# building my first hearth...  Mortar?



## beelinn (Nov 30, 2007)

Hello!  I'm a newbie!

My husband and I are getting our first wood stove soon.  We've wanted one for many years, but had some reservations because of the young mischief maker.

So, I'm building my hearth.  I've come into some confusion regarding mortars.  I'll be using mostly 12x12 slate rock tiles and maybe some smaller pieces around the outside and down the front.

The mortars.  What do I put under the tiles to stick to my cement board?  I've so far understood that I'll use fireplace mortar for between the slates to bond rock to rock, but what about rock to durarock?  Is it the same?  It's sounds like it would be, but what's this about fire curing?  Perhaps this is something the hardware store would be able to tell me.

I'm moving right along with my project, maybe I'll be able to figure out how to post pictures of all my steps!  I've got my 2x4 frame, then the 3/4 inch plywood on top.  I'll probably cut my durarock today and put that on.  Once I figure out this mortar business I'll be well on my way!


Thanks for any help you can offer!
Bee


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## moondoggy (Nov 30, 2007)

i think you need something like Versabond or Flex___  cant think of the dang name.....a flexible thinset.


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## Highbeam (Nov 30, 2007)

You don't need(want) to use fireplace mortar anywhere in your tile hearth. Just the plain old thinset sold at Home depot is what goes between the durock and tile. Apply the thinset with a notched trowel. Then put the tile down and let it set up. Then use either sanded or non-sanded grout between the tiles. 

No worries with the crumb snatchers. My 2 and 4 year old have no problem at all around the stove.


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## eba1225 (Nov 30, 2007)

Yes, "thinset" is what you need. Just ask for the stuff that is used for adhering tile to the substrate.  Any hardware store has the stuff.

Erik


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## JimWalshin845 (Nov 30, 2007)

beelinn said:
			
		

> Hello!  I'm a newbie!
> 
> My husband and I are getting our first wood stove soon.  We've wanted one for many years, but had some reservations because of the young mischief maker.
> 
> ...



I would start with a sheet of galvenized metal over the underlayment, set your durock in thinset, screwed 8" oc and roofing  nails in the field every 8 inches.

Use thinset to set your tile and then sanded grout for the tile joints.

Before you cut anything, do a dry layout for the whole project for proper fit and aesthetics.

Be very careful if you are using ungauged tiles/slate.  The tiles under your stove legs have to be very level.  

We recently complete a new hearth area and are very pleased with the results.

Good luck and welcome aboard,
Jim & Kathy


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## beelinn (Nov 30, 2007)

Thanks everyone!  If this thing turns out the way my brain thought it up, It should look pretty nice indeed!

Bee


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## tradergordo (Nov 30, 2007)

Unrelated to your question - but thought I'd mention it - make sure you check the manual on the stove you intend to purchase, and verify that the r-value on the hearth you are building meets the required minimum.  

It sounded like you were putting durock on top of plywood and slate on top of that.  Slate has basically NO r value unfortunately, and common durock cement board has an R value of just 0.26.  Those materials by themselves may not be enough to get your hearth pad up to code.  There are lots of materials you could use to raise it's r-value, micore 300 seems to be the most popular and is used in commercial hearth pads, another popular choice is simply an air gap, created by using metal studs.


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## mtcox (Nov 30, 2007)

Natural stone (slate included) requires a special thinset.  Using any type of thinset will not do.  Supposedly the slate will soak up the water in the thinset prematurely and not bond properly.  Home Depot carries the proper material.  

Be sure your slate is clean and dry when you put it down.  The mining process for natural stone has a tendency to leave it dirty.


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## JimWalshin845 (Nov 30, 2007)

tradergordo said:
			
		

> Unrelated to your question - but thought I'd mention it - make sure you check the manual on the stove you intend to purchase, and verify that the r-value on the hearth you are building meets the required minimum.
> 
> It sounded like you were putting durock on top of plywood and slate on top of that.  Slate has basically NO r value unfortunately, and common durock cement board has an R value of just 0.26.  Neither of those materials by themselves is going to get your hearth pad up to code.  There are lots of materials you could use to get it up to code, micore 300 seems to be the most popular and is used in commercial hearth pads.



1/2" Durock is an approved protectorant for floor and wall construction and meets the "K" factor for most installations.  I also suggested a metal heat shield since the subfloor is a combutable material  You can refer to their document of compliance:

http://www.usg.com/USG_Marketing_Co...d_Submittal_Sheets/CB198_Durock_Cement_Bd.pdf


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## JimWalshin845 (Nov 30, 2007)

mtcox said:
			
		

> Natural stone (slate included) requires a special thinset.  Using any type of thinset will not do.  Supposedly the slate will soak up the water in the thinset prematurely and not bond properly.  Home Depot carries the proper material.
> 
> Be sure your slate is clean and dry when you put it down.  The mining process for natural stone has a tendency to leave it dirty.



My Uncle Joe (by marriage) will be rolling in his grave.... he was an Italian stone mason, did beautiful work.  When I got my first home he helped me (really I helped him) put down a large patio of ungauged random slate.  We put it down with straight portland cement.  

There are additives but thinset is basicly just portland and silica (very fine sand).  There are latex additives and mold inhibiters, but I beleive thinset out of the bag will due.

Yes, to help avoid quick setup you may want to take a dry sponge to the bottom of the slate to avoid slaking and maybe even butter the back of the slate too since it is not gauged.


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## beelinn (Nov 30, 2007)

Ya know, I just spent the last half hour reading the manual for my future stove, and there is nothing in it about required K or R factors!

Maybe someone else knows more than me!  The stove is just a little guy; a Vermont Castings Aspen model.    I'll be getting the double wall pipe and rear heat shield options.  It already comes with it's bottom heat shield.

I don't understand why slate wouldn't be good.  I got the idea from a brochure that sells hearth pads! :bug: 

Bee


BTW, you all are giving me some good info!


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## Hogwildz (Dec 1, 2007)

Home Depot has 16" x 16" slate that all ready grooved to accept the adhesive. Used them myself and came out great.
Any joints over 1/8" wide, you'll def want to use the sanded grout.


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## JimWalshin845 (Dec 1, 2007)

beelinn said:
			
		

> Ya know, I just spent the last half hour reading the manual for my future stove, and there is nothing in it about required K or R factors!
> 
> 
> Bee
> ...



Bee,

Read the link I sent you in a previous post, it shows the "K" value you need, or atleast for the floor.

http://www.usg.com/USG_Marketing_Co...d_Submittal_Sheets/CB198_Durock_Cement_Bd.pdf

This is also a good link as per National Fire Protection Code:

http://www.nfpa.org/freecodes/free_access_agreement.asp?id=21106&cookie;_test=1


-Jim-


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## JimWalshin845 (Dec 1, 2007)

Hogwildz said:
			
		

> Home Depot has 16" x 16" slate that all ready grooved to accept the adhesive. Used them myself and came out great.
> Any joints over 1/8" wide, you'll def want to use the sanded grout.



I would not use a tile/slate adhesive here.  Thinset only due to the heat factor.

Also, and this is from experience, you are going to have large grout joints since you are using random slate.  You should *not* apply the grout as you would a normal tile job with a rubber grout float.  The portland will invade every nook and cranny and a real b_^%$! to clean up without using muriatic acid.  So make your grout mixture very stiff, almost crumbly and use a puddy knife or jointer to apply the grout into the joints.  It will take longer to do the job but will be much cleaner.


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## tradergordo (Dec 1, 2007)

Jim Walsh said:
			
		

> 1/2" Durock is an approved protectorant for floor and wall construction and meets the "K" factor for most installations.  I also suggested a metal heat shield since the subfloor is a combutable material  You can refer to their document of compliance:
> 
> http://www.usg.com/USG_Marketing_Co...d_Submittal_Sheets/CB198_Durock_Cement_Bd.pdf





I think they may have really bumped up the requirements in recent years.  1/2" Durock won't come close to meeting the k factor required for these installations.  I haven't read up on "most stoves" so I can't say for sure.  But the requirement for my stove (vermont castings dutchwest large) is an R-value of 1.19.  k is a function of R, so you can easily convert between the two.  Like I said, the R value of 1/2" durock is .26 (k value is 1.92).  R = 1/k * thickness.  If you were using nothing but 1/2 durock, you would need: (1/1.92)*thickness= 1.19 or thickness = 2.285 inches.  That means you would have to use 5 layers of 1/2" durock to meet code, or possibly 4 layers plus your slate on top. 

There are also minimum total hearth pad thickness requirements - in my case it is 1" minimum, so again 1/2" durock would not be adequate.

As for the use of slate - I never said it was a bad idea, I like slate.  People use all kinds of nice looking stuff on top of their hearth pads including decorative tiles.  All I'm saying is that tiles and slate add little to the total R value unless you are talking about substantial thickness.  An inch of slate has an r-value of .05, you would need two feet of it to meet code for my stove.  I did take a quick look at your Aspen owner's manual.  Yes, I don't see any mention of a required R-value.  I don't know if there is any particular default value.  It does say:
"For installations with the heat shield attached, use a noncombustible floor protector such as 1/4” nonasbestos mineral board or equivalent, or 24 gauge sheet metal. The floor protector may be covered with a noncombustible decorative material if desired."

Sounds like these are very minimal requirements compared to my stove! Might have something to do with when the model first came out?  Not sure.


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## JimWalshin845 (Dec 1, 2007)

These specs are for the current Aspen 30000369 2/07 Rev. 13:

The stove is tested under ANSI/UL-1482 for the United States and ULC S627
and CAN/CSA B366.2 for Canada.

Even when the bottom heat shield is installed, you must
provide special protection to the floor beneath. For
installations with the heat shield attached, use a noncombustible
floor protector such as 1/4” nonasbestos
mineral board or equivalent, or 24 gauge sheet metal.
The floor protector may be covered with a noncombustible
decorative material if desired. Do not obstruct the
space under the heater.
Protection requirements vary somewhat between the
United States and Canada as follows:
U.S. Installations: The floor protector is required under
the stove and must extend at least 16” from the front of
the stove (B, Fig. 14), and at least 6” from the sides and
rear (A, Fig. 14). It must also extend under the chimney
connector and 2” to either side. (C, Fig. 14)
In Canada: A noncombustible floor protector is required
under the heater. The floor protector must extend 18”
(457 mm) to the front (B, Fig. 14) and 8” (203 mm) from
the sides and rear. (A, Fig. 14)

Durocks specs are under CB198/rev. 7-07:

Durock cement board may also be used as a floor protector in place of one layer of 3/8" thick millboard. (Having a thermal conductivity of k ≤0.84 Btu in./(ft.2 h °F) in the minimum dimensions specified by the room heater/stove manufacturer.)

As suggested by the moderators and building code enforcers here on the forum, the application of sheetmetal over a combustable floor, then 1/2 Durock and a noncombustable decorative meterial should meet code when a stove has a bottom heat shield and is at least 6" off the floor.


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## JimWalshin845 (Dec 1, 2007)

> author="tradergordo" date="1196490270"  But the requirement for my stove (vermont castings dutchwest large) is an R-value of 1.19.




If I remember correctly, isn't the Dutchwest convertible as either stand alone or used as an insert?  Aren't the optional legs very short?  

Please correct me if I am wrong,
Jim


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## tradergordo (Dec 2, 2007)

Jim Walsh said:
			
		

> > author="tradergordo" date="1196490270"  But the requirement for my stove (vermont castings dutchwest large) is an R-value of 1.19.
> 
> 
> 
> ...



No its only standalone, legs are 6 inch, bottom heat shield is standard.  I do think the 1.19 r-value is overkill, very little heat under the stove, you could easily put your hand anywhere on the hearth pad even when its blazing away...


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## Hogwildz (Dec 2, 2007)

Jim Walsh said:
			
		

> Hogwildz said:
> 
> 
> 
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Grout float worked fine for me. Natural slate should be sealed, which I did.  It cleans up easy. Exactly what I did, and had no problems with grout in the nooks etc. Sealed it 3x pre grout, cleaned it off after grouting. A plastic bristle scrub brush works great for the indentations & nooks etc. The sealed 2 more coats. Will prolly do 1 more come spring. I used pre mixed adhesive and have had no problems with it at all. Not to mention the adhesive is more forgiving than thinset as far as movement  etc.
To each their own, I know what worked for me, and have had no problems.


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## Gooserider (Dec 2, 2007)

First off, there is an article in the Wiki on building a hearth or hearth extension - I highly reccomend it!  (I ought to since I wrote most of it...)

1. Trader and Jim, you are BOTH wrong in some cases, either of you could be right in others...  Each stove is tested and certified with a required level of floor protection.  The exact number depends on the stove, and must be met.
IF the requirements state only a "Non-combustible surface" or something like the 3/8" millboard mentioned earlier as called for by the OP's mentioning that she will be installing a VC Aspen, then ONE layer of 1/2" Durock will meet the requirements, especially if it is covered with a layer of slate or other protective tile.  It is "good practice" to put a layer of sheet metal under the Durock, but it isn't required.

2. If using a frame to make a raised hearth, you must make it out of steel studs and top it with 2-3 layers of Durock (for strength) IF you are wanting it to be part of the "thermal sandwich" for the hearth pad (often the case if making a pad with a high R-value).  If you are only seeking to raise the height of the stove for aesthetic reasons, and only need a "non-combustible surface" then the materials used in the platform don't matter, as long as they provide the required strength and the stiffness required to support the tile / slate floor.  In the case of the OP, I'd probably use 2x?'s on about 8" centers, topped with a layer of 3/4" plywood, then a layer of sheet metal, 1/2" Durock, and the slates.  I would size the hearth to be at least as big as the manual requires, then adjust to make it fit the size of slates being used with minimum cutting.

3. The material to use for bonding everything together is a "Mortar meeting ANSI spec. 118.4 or 118.1" according to the Durock tech pages.  This translates to a LATEX MODIFIED THINSET such as Flexbond, or Versabond.  Use a notched trowel to spread a thin layer between the subfloor and the sheet metal (I used roof flashing) and between the sheet metal and the Durock, place the Durock and screw everything down with cement board screws (NOT dry-wall screws!)

4. Use the same thinset to put the slates down, spread it with a 3/8" notch trowel (sometimes called a floor trowel) - you do NOT need special thinset for slate - you might for other stone types, but should still meet the ANSI specs above.

5. If the joint between the slates is less than 3/8", use a non-sanded grout, if 3/8" or greater, use sanded grout.  When I did my slate floor, I was advised NOT to seal the tiles ahead of time, as they said any sealant in the cracks might reduce the grout adhesion.  I used a grout float, and had very little trouble cleaning the excess grout off by sponging as I went along.

Gooserider


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## JimWalshin845 (Dec 3, 2007)

> author="Hogwildz" date="1196593161"...... I used pre mixed adhesive and have had no problems with it at all. Not to mention the adhesive is more forgiving than thinset as far as movement  etc.
> To each their own, I know what worked for me, and have had no problems.



Please don't suggest to people to use a MAPI type of adhesive on or around their stove.  It does work wonderfully for other purposes, but as per hearth applications it is a hazzard!

Your hearth job will look fine for a few years... 3...5 maybe 10 years depending upon how much and long you burn.. then it will fail.

Please don't advise this method again...

P L E A S E?????


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## JimWalshin845 (Dec 3, 2007)

> "Hogwildz" date="1196485300"]Home Depot has 16" x 16" slate that all ready grooved to accept the adhesive. Used them myself and came out great.



<Cough>... Bee is using UNGAUGED and RANDOM SLATE.... a very different animal than gauged quarry slate.  Please go back to your cave drawings... I am really surprised you are hanging out at HD or Lowes! :coolhmm:


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## tradergordo (Dec 3, 2007)

Jim Walsh said:
			
		

> > author="Hogwildz" date="1196593161"...... I used pre mixed adhesive and have had no problems with it at all. Not to mention the adhesive is more forgiving than thinset as far as movement  etc.
> > To each their own, I know what worked for me, and have had no problems.
> 
> 
> ...



Just curious - but what is the nature of the hazard?  If the concern is about flammable adhesives or mixes that produce toxic gasses when heated - that's one thing.  But if the concern is simply that the adhesive may lose its bond (which can and will happen with just about any thinset or mortar), that's not a hazard.  Remember that the noncombustible tops on most hearth pads (slate in this particular case) is simply DECORATIVE.  Most hearth tops (tile/stone/etc) are there to make it look good and aren't even included in calculating the total r-value of the pad.   If it falls apart you fix it.


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## JimWalshin845 (Dec 4, 2007)

> Just curious - but what is the nature of the hazard?  If the concern is about flammable adhesives or mixes that produce toxic gasses when heated - that's one thing.  But if the concern is simply that the adhesive may lose its bond (which can and will happen with just about any thinset or mortar), that's not a hazard.  Remember that the noncombustible tops on most hearth pads (slate in this particular case) is simply DECORATIVE.  Most hearth tops (tile/stone/etc) are there to make it look good and aren't even included in calculating the total r-value of the pad.   If it falls apart you fix it.



So after it fails because of heat this is what starts to happen





Some mastic adhesives are flamable and many do give off noxious/toxic gases when burned. 

They are not meant for the job and meet ANSI.  When it turns to dust you'll end up with toe kickers and maybe it won't be you, but when someone trips into a hot stove it usually isn't pretty.


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## tradergordo (Dec 4, 2007)

That's a great picture - I think it perfectly illustrates why they ramped up the r-value requirement for hearth pads.  A single layer of durock obviously doesn't cut it (OK - not really sure what that is exactly in the picture so I'm speculating a little).  I don't think anyone should be using a hearth pad that could do that if the docorative top were missing (much less only cracked or poorly sealed).  

I was thinking about how they came up with the r-value requirement for my stove since it puts almost no heat down to the floor.  I am starting to think they go by the assumption that someone is going to set some wood directly on the hearth pad, and that wood is going to catch on fire because it was too close to the stove (or maybe leaning against the stove) when the owner is not around, and its going to burn all the way to ashes - and that hearth pad is going to be good enough to withstand all that and prevent the house from burning down.


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## JimWalshin845 (Dec 4, 2007)

That is not Durock so please don't insinuate anything.


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## tradergordo (Dec 4, 2007)

What is it?  And how do you know durock wouldn't have had the same result given whatever conditions that material were under?


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## Gooserider (Dec 4, 2007)

tradergordo said:
			
		

> What is it?  And how do you know durock wouldn't have had the same result given whatever conditions that material were under?



As best I can tell from the picture, the peice that is tilted up is the tile or whatever is being used for the floor treatment.  The underlayment looks like some sort of cement board, covered with whatever adhesive was used to fasten down the tiles, can't tell if it's Durock brand or not.  The discoloration (I'm not sure it's enough to rate being considered "charring") I wouldn't say is necessarily a problem with the cement board, as it could be a stain from the thinset or other material.

What I can say is that Durock is "UL LISTED for use with UL Listed solid fuel room heaters and fireplace stoves" and that Durock's data sheets specify the ANSI 118.4 or 118.1 standard for the mortar.  When I was planning out my hearth extension, and doing some research for the Hearth Construction article in the WIKI, I spent considerable time on the phone calling the tech support lines for Durock, Hardibacker, Wonderboard, and Permabase.  Durock was the ONLY one that said flat out that hearth pad construction was a reccomended application.  Hardibacker said they were approved for making clearance reduction walls, but NOT for hearth pads, Wonderboard didn't really answer beyond saying that it shouldn't be exposed to more than 350*F, and Permabase said they were "not non-combustible"

When I asked for clarification on the mortar specs, Durock's tech support folks said that the standards basically translated to "Latex modified" thinset, and specifically suggested the Flexbond brand. (Which isn't one of their products BTW) I have subsequently heard from another poster that Versabond also meets the ANSI standards, however NONE of the "readymix in a bucket" products I've checked do.  Both Flexbond and Versabond are dry powder products that get mixed with water at the time of use (like concrete) - the reason given for the use of the latex fortified mortar is that it is better able to handle the thermal cycling that a hearth undergoes.

BTW, my Encore cat (w/ required bottom and ash pan door heat shields) requires only a "non-combustible surface" such as '1/4" non-asbestos mineral board or 24g sheet metal' - Durock plus tile far exceeds this requirement.  

What I've observed is that my raised brick hearth (that barely met the old NFPA spec as it turns out) gets hot enough to be on the edge of what is comfortable to stand on for any length of time.  The 18" Durock and slate tile extension gets decidedly warm, but not uncomfortable.

Gooserider


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## JimWalshin845 (Dec 4, 2007)

tradergordo said:
			
		

> What is it?  And how do you know durock wouldn't have had the same result given whatever conditions that material were under?



Gordo,

I have some things to do before I answer your questions in detail... but please, tell us in the meanwhile if you used out-of-the-can mastic or did you use mortar based thinset on your new hearth?

Just curious,
Jim


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## tradergordo (Dec 5, 2007)

Jim Walsh said:
			
		

> tradergordo said:
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> 
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Thinset.  I don't even know what mastic is - haha.  Every set of hearth building instructions I've read said to use thinset.  My hearth consists of a layer of durock, a layer of micore 300, another layer of durock, and ceramic tiles on top.


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## JimWalshin845 (Dec 5, 2007)

tradergordo said:
			
		

> Jim Walsh said:
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Hey Gordo, 

Nice stove and floor... but those curtains and wooden window/floor trim near rhe stove?.... 

Please tell us you modified that since you did the install.

We wouldn't want Bee, who wants good advice, to see that set up would we?


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## Hogwildz (Dec 5, 2007)

Jim Walsh said:
			
		

> > author="Hogwildz" date="1196593161"...... I used pre mixed adhesive and have had no problems with it at all. Not to mention the adhesive is more forgiving than thinset as far as movement  etc.
> > To each their own, I know what worked for me, and have had no problems.
> 
> 
> ...


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## JimWalshin845 (Dec 5, 2007)

I'll take this as a personal attack.  :coolmad: 

But again, your suggestion as to use a mastic is NOT appropriate, and as per Bee's application with random ungauged slate it is NOT recommended by any means. :shut: 

So your point is? :question:


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## JimWalshin845 (Dec 5, 2007)

OOOOOP's... and I forgot you asked how our hearth came out! :coolsmile:


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## colsmith (Dec 5, 2007)

My limited experience with hearth building, which we did exactly once two years ago, was as follows.  As is typical, hubby sent me to Menards to buy the stuff we needed, because he doesn't like shopping.  I REALLY wanted to buy the pre-mixed tile adhesive, since it came in much smaller containers, would be less messy (no dust), harder to screw up (if wrong amount of water added, etc.), easier to use (you don't need to worry about timing quite as much), plus  it was cheaper.  However, he had told me to buy some named kind of mortar and it wasn't called that.  I spent lots of time on the cell phone with the tech support of the mortar/adhesive companies, after deciphering their numbers on the packages.  Had to buy the latex modified thinset because the pre-mix stuff was not certified for hearth applications, had to do with heat tolerances over time.  That is not to say there couldn't be some pre-mix tile adhesive that is okay, but Menards, a large DIY chain, didn't have any like that.  I was bummed because the thinset mortar stuff that was appropriate and correctly rated for such an application only came in a 50 or 60 pound bag and I could hardly lift the darn thing in and out of the shopping cart and into our truck, it being an awkward shape and not having handles or anything.  And it was messy and dusty and all those things, but that was the only choice I had at that time at that store.   I'm not an expert by any means, but if you want some thinset mix we have plenty left. 

We had plywood, Micore 300, durock, and ceramic tile as our base, we needed an R value of 1.  As others have said, the hearth itself doesn't get very warm, but it is true that occasional sparks and hot bits of wood visit it, so it needs to be able to handle heat.


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## JimWalshin845 (Dec 5, 2007)

God I hope your hubby doesn't expect you to make pancakes in the morning? ;-P   Mixing thinset is 'just add water'.. you don't have to add the eggs and oil. :red:  :red:  ;-) 

Kathy and I worked on the project together, I could have sent her out and she would have picked up the 40# bag of thinset or batted her cute eyelashes and I am certain a masculine man would have loaded it into the van.




As per cost?.... 40#s of thinset is about $10.00 US.  For applying ~30sf and applying both the Durock and 13" lugged PEI-5 tile we still had some left.  Show me an out-of-the-can product that will do that with a heavy lugged tile (btw.. mastics are NOT recommended for heavy lugged tile).


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## tradergordo (Dec 5, 2007)

Jim Walsh said:
			
		

> Hey Gordo,
> 
> Nice stove and floor... but those curtains and wooden window/floor trim near rhe stove?....
> 
> ...



It meets all clearances to combustibles, and is up to code, thanks for the concern.  Again these are the things people need to check in their owner's manuals and/or with the local fire department and building inspector, before installing a stove.  As for the curtains, they were soaked in flame stop.  I personally burned a sample of the curtain, pre and post treatment, with a propane torch, to see the effectiveness of the product for myself.  Great non-toxic product, I highly recommend.  They have demo videos on their website as well.


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## kd460 (Dec 5, 2007)

In the tile business for a long time.  Unsanded grout (or also called dry grout) should NOT be used for joints OVER 1/8 wide (someone mentioned using it for larger joints). It is not strong/durable enough. Maybe with the old wet grouts (look that one up) but those have not been used for about 20 years. Anything wider than 1/8 inch should really get sanded grout (also known as joint filler). 

Latex additives in the thinset and the grout do a few things: INCREASE bond strength, reduce porousity, and allow for slightly more flexion over a span. They do have some built in meldewcides and help retain color. There are a few new additives on the market that do the same thing as the latex modifiers, but are not latex. Jury still out on them. Not even sure about heat applications.

As far as using mastics (most mastics are an organic based adhesive) to bond tile with. Anything over 4x4 inch will take forever to cure out. It requires solvent evaporation for the stuff to "dry". Any tiles larger than that takes along time, especially glazed tiles. No place for the solvents to go except in the joints ( no adsoprtion/vapour transmission. I personally would not want to use this type of adhesive in an area that receives heat, but, if it has the specs and says it's OK, then no blood, no foul. I have used heat many times to remove tiles that have been installed using mastics. The thinsets are just so much more stable and add strength to the entire job. I feel the thinset is better for this type of application. I am also actually surpized that the latex additives in the thinset are not an issue with heat, but all these products have been tested, so if the specs say it's ok, then it's OK. KD


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