Installing Durock correctly on a wood subfloor

[CWYfire]

This has probably been asked hundreds of times and I will be reading other threads in the mean time. But thought I would ask as I am about to undertake this in the next few days.

Could someone describe how to install Durock correctly on a wood subfloor? Does one need to pull a coat of something like thin coat prior to screwing down the Durock to the 3’4” plywood subfloor? Currently we plan to cover the Durock with some sort of tile. Any suggestions on that portion or what might be a better finished surface than tile would also be appreciated.
Thanks for all of your incredible advice!
Ed

 

[webby3650]

Some tile-setters lay the durarock in a bed of thinset before screwing the daylights out of it, this makes a rock solid base. Also, keep R-values in mind. Your stove may require more protection than others.

 

[LLigetfa]

As mentioned, it depends on required R values and how stiff the tiles need it to be. If you use really strong porcelain tile, it doesn't much matter how you lay the durock.

One way to get more R is to sandwich layers of metal diamond stretch mesh lath and durock. Diamond lath is often used for stucco. Two layers at right angles traps enough air to equal a couple of inches of masonry.

 

[Elfin]

Webby is right, but another thing that should be added is a plywood support layer. Durock adds no structural support at all, and if used alone over subflooring you are likely to ultimately develop cracks in your tile and grout.

But like webby said, the first and biggest consideration is the requirement for your particular stove... if only a non-combustible pad is required, plywood and durock covered with a layer of tile are adequate, with respect to all required clearances.

So if that's your only pad requirement, the way I'd personally choose to layer this over subfloor is:

1. 3/4" plywood, screwed into the subfloor but not into the joists
2. concrete board, set into modified thinset and also screwed into place
3. solid-bodied porcelain tile (much stronger than ceramic and unlikely to chip or show damage)

 

[begreen]

Good advice. CWYfire, can you add your Century stove to your signature?

 

[webby3650]

Webby is right, but another thing that should be added is a plywood support layer. Durock adds no structural support at all, and if used alone over subfloor you are likely to ultimately develop cracks in your tile and grout.

But like webby said, the first and biggest consideration is the requirement for your particular stove... if only a non-combustible pad is required, plywood and durock covered with a layer of tile are adequate, with respect to all required clearances.

So if that's your only pad requirement, the way I'd personally choose to layer this over subfloor is:

1. 3/4" plywood, screwed into the subfloor but not into the joists
2. concrete board, set into modified thinset and also screwed into place
3. solid-bodied porcelain tile (much stronger than ceramic and unlikely to chip or show damage)

I believe Durock installed correctly does provide plenty of support. I think the screws are supposed to be on a 4" grid. That is how I did mine, I didn't want a big elevation change since mine is in the middle of the room. Mine gets walked on occasionally, wood dropped on it and it has no signs of cracking or flexing. It's been about three years now.

 

[LLigetfa]

When I built my house, I used 3/4" Strudifloor OSB which is much stronger than plywood. I put down diamond lath in a 3/8" layer of modified thinset for the porcelain tile and have not had a single cracked tile or grout joint. I realize a hearth requirement is different but this does make a point for comparison.

If you have access to the subfloor from underneath, you can double up the subfloor and/or install blocking on the underside to make it more rigid.

 

[LLigetfa]

I didn't want a big elevation change since mine is in the middle of room.

In my former home, I wanted the hearth to be flush with the floor because it was in a traffic area and a raised hearth would have been a trip hazard. I actually cut out the original subfloor and a little off the tops of the joists and used blocking to support a sunken subfloor. My stove required four inches of masonry for the hearth but through the magic of the layered diamond mesh, I got it down to an inch and a half. The tile never cracked from foot traffic and even dropping a few big splits didn't hurt it.

 

[wellbuilt home]

I don't think most stoves need r value on the floor . My dog would dry out pretty bad under the stove and i would turn her over before i went to bed .I just lay a tar paper sheet under the wire lath and coat it with the cheep home depot thin set . Then i set the tile in a 2 ND layer of thin set . Ive done thousands of sf of tile like this and never had a problem . Porcelain tile needs to be set in the modified thin set . I like to screw down the existing floor. If the height doesn't matter you could add a layer of ply wood

 

[humpin iron]

Wellbuilt is incorrect, read your owners manual to find out what the R-value under your stove must be. There are also areas in this web site that help you understand hearth building and R-values.
If you do it wrong it"s like eating double cheese burgers everyday.....................eventually you meet a cardiologist.

 

[LLigetfa]

I don't think most stoves need r value on the floor...

My RSF is a zero clearance fireplace with glass door. The four inch masonry requirement was for the hearth because of the radiant heat. Most stoves are elevated enough not to require it.

BTW, the best way to apply blocking is to cut 2x4's at a very slight bevel so that they are a little longer than the distance between the floor joists. You can then lather on some PL and hammer them into place and you only need to use one toe-nail per side. Altenate their direction so you have kind of a herringbone pattern.

 

[ansehnlich1]

I got 4 layers of durock on 3/4 inch advantech subflooring with a couple LVL's underneath and crossbraced nice. The durock is layed in loose without screws to avoid heat transfer. You can see the brick edge, some mortar around the edges and seams, otherwise, no screws, no nothin'. Brick layed on top ain't budged.

 

[fossil]

Depends entirely on the stove and how it's built and tested and certified. Some stoves require nothing more than non-combustible ember protection for a combustible floor, while other stoves require considerable insulative (r-value) properties in the floor protection. Not something that can be generalized, it must be verified for the particular appliance and then the hearth built to comply with the manufacturer's requirement. Rick

 

[webby3650]

That's a great looking set-up there, I love my 500, bet you do too! That Advantech flooring is great stuff! A local lumber yard did a test the first year it was available, they put a 2'x4' piece in a 55 gallon drum of water, after 3 months it had no signs of swelling. That is tough stuff!

 

[LLigetfa]

You can see...

What is it with workmen and baggy jeans? Reminds me of babies with full diapers. At least we were spared any serious cleavage.

I would think the weight would ensure some heat transfer. The metal screws would add little more.

 

[Elfin]

Wow... I've never seen a hearth-building thread go SPLAT in so many directions before!

Webby, Durock and other concrete backerboards have no structural integrity and do not add to the structural strength of subflooring. They should also not be directly attached to any structural component such as joists. Don't take my word on it... google it, or ask the experts over at the http://www.johnbridge.com tile forum.

Durock is, however, one way to build up your R-value, as nicely demonstrated by ansehnlich1. Great photos of the process, and a nice-looking finished product too!

 

[webby3650]

Wow... I've never seen a hearth-building thread go SPLAT in so many directions before!

Webby, Durock and other concrete backerboards have no structural integrity and do not add to the structural strength of subflooring. They should also not be directly attached to any structural component such as joists. Don't take my word on it... google it, or ask the experts over at the http://www.johnbridge.com tile forum.

Durock is, however, one way to build up your R-value, as nicely demonstrated by ansehnlich1. Great photos of the process, and a nice-looking finished product too!

I am not talking about building a house out of Durock, I am simply saying, if you have only subfloor and it gives a little bit, after you install Durock over it with thinset and the amount of screws as recomended it will stiffen the area.

 

[LLigetfa]

Durock and other concrete backerboards have no structural integrity and do not add to the structural strength of subflooring.

That's like saying the web of an I-beam doesn't either. Tell it to an engineer. Anything between a bottom cord and a top cord is a factor.

 

[fossil]

If your wood subfloor is nice and tight and the underlying structure is adequate, then there's nothing in the world wrong with simply troweling a bed of thinset (using an appropriately serrated trowel) on the subfloor and screwing the Durock directly down onto that. It's standard building practice for floor tile jobs of many descriptions. If the single sheet of Durock and whatever you plan to finish it with on top provides all the floor protection required for your appliance (in terms of r-value), then there's nothing else you need do. Rick

 

[CWYfire]

WOW! I AM BLOWN AWAY! Never expected to get so much help so quickly! THANX EVERYONE!

Fossil, I have posted a similar question and appreciate you looking at the specs to assure there is no elaborate process necessary for a hearth under my stove. I am trying to be cautious rather than get over zealous and burn down my house.... LOL

I have built homes for years. But this is my first wood stove install and it is on my own home where I keep all my goodies.... LOL Rather be safe than sorry....

I do have a good solid 3/4" subfloor with plenty of support under it and doubt it will move. It sounds like probably all I need to do is trowel on some thinset and screw down the Durock about every 8".

Is there a specific brand or type of thinset that I need to use and could someone elaborate more on the correct serrated trowel and manner of using it, etc.?

Nice Pics of the ACTION shots ansehnlich1! That hearth install is on a concrete slab rather than a wood floor correct?

Thanks Everyone!
Ed

 

[Elfin]

Hey, it's your house... do it however you want. Many opt for the get-er-done-quick way, but that's certainly not the type of advice I'd feel comfortable giving.

No tile pro who is willing to guarantee their work is going to install durock directly on subfloor, especially when that install is followed by putting a heavy stove on top of it... repairs will be needed in short order. And technically, you also should be checking your joists for potential deflection of the substrate. Too much joist movement means some sorts of tile or stone will simply not be recommended unless the joists are reinforced.

But again, when it's in your own house, and you are assuming responsibility for all repairs and installation failures, you can do whatever you wish.

 

[fossil]

...Fossil, I have posted a similar question and appreciate you looking at the specs to assure there is no elaborate process necessary for a hearth under my stove...

That wasn't me in the previous thread who checked that out for you, I believe it was InTheRockies who pasted a page from the manual which said that a grouted ceramic floor tile surface, properly installed, is a suitable alternative to the 3/8" millboard. Credit where credit's due. On the floor, Durock and tile are sufficient, so long as it extends out the required distance all around the stove. The wall clearances sound like the things you really need to be careful with in your installation. Rick

 

[ansehnlich1]

Nice Pics of the ACTION shots ansehnlich1! That hearth install is on a concrete slab rather than a wood floor correct?

NO, that ain't concrete, that brick hearth is built directly on a 3/4 inch subfloor of stuff that's called Advantech. No screws, just laid in Durock as you see it. Same mortar used for brick was used for filling in around edges, and doing seams.

No brick has moved, but I gotta tell ya, we have engineered floor trusses and planned ahead of time for the structural support of the hearth, including some heavy duty LVL's underneath. No movement in the floor whatsoever.

I wanted to build the hearth so that if I should ever get rid of the Oslo and buy something else, chances are I'll have the "r" value needed for several options of stoves and won't have to tear up the whole hearth to start over.

 

[LLigetfa]

Personally, I don't have faith in 3/4" plywood. Are the floor joists cross-braced and strapped as well? If it's a small area and you have access to the subfloor below, add additional bracing against the underside of the plywood. I don't mean the cross bracing. You should have that too.

You can buy modified thinset or you can add latex. I'd pre-treat the wood with latex anyway so then I'd just add some latex to regular thinset rather than buy modified. You can use a 1/4" square notched trowel. I would back-butter the tiles.

 

[fossil]

...No tile pro who is willing to guarantee their work is going to install durock directly on subfloor...

Perhaps we're not speaking the same language here. When I use the term "subfloor", I'm talking about tongue & groove plywood or OSB or whatever, typically 3/4" thick, specifically engineered to be applied directly to the floor joists/underlying structure. It's the continuous surface that's underneath your carpet, or your hardwood, or your vinyl, or your whatever. It's your floor, basically. If you want tile on it, then the typical way that's applied is to affix a sheet of Durock or wonderboard, or other suitable tile substrate, either 1/4" or 1/2 " thick directly to that subfloor and then set the tile. It almost sounds as if you think I'm saying it's OK to use Durock as subfloor...not! Rick