Mortar for stone

[mjlambert]

I am in the process of installing a wood burning fireplace and the next step is the stone work. We are going with the thin cultured stone. Looking at the directions, since it is an interior installation, we put cement board up and planned on putting the stone right to the cement board, without metal lathe. I am trying to figure out what mortar to use but am not having any luck finding one that meets the specs required. I was hoping to just buy some pre-mixed bagged type N mortar but it doesn't meet the spec. It says I need some kind of modified mortar. Can someone point me in the right direction?

Looking at the installation directions, another option would be to put the wire lathe up because then I can just use the type N mortar. Any thoughts?

 

[double-d]

I believe that you will find that a "modified mortar" contains polymers that are formulated into the dry mix that increase its performance and strength. Since you are not using the Lath, you would need this type of mortar.

 

[mjlambert]

I agree with needing to use modified mortar but where can I find it?

 

[double-d]

Looks like Home Depot has one...
"Custom Building Products Complete Contact Gray 50 lb. Large Format Rapid Setting Tile Mortar"
Non-slump - for use as a thin or medium bed mortar up to 3/4"
Complete Contact LFT Rapid Setting Mortar is a polymer modified, full contact medium bed mortar. It offers full mortar transfer to the back of the tile

 

[mjlambert]

Thank you, that should work!

 

[Highbeam]

Isn't this just regular old thinset? Like you use for tile.

 

[mjlambert]

It does seem like it is more for floor tile, but it has the ANSI A118.4 spec that the stone install instructions mention. I'm not sure I like the fast setting though.

 

[double-d]

Isn't this just regular old thinset? Like you use for tile.

You are correct, it is Thinset, but they offer it it in standard and with the Polymers for extra strength, from what I have read

 

[bholler]

We always use lath and mortar mixed with 3 parts sand to one part type n. Then we add an acrylic modifier and we have yet to have any fail. You can then use the same mortar to point the stone with if you are pointing it

 

[Squirrel1410]

Thank you, that should work!

I just finished doing mine - Stoneyard sent their veneer with "Lacrete" hi-bond modified mortar. Went directly to the cement board like you plan. That's apparently fine as long as interior application.

The stuff does not come off.

 

[mjlambert]

Thanks for the information. I can't find a source near me for Laticrete.

The Menards here has a ProSpec product that sounds similar and meets the ANSI requirements. I will probably use that.

 

[bholler]

I have seen allot come off wwithout the lath. there just isnt much tooth to the cement board

 

[mjlambert]

Well now you have me thinking that I should use the lath. This is in a seasonal cabin that we use and we don't heat it over winter. So, it won't have the moisture like if it was an outside installation but it will have temperature swings. Can I install the lath right on the cement board? Do I screw it through the cement board and into the studs?

 

[bholler]

Can I install the lath right on the cement board?

Yes

Do I screw it through the cement board and into the studs?

Yes screw it fast then go over it with a scratch coat and rake the surface of that coat to give the setting mortar something to grab to. And make sure if you are using this as a heat shield you maintain the air gaps behind it and on the top and bottom. Without those gaps you wont get any clearance reduction.

 

[WoodyIsGoody]

Well now you have me thinking that I should use the lath. This is in a seasonal cabin that we use and we don't heat it over winter. So, it won't have the moisture like if it was an outside installation but it will have temperature swings. Can I install the lath right on the cement board? Do I screw it through the cement board and into the studs?

I'm usually a fan of overkill. However in this case the lath serves no purpose (although it will require more mortar which means more mixing and more difficult application). Just dampen the cement board using a plastic spray bottle and wipe down with a large damp sponge before applying the mortar. Done right it will never fail. Modified mortar is amazing stuff. The cement board can suck the water out of the mortar if it's not dampened first.

 

[bholler]

I'm usually a fan of overkill. However in this case the lath serves no purpose (although it will require more mortar which means more mixing and more difficult application). Just dampen the cement board using a plastic spray bottle and wipe down with a large damp sponge before applying the mortar. Done right it will never fail. Modified mortar is amazing stuff. The cement board can suck the water out of the mortar if it's not dampened first.

I work on stoves every day and see lots of veneer stone walls. And yes it does fail sometimes even when done right. It can even with lath. Behind a stove it is exposed to fast and large temperature swings which will be even worse for the op because it isva cabin. To me the little bit of extra money and time is worth the extra insurance.

 

[WoodyIsGoody]

I work on stoves every day and see lots of veneer stone walls. And yes it does fail sometimes even when done right. It can even with lath. Behind a stove it is exposed to fast and large temperature swings which will be even worse for the op because it isva cabin. To me the little bit of extra money and time is worth the extra insurance.

Like I said, I'm normally a fan of overkill (belt and suspenders approach). But putting lath under the cultured stone can make it worse, not better (because the thermal coefficient of expansion of metal lath is many times that of the under-lying cement board). Using either installation method requires that the substrate (cement board) be solidly supported and rigid or the install will fail. The definition of "doing it right" is it will not fail in normal use. Normal use includes wide temperature swings. Most failures will be due to one of three failures to "do it right", not whether lath is used or not:

1) Cement board improperly installed (not supported or affixed in a manner that makes it rigid). Extra blocking is good insurance regardless of method used.
2) Cement board sucks too much moisture from mortar.
3) Mortar not mixed to proper consistency or not applied within the allotted time.

Adding lath will not prevent any of these issues although it could reduce the impact of #2. If you use lath and have issue #2, you better have affixed that lath to the cement board like your life depended upon it or it too will fail. Pre-dampening the cement board is super easy compared to adding lath and doing two applications of mortar. Lath had it's place when plaster was popular (and is still required for many exterior applications) but acrylic latex modified mortars are a completely different animal compared to plaster.

For a woodstove surround I would spend extra money on a mortar that is highly modified with an acrylic latex admixture. This will greatly improve the ability of the install to handle thermal stresses (lath will not). You can either buy a highly modified mortar or you can use a liquid acrylic latex admixture made for this purpose. While highly modified mortars and latex additives do cost more, they don't increase the amount of mortar needed (as lath does) so the overall cost will be the same or less (while the labor will be greatly reduced and the durability greatly improved). Mortar is a general term for a wide variety of cement based products. Thinset is a type of mortar and would be fine here if you are going for a uniform "flat" appearance (but it's not suitable for applying in thicker layers). Being a "belt and suspenders" type of guy, I would buy a quality acrylic latex thinset and fortify it with additional acrylic latex liquid admixture to insure it handles the thermal stresses. And, yes, pre-dampen the cement board. This will make all the difference in the world.

Regardless of lath or direct install, it's critical that proper mortar mixing procedures are used and time windows are adhered to. Mix in small batches, allow the freshly mixed mortar to slake for the specified time (5-10 minutes) and throw out any mortar that cannot be used in the specified time window. Starting with very cold water can help extend the time window a little bit.

Properly installed it will not fail. Lath adds a lot of hard labor with no benefit.

 

[bholler]

Like I said, I'm normally a fan of overkill (belt and suspenders approach). But putting lath under the cultured stone can make it worse, not better (because the thermal coefficient of expansion of metal lath is many times that of the under-lying cement board). Using either installation method requires that the substrate (cement board) be solidly supported and rigid or the install will fail. The definition of "doing it right" is it will not fail in normal use. Normal use includes wide temperature swings. Most failures will be due to one of three failures to "do it right", not whether lath is used or not:

1) Cement board improperly installed (not supported or affixed in a manner that makes it rigid). Extra blocking is good insurance regardless of method used.
2) Cement board sucks too much moisture from mortar.
3) Mortar not mixed to proper consistency or not applied within the allotted time.

Adding lath will not prevent any of these issues although it could reduce the impact of #2. If you use lath and have issue #2, you better have affixed that lath to the cement board like your life depended upon it or it too will fail. Pre-dampening the cement board is super easy compared to adding lath and doing two applications of mortar. Lath had it's place when plaster was popular (and is still required for many exterior applications) but acrylic latex modified mortars are a completely different animal compared to plaster.

For a woodstove surround I would spend extra money on a mortar that is highly modified with an acrylic latex admixture. This will greatly improve the ability of the install to handle thermal stresses (lath will not). You can either buy a highly modified mortar or you can use a liquid acrylic latex admixture made for this purpose. While highly modified mortars and latex additives do cost more, they don't increase the amount of mortar needed (as lath does) so the overall cost will be the same or less (while the labor will be greatly reduced and the durability greatly improved). Mortar is a general term for a wide variety of cement based products. Thinset is a type of mortar and would be fine here if you are going for a uniform "flat" appearance (but it's not suitable for applying in thicker layers). Being a "belt and suspenders" type of guy, I would buy a quality acrylic latex thinset and fortify it with additional acrylic latex liquid admixture to insure it handles the thermal stresses. And, yes, pre-dampen the cement board. This will make all the difference in the world.

Regardless of lath or direct install, it's critical that proper mortar mixing procedures are used and time windows are adhered to. Mix in small batches, allow the freshly mixed mortar to slake for the specified time (5-10 minutes) and throw out any mortar that cannot be used in the specified time window. Starting with very cold water can help extend the time window a little bit.

Properly installed it will not fail. Lath adds a lot of hard labor with no benefit.

I completly dissagree i have yet to see stone coming off of the wall on a lath and scratch coat job and have seen tons that have come off of bare cement board. If there is no benifit why do some manufacturers require it. And yes we still use mortar with acrylic modifer in it even with lath. Infact we dont use any mortar on anything without modifier unless it is lime mortar on historic stuff

 

[Squirrel1410]

I completly dissagree i have yet to see stone coming off of the wall on a lath and scratch coat job and have seen tons that have come off of bare cement board. If there is no benifit why do some manufacturers require it. And yes we still use mortar with acrylic modifer in it even with lath. Infact we dont use any mortar on anything without modifier unless it is lime mortar on historic stuff

Just curious, have you seen it come off when grouted, or just dry stacked? I didn't use lath because the instructions said it wasn't necessary, but I am also using pointing mortar between the stones. Not like I can go back now, and I did wet the cement board first, but just wondering.

 

[WoodyIsGoody]

I completly dissagree i have yet to see stone coming off of the wall on a lath and scratch coat job and have seen tons that have come off of bare cement board. If there is no benifit why do some manufacturers require it. And yes we still use mortar with acrylic modifer in it even with lath. Infact we dont use any mortar on anything without modifier unless it is lime mortar on historic stuff

I've seen both fail, it's not uncommon because most amateurs (and some professionals) don't know what they're doing. Not sure why you've never seen a cultured stone job with lath fail. There's nothing magical about lath if you do the job wrong:





Lath is good if your substrate is not suitable for a direct install. But with solidly mounted cement board it's a waste of time.

 

[WoodyIsGoody]

Just curious, have you seen it come off when grouted, or just dry stacked? I didn't use lath because the instructions said it wasn't necessary, but I am also using pointing mortar between the stones. Not like I can go back now, and I did wet the cement board first, but just wondering.

Any of the install methods can fail if the mortar is not properly bonded (and lath makes it more difficult to get a good bond to the cement board because you can't "burnish" the mortar to the board). While mortaring between the stones may add a little overall strength, it's not really considered structural. If you did the job right it won't come off with or without pointing mortar.

Just make sure your pointing mortar is also latex modified to give it the necessary amount of resiliency.

 

[WoodyIsGoody]

I completly dissagree i have yet to see stone coming off of the wall on a lath and scratch coat job and have seen tons that have come off of bare cement board. If there is no benifit why do some manufacturers require it. And yes we still use mortar with acrylic modifer in it even with lath. Infact we dont use any mortar on anything without modifier unless it is lime mortar on historic stuff

Here's a shameful video that might explain why you have seen failed cultured stone jobs. It's a journeyman "mason" teaching high-school students how to set a cultured stone wall. The shameful part is he says when your mortar starts to get too thick you just add some water. Never add water to your mortar after it starts to become unworkable! It must be thrown away. That's why you only mix the amount that can be used before it thickens.

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It's heartening that he's inspiring youngsters and exposing them to a new skill, shameful that even a working pro doesn't understand his own trade and he is spreading poor work habits.

 

[bholler]

Here's a shameful video that might explain why you have seen failed cultured stone jobs. It's a journeyman "mason" teaching high-school students how to set a cultured stone wall. The shameful part is he says when your mortar starts to get too thick you just add some water. Never add water to your mortar after it starts to become unworkable! It must be thrown away. That's why you only mix the amount that can be used before it thickens.

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It's heartening that he's inspiring youngsters and exposing them to a new skill, shameful that even a working pro doesn't understand his own trade and he is spreading poor work habits.

Yeah we sat in on a masonry class at the highschool trade school when they were teaching them to build chimneys. He went through his lesson then asked if we had anything to add. I said yes disregard everything you were told and read the code book. He said nothing about clearances to combustibles gave them the wrong info on the height of the chimney. Said you didnt need to pour the hearth extension as pa rty t of the hearth ect. It is sad that even the teachers dont know how it should be done