I've worked in the construction business, and I tend to overbuild. Even so, how many times will you be doing this? More, in this case, in my view, is definitely better.


carioca 10-08-2006 12:30 AM

deep clay soil, warm temperate climate anyone?
Hello there!
Interesting discussion about those foundations - but you all seem to be in colder climes. We are sitting on deep expansive clay, starting at about 300 mm depth and going down 'forever'!

For the shed I built a few years ago from aerated concrete blocks I put in 500 mm x 400 mm perimeter trench foundations with six-rod rebar 'cages' - the slab itself was poured about 125 mm thick, with one layer of reinforcing mesh plus huge 18 mm lengths of rebar under the 2400 mm wide roller door openings.

I mention this because the slab STILL cracked neatly diagonally across each and every corner. Luckily the very deep and heavily armoured lintels I poured over the openings seem to stay in place :-)

So, for the putative Pompeii oven, I am considering a post & beam foundation on concrete piles...

Does anyone have any experience with this in clay soil? How deep, thick would the piles need to be?

As I mentioned elsewhere, I am thinking of supporting the round hearth on perhaps 4 to 6 radial rows of concrete blocks - considering the post&beam footings, I could perhaps do without a full slab and just tie the blockwork to the footings properly, and then tie the hearth into this setup with rebar in distributed in the concrete-filled blocks... does that make structural sense? (I wish I'd done more maths and physics in my earlier life!)



Marcel 10-08-2006 07:09 AM

Consider plotting a "graph" for foundation conditions .pdf
(M) I am no longer directly involved in the .pdf rewrite of the construction plans so perhaps my input may be seen as officious. But after reading the complex verbal descriptions of both concerned and experienced builders about how to handle the base slab foundation of your ovens, I'd like to suggest that the engineers on this forum get together and design a graph (or perhaps several for different parameters) that would plot a minimum recommended curve of, e.g. temperature (or latitude) against thickness of slabs. Since your .pdf directions would be in black rather than color, you could use --- for one parameter, ++++ for another, etc. This would allow the builders, in different regions, to extrapolate the thickness of slab, depth of pilings, etc. for the region in which s/he lives.



Versachi 10-08-2006 10:58 AM

Slab thickness, frost etc.
I work in construction here in Thunder Bay, Ontario where we see frost from mid Nov. to mid April. Canuck Jim would be correct saying that he lives in a "somewhat cold climate, though it is considered balmy by Canadian standards". I would see colder weather on a more regular basis than Jim. I don't know that thickness of slab needs to increase in relation to the coldness of the winter. The usual problem with frost heaving is poor soil conditions and too much moisture in the area. If the soil is poor it is best to remove it and replace with 16- 20 inches of granular A gravel and packed properly. Around here it is common to see 2-4 inches of styrofoam underneath the slab and extending outward 2-4 feet from the slab with a 2% slope on it to direct water away from the slab. I would agree with Jim that it doesn't hurt to go with a thicker slab and more rebar. You only have one chance to do the slab so overbuilding it is better than underbuilding. If I get the chance I would like to poor my slab before the snow flies. If I do I will post the pics and you will see some styrofoam under my slab. There is an article in the Canadian Building Digest relating to frost. Here is the link.

carioca 10-11-2006 05:27 PM

what are "sono tubes"?
Forgot to ask first time 'round: what are these 'sono tubes'? And, if they are some type of tubular formwork, why don't they get filled with reo and concrete?

The write-up and replies otherwise gave me a few more useful considerations to keep in mind...



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