Rocket stove floor and Sand and Clay

I am getting ready to build a rocket stove this spring in my studio building at my cabin. I may build one in my greenhouse first then the studio this one is a dirt floor so no worries there. Here are my questions.

I know that the structure will withstand the weight of a rocket stove and mass heater. The studio is buit on 10"x10" beams which i have placed in the right areas to hold this mass but my question is how do i protect the floor from the rocket stove. My plan was to build the rocket stove out of fire brick and my thought was to use two layers of cement board on the floor with the brick on top of that. Is this enough protection? or should i use something more like two layers of patio bricks then the fire brick for the stove? Also can i use fire brick instead of a barrel?

My other question is the sand and clay mixture. I live on an island in Alaska and it is very expensive to get things here so i want to use local sand and clay. However our sand is not silica sand our sand is black sand basically crushed shale rock will this work for the cob on the stove? Also or clay is blue clay glacial clay will this work for the cob as well.. If it does it sure will be gorgeous it is a really cool grey blue color.

Thanks for all the help

Robert,

Think insulation under the stove, not conduction. Concrete board, brick, etc. conduct. Some high density foam board will insulate your floor against the heat. Maybe 2" Maybe 4" with the brick over that. This will change the elevation of your whole assembly but your thermal mass should be insulated from the floor also. Your cob can come down over the exposed edges, or frame them with wood. Test the cob you can make with your sand and clay. If you can produce a tight, heavy, strong brick of it, it will work. Any grass straw around? Fibers will make stronger cobb. Make different recipes, dry them and see what is strongest. Sharp sand is better than smooth sand from rivers and beaches. You don't need expensive fire brick in your manifold. Good luck.

Hi Robert,

We have a place that we spend summers at (for now) on an island in SE, and I built a RMH last summer. Under the combustion unit and first 10' of bench I put down concrete board, then regular bricks on edge, filled the cavity with a perlite/clay mix, then another layer of concrete board. The remainder of the bench only has a layer of concrete board to build on. Under the combustion unit I put a layer of fire brick then built the J-tube on top of that which included another layer of firebrick on the floor of the horizontal burn tube. A caveat here: I ordered "fire brick" from Ace Hardware in Seattle and what I got was splits, not full bricks. This made for the vocalization of a few choice words followed by some head scratching. . . . but beyond requiring some care setting the bricks for the heat riser the build went well. There is no appropriate sand to be found so that had to be purchased from a road construction company. Found some pretty good clay. It is a two mile skiff ride from the town dock to our place so the sand and clay was in five gallon buckets, transferred from the truck to the skiff, thence from the skiff up the bank to the house.

Weren't there to use the stove all winter, but the subfloor never got more than slightly warm when the unit was fired for most of the final three days we were there as I was curing the cob bench.

Edited to add: There is a 2" gap between the bench and the wall behind it. My thinking is to fill it with a clay/perlite mix this summer. It is an 8" system. The bench is 18"x18" by 21' long before it exits the house. Time was a factor as our departure date was looming. The base of the bench is those dark shale rocks that it sounds you have. I was building by myself so I built a 5' form for the bench into which I shoveled the cob, worked it to pack it in, smoothed the top, let it rest overnight then moved the form the next morning. I would finish the prior days cob surface and mix and shovel in the next 5'.

This summer I'll finish the surface, still working out the method to use there.

Andy