The blocks are designed to interlock with adjacent blocks and the course above, so once the first course has been laid in mortar (to compensate for any unevenness in the slab), the remaining courses are dry laid. Mortar thickness varied from ¼” at one corner to a little over ¾” at the opposite corner, so the slab was reasonably level. The blocks are 8” high and are bevelled on the sides and top, to simulate a ½” mortar joint. Laying these blocks was very satisfying work, as it all goes up so quickly!
Midway through construction of the hearth stand, I decided to use ribbed steel decking, instead of the usual plywood formwork, to support the hearth slab. This is used extensively in the building industry for multi-story buildings, and supports the concrete slab - both physically and structurally. I used 1mm thick galvanised steel, which is able to span over 2 metres (6’ 6”) in a single span for the 5” slab thickness that I ended up with (my span is less than 4’).
At around the same time, I decided to change from 4” vermiculite concrete to 2” Calcium Silicate boards for my sub-dome insulation layer, with the result that I could increase the height of my hearth slab to accommodate more wood, and make it a bit easier to retrieve. The down side was that each block of the top course of concrete blocks had to be cut to 3” high. I achieved this by way of a diamond blade in my 5” angle grinder, and it was fairly straight forward, if a bit dusty! The internal webs of the blocks were drilled as much as I could from each end, then the block split quite easily. In this way, I successfully managed to get two ⅜-height blocks out of each whole block. I made sure that the overall height of the blocks was the same as a whole number of brick courses, because the steel decking for the vent landing would be supported by brickwork at the oven entrance – part of the brick bench structure in the gazebo.
