At 06:25 AM 8/6/2009 , David Williams wrote:

Hi Norbert,

Looking into pouring the slab and we're planning to just cut out the 52"x52" square out of the the floor and using the floor framing as the form to pour in the concrete.  Some questions for you:

4" of 3500 psi concrete, with 10mm rebar on 6" centres, in both directions.
Often, the slab just sits on the last course of blocks, so the thickness is whatever (4" min), rather than adjusting the block height.

Good question.

I have been ignoring it for many years, as with our heater there is very minimal heat transferred to the slab, because of the insulating base slab in the core.
However, ASTM E-1602 calls for it, and it is in the new IBC building code in the States.

I think Tempcast deals with it simply by making the slab 2" bigger, ie, filling in the space with concrete.
The by-the-book way to do it would be to fill the space with styrofoam, and then pull the styrofoam to create an air space.
Another way to do it would be to use 2" of rigid non-combustible insulation, such as Roxul Drainclad, and leave it in place.

Depends on how by-the-book your local inspector is. From strictly a fire safety viewpoint, with our core the 2" doesn't do anything useful,
because the temperature rise in the slab would be 20 degrees, or less.

I'd recommend just adding 2" to the hole, and filling with concrete, as a compromise.


Date: Fri, 07 Aug 2009

"Brian Klipfel" <b_klipfel(at)>,
Alex Chernov <alex_stovemaster(at)>,
Dan and Barbie Givens <stonecastle(at)>,
Jerry Frisch <lopezquarries(at)>,
Dave Misiuk <misiuk(at)>,
"Marcus  Flynn" <Contact(at)>,
Scott Goodman <goodman_s_r(at)>,
Steve Bushway <sbushway(at)>,
William Davenport <info(at)>,
Joel Dick <jdontario(at)>,
tristan fortin lebreton <tristanfortinlebreton(at)>

From: Norbert Senf <mheat(at)>
Subject: Fwd: Cape Breton

Hi Marcus:

Thanks for sharing your method. I'll forward this, as it is of general

Best ........ Norbert

From: "Marcus Flynn" <contact(at)>
To: <mheat(at)>
Subject: Cape Breton
Date: Fri, 7 Aug 2009

Hi Norbert,
           There needs to be 2 inches free space between a foundation slab
and the structural members of the floor.

           I have the hole in the floor, framed 2 inches larger all round
the slab, but have the sub floor cut to exactly the footprint of the heater
and chimney. Then I form from below with 2 by 8 planks, with removable shims
between the form and the framing of the floor. After the pour the planks are
removed leaving a nice 2 inch space, while leaving the sub floor in place to
support the finished floor above the 2 inch space.
          Before starting I remove half an inch all round from the sub
floor, so that it doesn't actualy touch the first row of facing.
           The 2 inches all round is trouble, but I like the fact that the
heater is free from the structure. I never considered this point a fire
risk, even with contact, but with the 2 inch space it is sure.

           I have never seen anyone else do this, or been reprimanded for not
doing it. And once the floor and ceiling ( in the basement ) are finished
the place in question will not be visible.


Date: Fri, 07 Aug 2009
From: Dave Misiuk <misiuk(at)>
To: Norbert Senf <mheat(at)>
Subject: Re: Fwd: Cape Breton

I have designed structural heater slabs that have the 2" air-space all around between slab and wood floor members.  Works great where there is no seismic code requirements.  But here the heater slab either needs to be stabilized by attaching it to the structural floor diaphragm or needs an independent seismic support all the way to the foundation.  I've detailed the edge of the heater slab by closing the 2" gap with steel bent plates screwed to the floor plywood and cast into or otherwise attached to the concrete.  This provides the code required air-space and has a non-combustible connection to the floor diaphragm.

As an inspector, and in my designs, I let common sense rule.  You know, what defines the "heater slab?"  What if the slab is 4 times larger than the footprint of the quickly becomes a moot point to provide a 2" air-space if the edge of the support slab ends say 4 feet from the heater.


Date: Fri, 07 Aug 2009
From: Dave Misiuk <misiuk(at)>
To: Norbert Senf <mheat(at)>
CC: (snip)
Subject: Re: Fwd: Cape Breton

Please see attached heater slab detail.  This project had a heater on both levels but could not be stacked due to seismic issues.  The upper heater has steel gravity columns and the slab is attached with the steel bent-plate to the floor system to provide the necessary lateral stability.


foundation detail

From: William Davenport <info(at)>
To: Norbert Senf <mheat(at)>
Subject: Re: Cape Breton
Date: Fri, 7 Aug 2009

My method is just about exactly the same as Marcus.  I like to drop 
1/2" cement board down on the blocks up to the edge of the formboards 
that have either been shimmed in place or screwed to the subfloor.  
Once the concrete is cured I take the screws out and the boards drop 
right down into the basement.  Sometimes I have to put in some 
temporary braces up under the cement board form to hold it up when the 
concrete cured,  depending on how much it cantilevers over the blocks.

So much fun!


Date: Sat, 8 Aug 2009
From: daniel pike <olworldmas(at)>
Subject:  foundation  breton
To: Norbert Senf <Mheat(at)>

Norbert,  My practice is I  make the opening the exact  size I need for the mass. Course the block work to give me a 6"' slab, using dura-rock at the bottom and the framing as my form. I wrap the sides of the form with a couple layers of sill form or felt  paper,  so wood and masonry are independent of each other. I don't worry about the 2" clearance for two reasons. One, as mentioned before slab does not get hot enough to be of concern and two, It's my belief code states the masonry and combustibles can meet as long as there is a minimum of 10" of solid masonry between them and the heat-kit meets this requirement. As of today I've never had a building inspector not accept this method. 


Date: Sun, 9 Aug 2009
From: Alex Chernov <alex_stovemaster(at)>
Subject: Re: Cape Breton

Hi guys,
Dave, thanks for the explanation of the seismic measures.
Adding to the discussion, I always leave 2" air gap as per ASTM with the exception when certain side of the slab is under unheated portion (wood storage etc). Arguably, there is no heat there, and as such it is easy to convince the inspector that no clearance is required. Depending on the inspector, however, it still may be safer to leave the gap.
Plywood/subfloor is cut to come right to the slab. There is a detailed explanation and a drawing in my heater guide available at my web site
I find that it is most important to explain proper treatment of the opening to the carpenters beforehand since it is difficult to fix it properly without lifting large sheets of the subfloor material if it is cut along the joists.
Most of the time I use 2" Styrofoam as temporary form attached lightly directly to the wood joists, and take it out after concrete has cured (I put plastic film on the concrete side so the Styrofoam doesn't stick to the concrete).

From: "Marcus Flynn" <pyromasse(at)>
Date: Sun, 9 Aug 2009

 Hello Norbert,
                 I am sending this link to a bilingual page on my site where
I have two image sequences detailing the way I build foundations. You are
welcome to post it in relation to the foundation discussion if you find it

                 It is a page I have had on my site for about 3 years. Not
perfect, but the best I can come up with at the moment.