I may have screwed up the insulating layer

I’ve been working on weekends in 90+ degree heat, so that’s my excuse…

I poured my hearth slab in situ, as I don’t have any large level surfaces to make the three sections of hearth slab that the directions indicate. I was doing the math to create the void for the insulating layer of perlite and portland cement, and somehow I left off 10 inches front to back. So my sand and firebricks are going to have 5 inches at the front and rear that are sitting on the slab rather than on the insulation.

I really don’t want to try to grind out trenches at both ends of the insulation section. Will leaving it be really hurt me in the long run?


Wow gdisdenza! I’m with you. I’m in AZ and getting ready to pour my hearth slab this weekend! I’ve got my “Southwest Sauna” setup over the site, so it stays a cool 80F inside (lower in the mornings)…but I’m concerned about the curing process! Good luck on the insulation layer. I’m trying to figure out my cut-out form tomorrow!


Hi gdiscenza

Ah man. I feel your pain. I made a few critical errors when I built mine. You say to yourself “Why did I do that? Now what?”

So here are your options. If you leave it as is, the firebrick that will be on top of the slab will get the heat sucked out of it much faster. That may not be a problem in the front where the opening is because most likely your not cooking there. But in the back where you might have fires there it will be trouble some. Not to mention the slab can get brittle over time. So food for thought there.

Or. Ugh. You cut out what you need too. You mentioned that you prefer not too. I would mark with a marker where I need to cut out. Then with a angle grinder with a diamond blade cut along thise lines hopefully reaching down to the 2 inch depth. You can use a circularsaw with a diamond blade and set the depth but your risking over running the corners. These diamond blades are a wonder. As long as you don’t have rebar they cut pretty good thur masonry products. Even then I hear they go thur rebar.

Once you have the half box cut you can then cut slots say an inch apart or two inches apart. Chiseling then should be easier. I would advise this because its very critical that you have a insulated area under you dome.

Maybe someone else’s has a better idea here but thats the best advice I can give you. All mistakes are fixable. Don’t sweat it. (No pun intended)

Hope this helps. I wish you all the luck.