Will three sheets of durock need an underlayment? Hearth build

[rwhite]

So in trying to meet the updated requirements for an Osburn 1600 (mfg. 1990) that I just got I need an r value of 1 to extend 16" in front. Current hearth is non-combustible but will be short by 2-3". I want to make the hearth extension removeable so I don't have to tear into the hardwood floor. If I screw 3 sheets of durock together (staggering the seams) will I still need an underlayment if I decide to tile it? Other easy option to get me through the year is just screw the 3 sheets together lay them on the floor and throw a hearth rug over the top. This would be the easiest to move around and I could just pull it up in the summer and throw it in the garage without the worry of tile cracking.

 

[BlankBlankBlank]

As I understand your floor situation, you have 3/4" thick solid wood hardwood flooring. This will typically be installed over plywood in newer homes or 1"x6" boards in older homes. If your home is old enough, the subfloor might even be what we today call 5/4 stock.

Provided that your hardwood is installed over a 3/4" thick plywood or similar thickness product, installing Durock over the top should be fine.

 

[begreen]

Yes, you could build your own removable hearth pad. 3 layers of Durock nexgen will exceed the required R=1.0 protection. Back it with 1/2" plywood for stiffness and rigidity. The manual shows that it needs to be 8" wider on each side of the door opening. You'll need to measure the door opening, but if it is 20" or less you should be able to make the extension from a single 3'x5' sheet of Durock. That would yield 3 - 20" x 36" pieces. If it's larger, you'll need a second sheet.