DIY Pizza Oven Foundation Slab - Rebar and Reinforcement - May 2024

Last week, I ran through the details of the final dimensions of our backyard wood-fired pizza oven including the reinforced slab, the block stand and the insulated hearth.  I had been thinking of a 39" diameter oven and after looking through the actual dimensions of the block stand (less than 16" per block), I've settled on these final dimensions:

Reinforced foundation slab: 72" wide x 80" deep.

Stand: 4 blocks wide x 4.5 blocks deep - 62.5" wide x 70.312" deep.

Hearth: Same as stand.  Framed with 2x6 dimensional lumber.

Oven: 39" interior, 51" exterior side-to-side x 64" front-to-back.

The first major task is to pour the foundation slab.  The steps to get that done include:

Excavate the site

Lay down 1/2" gravel and compact the base

Think about drainage on/around the foundation slab 

Set up the forms and stake them level and square

Buy the material (concrete, rebar, cinder blocks, angle iron)

Lay down a 6 mil vapor barrier

Install 1/2" rebar and reinforcement grid

Pour concrete

I've done the first four of those (excavate, install gravel, set up the forms) and because I've decided to build this on top of a 4' deep 'dry well', I only excavated about 12" deep and only had to install about 4-5" of gravel.  I used a little bit of river rock that will help connect the edge of the slab with the drywell underneath.  I also bought some of the material to spec'-out the size of the stand/slab.  

The whole series of posts on my backyard pizza oven construction project can be found here at the [pizza oven] tag.  

Today, I'm moving on towards a little bit of progress:  vapor barrier, rebar and reinforcement all installed.

I've poked around the Forno Bravo forums, read all the instructions an watched dozens of YouTube builds and decided on using the bent rebar corners (at 90 degrees) in all our corners approach.  The idea is that you put the rebar in the slab BELOW where the stand is going to sit, so the slab can be reinforced below where the force is being applied (straight down, I think).  

I went with a 'double' row of rebar all the way around the foot print of the slab and I used a pair of galvanized pipes to bend the bar.  This is 1/2" steel rebar.  Here, below, is a close-up of the double rebar bent as it meets the corner of the slab.  This detail (below) is what the plan calls-for.  But....that seems like the corner of the slab is left un-reinforced, no?  

I thought about some other options - like this one below where I add a pair of pins that meet in the corner at 90 degrees.  I'd 'tie' these to the double frame.   After thinking about this approach (below), I opted to not go this route.

Below, you can see how I pieced it together with double bars around the perimeter and then I started to go down the middle.  This first photo shows the rebar laid out - but it doesn't show the rebar across the middle side-to-side.  That comes next. 

This was my first time bending rebar and while they sell 'rebar bending tools', I found this YouTube video that shows an easy diy-hack for bending bar with those galvanized pipes.  It worked great.  I bought 10' sections of rebar and used a angle iron with a metal-cutting disc (5.5") to cut the 1/2" rebar to size.  

Here, below, is another look.  I was using the cinder blocks in two spots to sort of 'bend' the bar back flat as it was slight out of square.  I also added a couple more supports to the edge of the forms - three more - making a total of 13 in use.

I was able to complete the project by installing a number of these black plastic rebar chairs.  They hold the rebar at 2.25" height - exactly half-way up the 5.5" slab.  Below is a closer look at the plastic rebar chair:

Once I had the rebar cut the right dimensions, I picked all of it and set it aside.  Then, right no top of the gravel, I laid down the 6 mil vapor barrier and cut it to size.  I had to cut it open at the drain pipe location and the closed it back around the throat of the pipe.  On top of the vapor barrier went the rebar chairs and 1/2" rebar.  Doubled-up around the perimeter and criss-crossed down the middle/across the middle.   I used rebar wire and some zip-ties to pull everything together and taught.  

Then, I used a pair of 4' x 8' remesh 10-gauge grids on-top of the rebar.  I had to cut down the length on both and around the drain pipe.  I overlapped the middle section and then laid-in the cut-downs in the top corners to get that 'corner reinforcement' that I was seeking.  

Here, below, is the final product.  4'-deep of large rocks, 5-6" of 1/2" gravel, 6 mil vapor barrier, 1/2" rebar supported by 'chairs' and a pair of 10-gauge reinforcement grid laid on top.    Once I cap that drain pipe with some duct tape and consider installing other drains, this will now ready for concrete.

Next up is the delivery of the concrete and block for this slab, the stand and the insulating hearth.  Once delivered, I'll begin to 'stage' it down by the oven project.  Then, I'll go rent the mixer and get slab poured.    I'm figuring 40-or-so bags of concrete will get this slab filled, so I'm going to try to find a 48-72-hour window when there's no rain in the forecast to first stage the material from the garage to the site and then pour the concrete.