Hearth Construction 101 for Fisher.

[WeldrDave]

Hello all, I would like to get some input please, "have a tendency to get arguments started" but here goes.

I'm building a corner hearth for my Fisher baby bear which will be installed in spring; as per the pictures the frame work is 2"x 6" and the plywood is 3/4". Its all glued and screwed together, now here's where I will ask the experts, "all of you" the next question? 28 gauge sheet metal next? durarock next? then tile? "I wanted to do a nice sand stone tile design" Insulation under the framing? leave the air space? I'm reading to much conflicting things. My frame work will be sitting on a hard wood floor, floor joists are 2"x 12"s no worries there. Any way, I would like to here all suggestions please Thank you!

Dave.

 

[Dakotas Dad]

I don't know anything about Fishers need for CtoC, or hearth requirments, but no way shape or form would I do that.

 

[Bret Chase]

just as a matter of tile support (stoves make for a very high point load), you would want 2 layers of 3/4, preferably fire resistant plywood, then the durock, then the tile. That's just regarding properly supporting the tile... I'll have to review my copy of NFPA211 for the rest of it

 

[Stax]

What does the Fisher manual say about hearth requirements?

 

[StihlHead]

You need to know what the factory clearances are (front, left/right and back), and what the R requirement is for the hearth before you build it. They are generally on the metal panel on the stove, or in an installation guide. No metal needed, use Micore 300 over the deck and then Durock Next Gen screwed to the decking and then tile over the Durock NG using thinset. That will give you an R value of 1.44 using one sheet each of half inch Micore and Durock.

 

[begreen]

Move to the classics forum. Ping coaly for hearth requirements for this stove.

 

[WeldrDave]

Thanks all so far, I have the side and back distances covered, it's a old, 78-79 Fisher baby bear, no UL plate on it. Anyway I got all things figured out for the most part, just the rest of the base. It's a very small and light stove, weighs about 275lbs. I have a Grandma I'm burning in my down stairs, on a brick hearth I built years ago, I built the wood hearth because this stove will be sitting on a wood frame floor, not brick. And getting under the house to reinforce the joists to make a brick hearth would be "very difficult".
Thanks all.

 

[WeldrDave]

just as a matter of tile support (stoves make for a very high point load), you would want 2 layers of 3/4, preferably fire resistant plywood, then the durock, then the tile. That's just regarding properly supporting the tile... I'll have to review my copy of NFPA211 for the rest of it

Thank you Bret, As I'm seeing I may do a double durock layer.

 

[WeldrDave]

Move to the classics forum. Ping coaly for hearth requirements for this stove.

Thanks Begreen, I did just that! he is the guru

 

[coaly]

Page 2 and 4 here;
https://www.hearth.com/econtent/index.php/wiki/1803ed61c450729660b7c4c916156793/

 

[playoutside]

Hello all, I would like to get some input please, "have a tendency to get arguments started" but here goes.

I'm building a corner hearth for my Fisher baby bear which will be installed in spring; as per the pictures the frame work is 2"x 6" and the plywood is 3/4". Its all glued and screwed together, now here's where I will ask the experts, "all of you" the next question? 28 gauge sheet metal next? durarock next? then tile? "I wanted to do a nice sand stone tile design" Insulation under the framing? leave the air space? I'm reading to much conflicting things. My frame work will be sitting on a hard wood floor, floor joists are 2"x 12"s no worries there. Any way, I would like to here all suggestions please Thank you!

Dave.

We also have a baby bear. What did you finally build? I'm still confused about the required r value for a hearth. I've read the fisher manual but it doesn't specifically list a required r value for hearth. I have been told a wide range of rvalue from as low as 1.5 to as high as 2.5. That's a wide range so what is the actual r value required for the baby bear hearth? And how far does the hearth need to extend. The manual says 36inches from combustables so do i need a hearth that extends 36inches to the right, left and front of the stove?

 

[coaly]

This will give you the size required with a diagram showing floor protection.
https://www.hearth.com/econtent/index.php/articles/wood_stove_clearances_installing_it_safely

The manual also gives a minimum floor protection size of 31 1/2" X 55" for Baby Bear as well. Codes are written to include "manufacturers installation instructions" so you must install under manufacturers guidelines as well.

The requirement given in manual is the equivalent of 3/8 asbestos millboard. (with additional brick covering)
This material is no longer made, and materials can be tested using Thermal Conductance (C) Thermal Conductivity (K-factor given with thickness =T) and Thermal Resistance (R). So you will have to use a formula for figuring R value when R is not given.
UL listed stove boards will give R value and other materials value can be found from manufacturer. 4" brick with C factor of 1.25 = r value of 0.80. So you add the materials you're using like cement board, tile, or brick until you have at least the R value of 3/8 millboard = 0.45 + brick = 0.80. This also needs to be over a piece of sheet metal like galvanized sheathing of 24 gauge.
Millboard thread here; https://www.hearth.com/talk/threads/millboard-the-truth.7226/

** To keep the floor under the center of stove cool to cold, you will find a sheet metal shield with an inch or more of air space under stove is the most beneficial. This is what was added on the UL listed stoves bottom and rear for reduced clearances. This does not reduce the R value requirement for your stove since yours was not tested with the shield. **

The manual states 36 inches to combustible due to NFPA 211 standard. (the standard codes are now written from) This has become the standard for all unlisted (untested) stoves. There is also a general clearance reduction rule given in NFPA 58 for reduced clearance when a heat shield is used behind or at the sides of unlisted stoves. Using the search feature at top of page will give you plenty of info on construction and air space required behind it.

 

[bigblulbz]

Ok. Kind of hijacking/adding to the original post. I'll be removing my existing unit and building a hearth also. The location of the unit is in a finished basement. I am dropping the wood framed wall behind the stove and am installing a steel studded wall with fire resistant insulation, then sheathing it with 1/2" Durock. Behind the steel studded wall is the concrete foundation. Th hearth that the stove will sit on will be field stone on top of the concrete slab foundation as a sub-strate.

In my head, I keep telling myself I have no combustibles present, other than the ceiling. I want to be able to push the stove back a few inches. Does anyone see any reason I will not be able to?

Thanks in advance.

 

[coaly]

Not sure about the insulation. I believe it's considered a combustible ?? "Resistant" is not non-combustible just like sheet rock is a combustible, but considered "resistant".

Wouldn't it be easier and make a nicer finish to apply brick face, or solid 4 inch brick if you need the thickness, directly to the masonry wall? Then you know it would be noncombustible. Considering it's a finished basement and you will end up with cement board behind the stove that will bother you until you finish it with a facing of some sort anyway.
Or omit the insulation and make it a dead air space that would give more insulating value than brick face only. Then you're noncombustible for sure.

 

[bigblulbz]

Excellent point.