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ARTICLES - - Hearth Design for Wood, Pellet and Coal Stoves

OK, you have decided to get a stove or an insert, and are needing a place to put it.  You may already have an existing fireplace, or you may be creating an entirely new installation.

This article is intended to deal ONLY with the Hearth, or the floor surface that the stove or insert is sitting on.  The walls around and behind the installation are a separate subject, and are discussed in this article and a second article as well as in many threads on our forums.  This article is an effort to summarize these topics and come up with a list of general “best practice” recommendations on how to design a hearth that works well and promotes maximum safety.

As usual all the standard disclaimers apply - while we attempt to give the best advice possible, we can’t see your setup or tell what will work in every case.  Your local code enforcement folks (AHJ) have the final word, and we advise consulting with them and other qualified building professionals about the details of your particular installation.  Neither Hearth.com, nor any of the contributors to this article accept any responsibility or liability for the results of your installation, as you alone are responsible for the results of your work.

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A FInished Hearth (details on this hearth)

The hearth structure is also sometimes referred to as a “Hearth Pad” or just “pad” for short, the terms will be used interchangeably throughout this article.

If one is installing on top of a concrete or other inherently non combustible floor, little in the way of a hearth pad is needed, other than possibly for aesthetic reasons.  However most of us are going to be installing on or adjacent to wood framed construction which is considered combustible. This article is therefore intended to deal with building a hearth that will work on a standard “stick built” home floor, with wooden joists, subfloors, etc.

The first thing that you need to check is the manual for the stove or fireplace you are planning to install, and look for the specification on the SIZE of the protected floor area as well as the suggested minimum R-Value (insulation properties).

SIZE
Size is the first consideration.  Stoves and inserts put out a lot of heat, and must be placed on a properly sized non-combustible surface.  Wood stoves also have an ever-present risk of spilling hot embers or other material out an opened door, so building codes and certifications of the unit will cause the manufacturer to specify a MINIMUM size requirement for the area the stove sits on that MUST be “protected” or made from non-combustible materials.  Typically, this area will be composed of the “clearance to wall” distance on the back and sides, plus the dimensions of the stove itself, plus a minimum of 18” in front of any door opening.  However the manual MUST be checked to verify the dimensions required, as the tested clearances overrule the more generic code specifications.

(Note, the newest NFPA 211 standard has increased the required minimum clearance in front of loading doors to 18 inches from 16 inches previously. Even if you are installing an older stove which says 16” on the label or manual, you should build to the required newer standard of 18”.)

These are MINIMUM dimensions!  I advocate using more than the 18” clearance, for the added safety margin it provides.  Many people make the hearth wider for aesthetic or practical reasons.  A larger hearth can provide hard surfaced, easier to clean storage areas for firewood and other fuel supplies, stove tools, and other related items.  Another potential advantage to an oversized hearth is it’s ability to handle replacement stoves that might have larger size requirements without needing to be modified.  It is not a bad idea to build large enough to fit any stove you could reasonably expect to install in the space.  Other reasons to build oversized - If planning to cover the top with tiles, building to fit the tile size can result in not having to cut tiles, which will look better and be easier.  If the pad is oversized, you can trim the edges with wood molding as the wood will be far enough from the stove to meet code.

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Even if you have an existing hearth or fireplace, it is important to check to verify that it is large enough to accommodate the new appliance.  This is particularly true with fireplace inserts and installations that place a stove in front of a fireplace.  The existing hearth may not provide the required clearance in front of the door opening.  If this is the case,  you will need to either replace the existing hearth, or build an extension to get the needed clearances.  (Hearth rugs DO NOT meet the requirements, and many inspectors will not approve an extension that is readily movable on the suspicion it might get picked up and stuck in a closet somewhere as soon as they leave…)

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Oversized Hearth in front of fireplace

R-VALUE
This number is the “insulation value” (Resistance to heat or cold)  that the floor protection needs to prevent excessive temperatures from reaching combustible floor material (such as floor joists, subfloor, etc.).  It varies from stove to stove, and can range from requiring only a thin covering such as a layer of ceramic tile over plywood, to very heavy insulation in excess of R-2.0.  In all cases you should check the manual to find the required value.

Some manuals may not specify a an exact R-value, but may specify materials that it is made from, or use a description such as “UL Listed Floor Protector, or equivalent.”  In such cases, one may be able to get an equivalent value from the R-Value Table, or one should check with the dealer or manufacturer tech support for better information.

Other manuals may specify the insulation value in other units, or you may find hearth materials specified in other units.  We recommend covering everything to R-value units as they seem easiest to work with.  Some of the other units you may encounter, and their conversion formulas are:

k-factor (note, lower case “k”) is given with a required thickness (T) in inches R = (1/k) x T

K-factor (note, UPPER case “K”) is given with a required thickness (T) in inches R = (1/(K x 12)) x T

r-factor (note, lower case “r”) is given with a required thickness (T) in inches R = r x T

Typically, pellet and gas burning appliances will have minimal requirements, and cordwood stoves have the highest requirements.

Once again, this number represents a MINIMUM value, a higher value will do no harm.  Note that once built, it is for all intents and purposes, not possible to increase the R-value of a hearth without completely dismantling it and rebuilding it from scratch.  Therefore, it is a wise decision to “over-build” the hearth to meet the requirements of any stove that might be installed on it in the future.  If doing this, an R-2.5 hearth will exceed the requirements of almost any stove this author is aware of.  (Exception: stoves using short leg kits which can cause unusually high requirements)

There are many different ways of determining if a hearth meets the required R-value, however one of the easiest is to simply add the R-values of each layer in the hearth pad “sandwich” and see if the total is greater than or equal to the required value.

Here is another Hearth.com article on R and K values.

R-VALUE TABLE
This chart lists the average R-values of many common building materials, however if possible the actual R-value of the materials used should be determined and used in calculations, unless the source is a mfgr website or other “official” source.  Note that the R-value is for the thickness of material specified, if your materials have a different thickness, then you will need to adjust the R-value appropriately to match.

Material Thickness R-Value Source Notes
Air Space (ventilated) 1” 1.43 4
Cement Board (Wonderboard) 1/2” 0.2 1
Cement Mortar 1/2” 0.1 1
Ceramic Board (Fibrefrax) 1/2” 1.1 1
Ceramic Board (Micore) 1/2” 1.1 1
Ceramic Pavers 1” 0.08 1
Ceramic Tile 1/4” 0.01 5
Common Brick 4” 0.8 1 (one layer, on edge)
Common Brick 2-1/4” 0.45 1 (one layer, flat)
Concrete 1” 0.095 1 3
Concrete Block empty core 8” 1.0 4 3
Durock Cement Board 1/2” 0.26 7
Flagstone 1” 0.079 1
Glass Block 4” 2.0 4
Granite 1” 0.083 1
Gypsum or Plaster Board 1/2” 0.45 1 3, 4
HardieBacker 1/4” 0.13 2 2
HardieBacker 1/2” 0.26 2 2
Horizontal Dead Air Space 1” 0.97 3 1, 7
Horizontal Dead Air Space 1/8” 0.92 6 1, 6, 7
Limestone 1” 0.153 1
Marble 1” 0.09 1
Rockwool or Fiberglass Bats 1” 3.33 4 4, 5
Sand and gravel 1” 0.59 4
Sandstone 1” 0.079 1
Slate 1” 0.1 1

NOTES:
1)May be enclosed, need not be vented, may contain support structures.
2) At this time questions have been raised about the use of HardieBacker, it is not recommended until we receive further information from the manufacturer.
3) Value for this item needs confirmation
4) Any paper backing must be removed to be considered non-combustible
5) Material must not be compressed significantly, it is intended to fill otherwise empty spaces, and should not be used as a structural part of the hearth.
6) You can’t “stack” horizontal still air to accumulate R-values, you must separate each layer of the horizontal still air with another non-combustible material.
7) There is currently debate about the use of dead air space, it is probably best to consider alternatives.

Materials NOT to use -
- PermaBase brand cement board by National Gypsum; looks good, and has an R-value of 0.2, but instructions on their website say it should not be exposed to temps over 220*F, and according to their tech support phone line, the product is NOT considered “non-combustible” because it contains polystyrene beads.

 

Hearth Pad Types.

There are several options for types of Hearth pads, but basically they break down into three classes; Portable, Raised, and Flush.

PORTABLE
Portable pads can be either manufactured or home built.  Manufactured pads should be UL listed, be at least as big as the required minimum size and have an R-value at least as high as the stove they will be used with, or be specifically approved for that stove.  Home built pads need to follow the same construction rules as the other pad types, meeting size and R-value requirements.

Portable pads are the easiest approach, as they merely need to be placed in position and have the stove placed on top of them.  Some may require removal of carpet or other combustible floor material under them, depending on mfgr instructions.  However they can be very heavy and awkward, and so are generally suitable only for smaller stoves with low R-value needs.  Be sure to verify that the size and thermal specifications of any pad being considered are suitable for your stove.

RAISED
Raised hearths may be the easiest to build, and are the simplest and most flexible to design.  As the name implies, they are built on top of the existing floor structure, and can be as simple as a stack of cement board, or other non-combustible board as needed to reach the required R-value, covered with a layer of tile or other non-combustible material for appearance.  More elaborate designs can be taller, made from sheet metal studs, or other non-combustible supports, and can incorporate wood storage areas or other features.  Since it has no strict limits on it’s thickness, it can be built from whatever appropriate materials are most readily available, without needing exotic materials such as Micore.

One easily overlooked aspect to keep in mind, is that many stoves have required OVERHEAD clearances, one must be careful not to build a raised hearth so high that the stove will be unable to meet any required overhead clearance requirements as stated.

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Simple Raised Hearth

FLUSH
A flush hearth is one that has a surface that is approximately even with the level of the surrounding floor.  It is a particularly popular choice when extending an existing hearth.  It does have the potential problem of encouraging people to put combustible objects on it that are closer than clearances allow, but it avoids the trip hazard posed by a large raised hearth.

Because a flush hearth is inherently limited in it’s maximum thickness, it can be tricky to design and build, especially if a high R-value pad is required.  Often more exotic materials may be required, such as Micore or other ceramic insulation, and in many cases significant modifications to the existing floor must be made.

First, it helps to understand the normal modern floor construction.  Starting from the bottom and working up, you will find the floor JOISTS, typically 2x8 or wider, on 16” centers, which support the entire floor structure, and which should NEVER be removed or cut into unless you know exactly what you are doing.  Running across the joists is the DECKING, usually made of 3/4” planks, spaced fairly closely together.  The decking stabilizes the joists and provides structural integrity to the floor.  It is best not to remove the decking, but it can be done if alternative supports for the joists can be installed (see below).  On top of the decking is the SUBFLOOR, usually made from 1/2” or 3/4"sheet material, either plywood, OSB, or particle board.  The subfloor is there to provide a smooth and level surface for the floor covering, and can be removed as necessary.  Last there is the floor covering itself, which might be carpeting, tile, hardwood, etc. and which can certainly be removed.  It’s thickness will vary depending on the material and installation.  This author assumes a carpeted floor is about 3/4”-1” thick, other materials need to be measured.

Thus, with a carpeted floor, removing the carpet and pad gives about 3/4” to work with, removing the subfloor gives about 1.5” and removing the decking about 2.25”.  In that space you need to stack up a “sandwich” of materials that will give the required R-value, along with a cover layer that will protect the pad materials from damage and be aesthetically appealing.

As a further limit, Micore and some of the other high R-value materials are rather soft, and must be covered with a layer of HardieBacker, Durorock or other rigid board in order to reliably support a ceramic tile layer without the risk of cracking.

There are a lot of ways you could qualify the distance required, but in an inspection situation you always have to present these to the official for his perusal. Here are a couple of what might be guidelines:

1. NFPA clearly addresses the hearth requirements based on the leg height. Well, it would seem that hearth height would function in the same way.

2. Some stoves, such as the HearthStone we mentioned earlier, actually have published the specs for the “spark guard” part of the hearth.

3. Premade stoveboards, made for this purpose and approved start at $59 and go up from there to $500+. The low cost boards are typically a piece of micore-like material, with one sheet of sheet meal over them.
Each manufacturer decides how few or how many situations they are going to test to. That is why NFPA as well as common sense and experience (and other codes) enter into the equation. It seems simple - follow the book - but as we can see here it is often a combination of many books!

A good real world example is the HearthStone Homestead, which addresses a lot of these point in their manual. In this case, then the hearth is raised 9” or more, no R-Value (just non-combustible) is required. When the hearth is only raised 2”. the R-value is still reduced from that needed under the stove (by about 60%)

This is almost a worse-case stove, since it sits on mini-legs, sits out near the end of the hearth (stove and glass can radiate down easier), etc.

See enclosed or go to HearthStone site

CONSTRUCTION TECHNIQUES:

DECKING REMOVAL INSTRUCTIONS: (By Elk, interpreted by Gooserider)The Decking can be removed from joist centers.  This is a support issue,  A lot of weight above.  Since it can be gotten to underneath, what I suggest tis to remove the first sub flooring. to the floor joist ,then solid block with 2 x 8,  2 x 10 or any combination flush to the top of the floor joists between the joists.  The nailing or support is the tricky issue of the blocking.  But toe nail and side nail through the joists as best you can.

Keep the blocks flush to the top of the joist then using 2 x 4’s sister to the original floor joists to add support to the blocking.  Nail the 2 x 4 supports with 16d common nails every 8” or less . You then can nail down the blocks from the top to the 2 x 4’s.  You now have replaced the support the sub floor decking provided. You can install cement board or other insulating boards to your desired height.  This will gain you 3/4” more inches and you can build up what you need. The support issue has been taken care of. In fact much stronger than the original decking. Naturally more work, but a doable solution to the problem.

Presumably, if the joists can’t be accessed from beneath, it would be possible to install the 2 x 4 support blocking first, then place the 2 x 8 or 2 x 10 solid blocking in from above.

Other good practices…

Include a layer of sheet metal - this is specified in NFPA standards when hearths are constructed of “loose” materials (like brick or block and sand in-between), but using a sheet between layers of cement board can cut heat radiation transfer. 24 gauge is usually specified for the loose hearths, but 26 (thinner) would be fine for a rigid hearth (glued thin-set or cemented).

Stagger fasteners - In theory a metal fastener such as a screw or nail is potentially a conductor that could transfer heat from a hot stove into combustible materials under the pad.  To avoid this potential problem, use fasteners that are just long enough to fasten one layer to the layer under it, and off set them from each other.  This will break up any potential heat transfer path.  Another useful thought is that your hearth pad is not going anywhere with a stove sitting on top of it - many people use fewer fasteners than usual in the area under the stove, as they are really only needed around the perimeter of the pad.

MATERIAL COMMENTS:

MICORE
If you are building a high R-value hearth and want it very close the floor (or flush) chances are that you will need to use some Micore or other ceramic board insulation.  Unfortunately, this material can be difficult to find.  The “big box” building stores do NOT carry it (as of this writing)  Some sources that may be able to supply it -
1. Your local stove dealer - ask the installers, not the sales people, as this is often not an officially sold product, but if the store builds hearths they may have it and be willing to sell a sheet.
2. Drywall / Sheetrock specialty dealers - they carry lots of unusual materials, some of which may be of interest.
3. US Gypsum manufactures Micore, their State Distributor List may help you find a source, though at least one forum member reports that the dealers listed are wholesale only, who aren’t willing to sell less than pallet lot quantities.
4. One member recommended Kamco Supply if you are in the New England or Eastern Seaboard area down to Virginia. in the US.
The last two sources may be good, but I am unsure how to verify that the material is actually the right stuff…
5. Some members have suggested that certain ceiling tiles may be made from it.
6. Other members have reported that it is used in the centers of many office cubicle dividing walls, which can be found cheaply at used office furniture places.


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