how thick is thinset in a hearthpad?

zachhandler · Nov 30, 2009

Hey everyone.

Here's my question: As I rebuild my hearth to bring it up to code, how much depth should I allow for thinset? I want it to end up flush to the floor. My layers are 3/4" plywood, 1/2" micore (thanks ebay), 1/2" durock, 3/8" slate tile. That's 2 and 1/8" total. But how thick does thinset usually end up, and do I need any thinset between the micore and durock?

By the way, after getting thoroughly "wrapped around the axle", I finally committed to an insert. Went the budget route with the Drolet wood burning insert. It was on display at menards, delivered to my door the same day for a $30 fee.. Seemed very well built, 1/4" steel topplate. I wanted either a big insert or even better a big cast iron hearth stove, but a small insert was the only way to go without ripping apart the 100 year old mantle. Since I may only be a few more years in this house I gotta keep things attractive to someone who doesnt care about wood heat. Here's a couple cruddy cell phone pics of the insert and the fireplace. Prepping the fireplace for slate as well.

Highbeam · Nov 30, 2009

Thinset thickness depends on the notch trowel but being thinset, it is really quite thin. Normal 12" tiles you can expect 1/8" to 1/4" max.

zachhandler · Nov 30, 2009

Highbeam said:
Thinset thickness depends on the notch trowel but being thinset, it is really quite thin. Normal 12" tiles you can expect 1/8" to 1/4" max.

Thanks! want to be sure I cut the right depth with the circular saw tonight.

I do have another question for any veteran out there: If I nail or screw the cement board and micore to the plywood subfloor, then does that lower the R value in all those spots? Is there a better way to stick the micore to the subfloor and the cement board to the micore?

Bobbin · Nov 30, 2009

I used a 1/4" trowel when I set the tiles which are 14" square, on my hearth. The husband routed the trim band (that fit over the duroc and contained the thinset) the depth of the tile's thickness (a quarter inch, thereabouts). The tiles are now flush with the top edge of the trimboard. Makes clean up easy and it looks nice.

I tried to trowel thinset over the entire hearth area before setting any of the tile and made a ton more work for myself than was necessary.

I felt really hurried and unecessarily so. The "open time" for thinset is really very forgiving and I had more easily workable time than I thought I did. Knowing that really helped me a lot when I tiled the shower in the bathroom.

The other mistake I made was in grouting. I grouted along the joint between the tiles and the chimney... evidently there is a heat resistant caulking you can use that will resist cracking with time and heat changes. My grout line there has cracked along the chimney. No other area of the grout has failed, however.

Highbeam · Nov 30, 2009

Buckthorn Burner said:
Highbeam said:

Thinset thickness depends on the notch trowel but being thinset, it is really quite thin. Normal 12" tiles you can expect 1/8" to 1/4" max.

Click to expand...

Thanks! want to be sure I cut the right depth with the circular saw tonight.

I do have another question for any veteran out there: If I nail or screw the cement board and micore to the plywood subfloor, then does that lower the R value in all those spots? Is there a better way to stick the micore to the subfloor and the cement board to the micore?

Don't worry about the screws effect on R-value. Use the screws per durocks spec. You are putting a heavy stove on this hearth and the micore is squishy. You want sufficient srcrewing. The tops of the screws will be covered by thinset and tile.

Be sure to properly wiggle and set the tile down into the thinset. If you're trying to hit it dead on with level to the floor then you'll need to be very consistent with the methods of application, the viscosity of the thinset, and the flatness of the substrate. I suggest you spend 10$ on extra tiles and practice on a piece of plywood. This will give you some practice and you can measure for final tile height. Mix the thinset to peanut butter thickness. I tend to mistakenly get too thin with it. Be sure to use those little spacers for consistent grout lines but I place them standing up so I can pluck them out of the tiles before the thinset sets.

Oh and wipe the tiles before that thinset sets up on the tops. It is much easier to get it off when it is wet.

tickbitty · Dec 1, 2009

Cool, I am glad you posted this because I am looking at doing about the same thing with my very similar hearth (slightly larger fireplace/clearances than yours but not by too much.) I started a thread last night asking what a good "sandwich" would be for the hearth materials. I know "Micore" is recommended but I've no idea where to get it. Also, "Hardibacker" and "Durock" - are those the same thing and redundant? I want my hearth to have good R value overall. I don't mind if it's not totally flush but I don't want it more than an inch or so more than the current floor, I think. I am extending mine out to 36" I think.

So, I need to find Micore and figure out what all else needs to be in the sammich and in what order. And I need heat resistant grout? So glad to have the benefit of all your experiences here!

Highbeam · Dec 1, 2009

I couldn't find micore so I had to use one of the other materials. You don't need special grout, just get the home depot stuff and mix it up. Hardibacker, durock, and wonderboard are all tile backer boards but they are made differently. The hardi is a wood fiber stuff and not a good choice for the hearth. The durock and wonderboard used to be a cement product with decent R-calue but I had heard that one of the brands changed their formulation making it a no-go for hearths. I used wonderboard three years ago for my hearth and it is good heavy stuff.

The most important thing to consider up front is your desired and required R-value. Then you have choices on how to get it. None of it is very expensive.

A 36" deep or wide hearth is tiny. mine is 4' deep by 6' wide and I only wish it were deeper.

firefighterjake · Dec 1, 2009

I figured the thinset added a little less than a 1/4 inch as I was using a 1/4 notched trowel, but of course the tile squished some of that down.

Screws . . . a non-issue. covered in tile and thinset and the heat should dissipate relatively quickly. In my opinion, it is far more important to know what you need to reach the R value requirement . . . well that and use plenty of screws to make sure the hearth is not rigid . . . and use something like Flexbond or Versabond which allows some flexibility to the tile/slate.

Durock . . . I can't speak to the Hardibacker or Wonderboard . . . I do know that the local Home Depot up here is still carrying the Fire rated/listed Durock. It's the 1/2 inch stuff and has a sticker on it stating that it is allowed for hearth construction.

tickbitty · Dec 2, 2009

Highbeam said:
I couldn't find micore so I had to use one of the other materials. You don't need special grout, just get the home depot stuff and mix it up. Hardibacker, durock, and wonderboard are all tile backer boards but they are made differently. The hardi is a wood fiber stuff and not a good choice for the hearth. The durock and wonderboard used to be a cement product with decent R-calue but I had heard that one of the brands changed their formulation making it a no-go for hearths. I used wonderboard three years ago for my hearth and it is good heavy stuff.

The most important thing to consider up front is your desired and required R-value. Then you have choices on how to get it. None of it is very expensive.

A 36" deep or wide hearth is tiny. mine is 4' deep by 6' wide and I only wish it were deeper.

TY for the information! That last part must be to me - my current hearth is only 18" by 64" so I am doubling it. It's probably for an insert or the unlikely event of a small freestanding that I could put in front of the fireplace but since most of those don't seem to list the clearances for a mantel etc I am going to assume that I should err on the side of caution and go with an insert probably.

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