We are thinking of bringing the rock facing of our masonry stove up above the firebrick core. Wondering the best way to attach a wood frame on top of the core?
@dclaire, my answer would depend on whether you were planning on bringing it all the way up to the ceiling.
First, it’s useful to review a typical scenario:
In this typical scenario, you build your core, install your expansion joint, build your facing, install a minimum 2” of a refractory insulation to keep the heat down in the body of the heater, and then we like to install a cap to nicely finish off the top and act as a redundant seal. Note the minimum 8” vertical clearance to combustibles which is called for in the ASTM 1602:
(There is some ambiguity in the term “masonry heater capping slab” as to whether it refers to the capping slab in the core or the finished capping slab.)
I am going to have to end my reply here, for now, addressing a typical situation. I will try to address your specific question soon…. (I’m guessing you have a number of days of stone work to do before you get to the top! 
)
@SolidRock might also have some good input on the matter…
We are thinking of expanding upward at an angle, paralleling the angle of the roof beam above the stove, but not going that high. Thanks for the diagram of the top of a heater, looks like we build a form and cast a slab of concrete? Can I use clay bricks (we have a lot left over from building) around the top of the masonry heater core to create the 2" spacing for the insulation, and set the cap on? Or do you use fire brick up there also? Do you have preferences or recommendations on the refractory insulation material?
It seems like there is some overlap between this topic and the other one we have going….
Here is a drawing of a scenario where you extend the facing significantly above the core, in some cases to the ceiling.….
In your case, @dclaire, it sounds like you are thinking of actually tilting the facing capping slab to match the pitch of the roof. And, as you are considering, it may be necessary to create additional structure in order to extend the single wythe of masonry up. You would have to remove anything combustible. I have considered a steel stud and cement board form above that I could leave in place and build the facing against but have never had a project where it was called for thus far.
Hope that helps!
Max,
do you have a recommendation on the mineral insulation? Perlite and lime, clay or cement? And is there a recommended ratio of the mix?
Claire
@dclaire, we have most often used a mix of 4 perlite : 1 lime . Mix the two ingredients dry first and then slowly add water until you can make a ball in your hand (with gloves on! ) that sticks together but before the mix gets gooey and over-saturated. Then pack it on top of the capping slabs and inside the facing masonry.