# Rear clearance for hearth with cement board?



## Johnkimble (Aug 29, 2015)

I'm wondering if someone can tell me what the rear clearance needs to be for a wood burning stove? The new hearth will be built up with 2x4s, plywood,  and multiple layers of cement board.  The rear wall will be built up the same.  Does the rear still need to be 18" from the wall,  or can it be closer?  The front will have 18" of clearance,  and the sides will have 24". Ideally I'd like the rear to be closer to save some living room real estate.  Thanks!


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## jeff_t (Aug 29, 2015)

Depends on the stove. What do you have?

And welcome.


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## Johnkimble (Aug 29, 2015)

Not too sure exactly what stove I have.  It came with the house.  Here is a picture with a serial number.  



jeff_t said:


> Depends on the stove. What do you have?
> 
> And welcome.


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## Bioburner (Aug 29, 2015)

You have a Earth stove but that's only half the story as the plate does not say what model Post a picture of the stove proper and someone should be able to ID it so you can track down a owners and installation manual.


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## Johnkimble (Aug 29, 2015)

I think from what I found its a model 101?


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## begreen (Aug 29, 2015)

The diagram on the back label shows the clearance requirements. That is 36" in all directions from the nearest combustibles back or side. You can reduce the rear or side clearances by using proper wall shielding. 4" of brick will get you down to 24". A properly vented NFPA wall shield will get it down to 12". 
https://www.hearth.com/econtent/index.php/articles/stove_wall_clear


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## Grisu (Aug 29, 2015)

With a new stove you will have close clearances (like 6" in the rear) without any cementboard, potentially ember protection only in the hearth, a nice fireview, better heat output, and a happier home insurance. 

If that does not convince you here is the manual for your stove: https://www.hearth.com/images/uploads/Earthstove100_manual.PDF


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## Johnkimble (Aug 29, 2015)

Well I'm thinking 18" in the front,  12" in the back,  18" on each side with 1" of durock underneath and against the wall.  3" of dead air underneath and against the wall.  Sound suitable?


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## begreen (Aug 29, 2015)

No, dead air is not covered by NFPA specs. The rear wall needs to be a ventilated, 1" air gap, open at top and bottom.


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## Johnkimble (Aug 30, 2015)

Any references of what a 1" air gap looks like? I'm planning on tiling around the whole thing.  Is that possible with an air gap?


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## begreen (Aug 30, 2015)

Sure. It just requires that you leave a 1" gap at the bottom and top of the wall shield so that air can freely flow behind it.


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## Johnkimble (Aug 30, 2015)

Can anyone provide me with a picture or two of what that looks like? It's hard for me to picture it.  Would I cut a 1" hole in my 2x4 framing to vent the air, and tile around it with an exposed hole?


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## Grisu (Aug 30, 2015)

The link that BeGreen posted has some good schemes in there how to construct a ventilated wall shield. You set the cementboard 1" away from the combustible wall. Screw it with non-combustible spacers into the wall left and right from the stove. Leave a 1" gap at the bottom and leave the top open. After that you can put tile on the cementboard but leave the 1" gaps that air can freely flow behind the wall shield. You can close off the sides. You create a chimney effect where cool air comes in at the bottom and warm air leaves from the top. That air flow will cool the wall that is behind the wall shield.  

Maybe this post helps: https://www.hearth.com/talk/threads/in-search-of-wall-shield-ideas.124840/page-3#post-1681671


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## Johnkimble (Aug 30, 2015)

Okay that is making much more sense.  If I shim my floor and wall pad with 1" spacers,  say out of plywood,  and build it up in the order of: plywood spacers - > 2x4 - > plywood - > 2 or 3 layers durock -> tile,  would that be adequate? What would the minimum safe clearance be to the rear wall?


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## Grisu (Aug 30, 2015)

No, there cannot be anything combustible between the wall shield and the wall. So no plywood or studs etc in the wall shield. The spacers from the wall could be strips of durock or metal studs. You also need only one layer of durock under the tile unless you are concerned that one alone won't hold the weight. Adding several does not change the clearance requirement. Thus:

wall -> non-combustible spacer at least 1" thick -> one or more layers of durock -> tile
Don't forget the air gap at the bottom and the top. 

In that case the stove can be as close as 12" from the wall (not the shield) provided your local building inspector and home insurance agree with your installation. They always have final say whether a stove installation is considered safe.


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## Johnkimble (Aug 30, 2015)

What about these other examples? 

https://www.hearth.com/talk/threads/can-i-use-wood-studs-to-build-frame-for-hearth-pad.134464/


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## Grisu (Aug 30, 2015)

A ventilated wall shield is different than a floor protection hearth. For the wall shield you have to follow NFPA 211 guidelines for wall shields. Here is an older version but I don't think that section has changed: https://woodheatstoves.com/free/NFPA211_old.pdf (page 211-38 f)
Please note that if you don't do a ventilated wall shield (like a course of bricks over the wall) then your clearance reduction will be lower.


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## Johnkimble (Aug 30, 2015)

So for the wall,  I could do 1" thick strips of durock between the entire sheet of durock and the existing combustible wall? Allowing ventilation from top to bottom,  then tile?  Would the same go for my floor,  or should I just do metal studs for the floor and wall shield,  then durock on top?


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## Grisu (Aug 30, 2015)

Johnkimble said:


> So for the wall, I could do 1" thick strips of durock between the entire sheet of durock and the existing combustible wall? Allowing ventilation from top to bottom, then tile?



It should approximately look like this: http://i40.photobucket.com/albums/e244/akcooper9/June Dr/2014-02-26180946_zps194da54b.jpg
Same on the bottom with a 1" gap between the hearth and the bottom of the wall shield. Then you could put tile on that cementboard. 


Johnkimble said:


> Would the same go for my floor, or should I just do metal studs for the floor and wall shield, then durock on top?



For the floor/hearth, I don't know what kind of r-value your stove needs or if it has ever been tested in that regard. If not you may need to follow again NFPA 211 guidelines but ask also your local code authority regarding their requirements and for the current local code. The 2003 NFPA 211 states the following for a stove that has an air space of 2" to 6" underneath:


> 12.5.1.2.2 Room heaters, fireplace stoves, room heater/fireplace stove combinations, or ranges that are set on legs or pedestals providing 2 in. to 6 in. (51mm to 152mm) of ventilated
> open space beneath the fire chamber or base of the appliance shall be permitted to be placed on floors of combustible construction, provided the following conditions exist:
> (1) The floor under the appliance is protected with one course of hollow masonry units not less than 4 in. (102 mm) in nominal thickness.
> (2) The masonry units are laid with ends unsealed and joints matched in such a way as to provide free circulation of air through the core spaces of the masonry.
> (3) The top surface of the masonry is covered with sheet metal not less than 24 gauge [0.024 in. (0.61 mm)].


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## begreen (Aug 30, 2015)

The air gap is only for a wall. Air needs to be able to freely enter at the bottom of the wall shield (min 1" bottom gap) then travel behind the wall shield (also a 1" gap behind it) then out the top through a 1" gap. You can make your spacers for the wall shield out of long 3" wide strips of 1/2" cement board, doubled up to create the 1" furring shims behind the wall shield. Or you can use ceramic spacers and make the wall shield out of a lighter material like sheet metal. As long as it is totally non-combustible the material doesn't matter. Some folks have used copper or stainless steel. Cement board will be required if the surface is to be tiled.

Leave the floor pad on the floor. No air gap needed there. One generic approved hearth method is to put 4" brick on edge with sheet metal underneath or on top. 4" of brick is R=.80. If you use two sheets of 1/2" Durock NexGen the R value will = .78. Close enough. If you want extra peace of mind you can add another layer. Stagger the seams of the layers of Durock.


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## begreen (Aug 30, 2015)

Copper wall shield example:
https://www.hearth.com/talk/threads/copper-wall-shield-installed.140332/
Nice tiled wallshield example on page 3 of this thread:
https://www.hearth.com/talk/threads/in-search-of-wall-shield-ideas.124840/page-3


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## Johnkimble (Aug 30, 2015)

Okay so my 2x4 framed pad is okay to stay and layer 2-3 layers of durock on top with the dead air.  And for the wall,  1" gap to and bottom,  shimmed with durock, with a sheet of durock across the shims acting as my shield.  This sound suitable?


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## begreen (Aug 30, 2015)

Yes, that sounds better. What is the spacing of the 2x4s for the hearth pad? Is there a sheet of plywood on the top under the cement board?

Before hanging the wall shield be sure to account for the thickness of the tile or stone on top of the hearth so that when all is done the wall shield bottom starts 1" above the final hearth pad surface.


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## Johnkimble (Aug 30, 2015)

Copy that.  The hearth pad 2x4s are on their side,  so 3.5", then plywood,  then I'll do 2 layers of durock.


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## begreen (Aug 30, 2015)

Should work out fine. The old Earth Stoves are heavy suckers.


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## Johnkimble (Aug 30, 2015)

It looks like my only option from local home improvement stores is durock next gen 1/2", or hardiebacker.  I picked up the next gen,  but read it's not made for hearth construction, but is okay for a wall shield.  Is this true?


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## begreen (Aug 30, 2015)

Durock NexGen is cement and pumice. I can't see anything there that would not work for hearth or wall shielding. Hardibacker board has cellulose in it. That would not be my first choice for backer board though some have used it without issue.


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## Johnkimble (Aug 31, 2015)

So NFPA states:

1⁄2-in. (13-mm) thick noncombustible
insulation board with ventilated air space.  Minimum clearance as wall protector 6". Maximum allowable reduction in clearance 66%. So could I go 6" clearance in the rear or stick with the 12"? Thanks.


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## Grisu (Aug 31, 2015)

12". That's the minimum distance allowed by NFPA guidelines for an unlisted stove. If you want to get closer than 12" you need to get a different stove that has been tested with less rear clearance. 

From which version of the NFPA are you citing?


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## Johnkimble (Aug 31, 2015)

NFPA 211.  So for an unlisted stove,  does 18" front and sides with 12" in the rear sound okay.  I've got my durock and plan on building the pad and shield tomorrow.


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## Grisu (Aug 31, 2015)

Yes, that sounds ok. 12" in the back is ok with a ventilated heat shield. With 18" in the front and sides you mean the size of the hearth or is that an alcove?  

I was asking for the NFPA version as that citation was not in the 2003 and 2006 editions I have. Have you been looking at the 2013 version?


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## Johnkimble (Aug 31, 2015)

18" each side of the stove on the hearth.   2003 edition,  page 27.


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## Grisu (Aug 31, 2015)

I could not find this one:


> Minimum clearance as wall protector 6".



But I think you were looking at the wrong section. The table on page 27 is for connectors. You need to look on page 38, paragraph 12.6 for solid fuel-burning appliances. Floor protection is in figure 12.6.2.1(b). Minimum clearances in table 12.6.2.1.


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## Johnkimble (Aug 31, 2015)

Oh my mistake.  You are correct.


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## Johnkimble (Sep 5, 2015)

I had another question.  When creating my shield with 3" wide strips of durock doubled up as spacers,  how many should I space between my 60" wide shield?  Does it need to be entirely empty space between the two strips of durock on each end,  or can I put a few more vertical strips to increase the strength of the shield?


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## coaly (Sep 6, 2015)

Ceramic insulators are best. (for fencing or available in kit form) If you prop up a piece of cement board near a heat source you will find it gets very hot on the back as well. This is because it transmits heat very well. Its resistance to flow is very low. Also don't put any in the center line of stove. You set the heat shield on bricks, spaced for air circulation under it, (air intake) so the spacers do not support weight, they only hold it from the wall. Simply run a screw in to hit studs.

https://www.hearth.com/econtent/index.php/articles/stove_wall_clear


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## Grisu (Sep 6, 2015)

If you look closely at NFPA 211 you will see that the spacers should not be directly behind the stove but off to the side. The same should be true when you use several layers of cementboard. I second Coaly's suggestion of putting some bricks or other support under the wall shield that the spacers do not need to carry the full weight then you won't need to worry about having them only on the side.


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## Johnkimble (Sep 6, 2015)

Thanks for your replies! I was wondering about supporting the load of the wall shield.  Instead of bricks supporting the bottom of the shield,  what about using the entire sheet of durock, and cutting 1" air gaps along the bottom? Leaving maybe four 3" wide "footings" of the durock shield to sit atop the pad? 

Also,  where is the best place to buy ceramic spacers? Do local places like home Depot or lowes sell them? Thanks again!


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## Johnkimble (Sep 8, 2015)

I attached a picture of what I'm hoping to do with the shield.  I'd appreciate some input, specifically how the durock board will be resting on the hearth pad, while spaced out with 3" strips of durock to create a 1" air gap.


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## begreen (Sep 8, 2015)

Yes that should work. Making a 1" thick spacer out of two 3" x 36" furring strips snapped off the Durock sheet is fine. Maybe make the bottom air slots a little more generous at the base, like 1.5-2" high.


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## Johnkimble (Sep 9, 2015)

Thank you.  Also,  in determining my shield height,  is it correct to assume since my stove requires 36" distance from unprotected wall,  the height of my shield must be 36" from the rear top height of my stove? 

12" rear distance to the shield,  36" unprotected requirement = 34" on top of my stove height using a^2+b^2=c^2.

27" stove height, means 61" shield height?  A bit tall for my liking,  but I understand if that's my only option.


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## begreen (Sep 9, 2015)

6" above the stove top should suffice unless single wall stove pipe is used. That has an 18" clearance requirement.

PS: The stove manual is located here:
https://www.hearth.com/images/uploads/Earthstove100_manual.PDF


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## Johnkimble (Sep 16, 2015)

I've decided to go with the Englander Madison stove.  The manual states it only requires ember and spark protection only.  So I'm confirming I don't need to bother with a ventilated shield anymore,  and a sheet or two of durock flush against the drywall is fine.   Also,  the manual states double wall chimney distance from combustibles is "N/A"   does this means there is no distance requirement,  and my double wall chimney can be 7" from the wall? 

Thanks.


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## Grisu (Sep 16, 2015)

Johnkimble said:


> Also, the manual states double wall chimney distance from combustibles is "N/A" does this means there is no distance requirement, and my double wall chimney can be 7" from the wall?



I was wondering about that one myself. This post and the following may help: https://www.hearth.com/talk/threads/madison-in-my-burn-trailer.128150/page-2#post-1742182 but it would probably be best to contact @stoveguy2esw directly.


Johnkimble said:


> The manual states it only requires ember and spark protection only.



That's for the hearth underneath.


Johnkimble said:


> So I'm confirming I don't need to bother with a ventilated shield anymore, and a sheet or two of durock flush against the drywall is fine.



Even drywall by itself is fine but you will need to have the minimum distance between wall and stove as listed in the table in the manual. The durock won't reduce your clearances but you can still do it for looks (e. g. with tile on top) and added peace of mind.


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## begreen (Sep 16, 2015)

Johnkimble said:


> Also,  the manual states double wall chimney distance from combustibles is "N/A"   does this means there is no distance requirement,  and my double wall chimney can be 7" from the wall?



I have taken N/A to mean "not applicable". That is the minimum stove clearance requirements do not change in correlation with the clearances of the double-wall connector pipe. There is no gain in clearance reduction due to the double-wall pipe.

Note that it does not hurt at all to exceed minimums


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## stoveguy2esw (Sep 16, 2015)

begreen said:


> I have taken N/A to mean "not applicable". That is the minimum stove clearance requirements do not change in correlation with the clearances of the double-wall connector pipe. There is no gain in clearance reduction due to the double-wall pipe.
> 
> Note that it does not hurt at all to exceed minimums




this is correct^^^^^^^^^^^^^^^^^^^^^


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## catriverrat (Sep 19, 2015)

On ES Stove install I'm going with galvanized corrugated roofing with recycled garage door track fastened to the wall  for my one inch space all the way the chimney.


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