# Hearth thickness



## Pemaquid (Oct 19, 2014)

Hi all,
I moved into a house with a hearth already made.  But it is slate and only 3/8 inch thick  installed over plywood.  My old house was standard brick.  I'm installing my baby bear fisher on this.  Is it thick enough?  If not can I just line bricks onto or do I need a hearth pad?


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## pen (Oct 20, 2014)

That sucker is going to get hot under there.  

To my knowledge, the manual calls for 3/8 inch asbestos millboard under the stove.  Since that isn't made anymore, here's a link to an older thread giving some advice on building a hearth up to that equivalent.  https://www.hearth.com/talk/threads/r-value.53445/

Additionally, it wouldn't hurt to consider making a sheet metal heat shield for under the stove, something that may mount between the legs and leaves an air space above it as well as below it.

Above all, make certain that the homeowners insurance company is OK with installing this stove.  This can become an issue if it is an old enough model that it does not have a UL tag on it.  I had a heck of a time keeping my fisher in my home as well as at my cabin because of this,,, and keeping the insurance up.  Just a head's up.

Good luck,

pen


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## fossil (Oct 20, 2014)

It's not the "thickness" that matters, it's the insulative value.  Materials like stone and masonry have really little heat insulation characteristics, so it takes a whole bunch of thickness to provide any protection for the combustible material under the very hot surface of the stove beneath the firebox.  This is a matter to be taken seriously and not fudged on or just intuited.  Search for r-value requirement for the floor protection under your stove model.  Refer to the label affixed to the back of the stove, contact the manufacturer, try to find a copy of the owner/installation manual...whatever.  Final option is to build a hearth in accordance with NFPA 211 recommendations, which probably isn't going to exactly thrill you.  We can help you with all of these paths to success.  Tell us more about the stove, especially how old it is and whether or not there's a label on the back.  If there is, then by all means post a pic of it here.  Rick


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## johnpma (Oct 21, 2014)

One of these has a R value of 1.5 

http://www.tractorsupply.com/en/sto...ve-board-36-in-w-x-52-in-h-slate?cm_vc=-10005


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## disamatic (Oct 21, 2014)

fossil said:


> It's not the "thickness" that matters, it's the insulative value.  Materials like stone and masonry have really little heat insulation characteristics, so it takes a whole bunch of thickness to provide any protection for the combustible material under the very hot surface of the stove beneath the firebox.  This is a matter to be taken seriously and not fudged on or just intuited.  Search for r-value requirement for the floor protection under your stove model.  Refer to the label affixed to the back of the stove, contact the manufacturer, try to find a copy of the owner/installation manual...whatever.  Final option is to build a hearth in accordance with NFPA 211 recommendations, which probably isn't going to exactly thrill you.  We can help you with all of these paths to success.  Tell us more about the stove, especially how old it is and whether or not there's a label on the back.  If there is, then by all means post a pic of it here.  Rick


 
Grrrrrr, i am in the same boat as "pemaquid", i bought the pleasant hearth medium 1800sq. stove, yeah i know its not the best, plan is to move it to shop next spring and buy something nicer, but we started with, no stove, no chimney, and no hearth. What has confused me is the back of the stove says, place on non combustible surface, then the manaul says it should have a r-value 2.0, then what is even more confusing is the large stove only requires a r value 1.19? So yeah, i guess i will have to get on the horn as well today. Its a free standing stove, seems crazy for a 2 on the r-value.


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## johnpma (Oct 21, 2014)

The stove companies have to protect themselves, you have to protect yourself, your homeowners insurance company has to protect themselves, you need heat, and sometimes the wealth of knowledge you acquire from the net can be overwhelming.

I have my stove on the pad I posted above and I have altered the leg height. The pad sits on a brick hearth which was built flush to the floors. Even with a roaring fire going I can hold my hand to the stoveboard beneath the stove. I really don't know why stove builders require such a high R value for many of the stoves on the market. And try not to let a lot of what you read frustrate you or scare you. Just be smart and safe.

And for the record I knew more about the safety of my stove, and it's requirements than my local building inspector......but the town needed my forty bucks.


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## begreen (Oct 21, 2014)

An old rule of thumb I have read in some old manuals is to stack bricks on the hearth, on edge, so that it is 4" thick. Mortar the bricks in place or frame around them and fill the cracks with dry sand.


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