My new 30 NC is here and it's time to build a hearth for it. My plan is to build using 2X10 lumber because I would like it to be roughly 12" off the finished floor. I know I'll have to cover it with material to achieve the 1.5R rating required and considering dura rock but is it typical to put down 4 layers of 1/2" to get the rating? I notice that gypsum board has a higher rating why not just use that? Final surface has yet to be determined but I heavily favor natural stone to match the rest of our cabin. Would 2 layers of gypsum, a layer of durarock and finally the stone be enough?
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Nope. I just went down the road you're traveling (NC30, raised hearth), and I did a lot of research. I think you are going to find the materials you mentioned won't get you the 1.5, and the gypsum board can't be used anyway if it has combustible components. For what it's worth, I think your 4 layers of Durock plus whatever non-cumbustible finish layer is going to be where you end up. Lots of folks here know more than me, but I think if you are raising it up that high anyway, just use the layers of durock and finish it off. Don't forget to follow the other clearance requirements too.
Sprinter
Minister of Fire
Right. Regular gypsum board with a paper face is combustible. Durock is where it's at for this purpose.
When I built the hearth pad for my quad, I ended up with three layers of durock and some ceramic tiles on top. Two layers was close, three was overkill, and the ceramic tiles add a little R factor as well (and look nice!). Double check if the tiles add anything to your heat protection - it just might be enough to get by with three layers. Then again, another layer of durock is a lot less expensive than burning up your house.
I am installing an NC13 currently and it requires an r value of 2. I'm going to use mglass made by American Gypsum. Found it at a local drywall supply place. Has an r value of .61 for a 5/8 thick sheet. Only cost 21$ for a 4x8 sheet
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