Im putting up stone veneer and i tar papered, and metal lath. why must i do a scratch coat everything will be sealed when i grout. Can anyone explain to me im just an hvac guy!!!! Thank you.
outdoorsmylife
I have been doing this type of work for years and in my opinion the only reason for a scratch coat is to help in the application of heavy stones that otherwise slide or fall off while installing them. While doing a drystack look stone I never use a scratch coat, but just coat the wall ahead of myself and butter the stones. I think this makes for a better job in my northern climate and I never have stones pop off. In my area a lot of Taco Bells got redone with a drystack look and a lot of stones popped off after one or two winters. I believe this was because of bond failure between the scratch coat and application coat and not intrinsic to a drystack application. Of course the later Taco Bells have the same stone layed with a mortar joint which helps the problem. If using a scratch coat be sure to really wet the wall ahead of yourself and sometimes you have to wet the stones too to get a good bond. I have removed stone applied both ways and I can tell you that the one with no scratch coat is harder to tear off.
Stone, the issue with those Taco Hells was the drystack, not the method of attachment.
With a properly done scratchcoat, the mechanical adhesion is plenty strong enough, to say nothing of the bond strength. The fact that you could use super glue and stick rocks to plywood and it would look the same does not make it the right way to do it.
With a properly done scratchcoat, the mechanical adhesion is plenty strong enough, to say nothing of the bond strength. The fact that you could use super glue and stick rocks to plywood and it would look the same does not make it the right way to do it.
You are correct it is the most important. In that instance (the drystack on Taco Bells), it wouldn't matter how well it is attached; the expansion of water into ice will exceed pretty much anything available. Exposed drystack adhered veneer is simply a bad idea in a heavy freeze thaw environment.
I have to disagree. With or without a scratch coat, properly applied drystack is just fine in a freeze/thaw environement. I can point to thousands of square feet of drystack that is just fine after many harsh winters. My point in bringing up Taco Bell was that a scratch coat can be a point of failure if not handled right and in my opinion you don't always need a scratch coat.
You may be right, we are lucky to get one or 2 freezes a year here. I saw quite a bit of it in Colorado though, and also quite a bit of pop-offs on new (2-5 year old construction).
We personally have always done a scratchcoat. I think it adds extra integrity to the install & creates less "sag" when setting stone between stud staples. We normally try to plaster the day before, backtrowelling with a 1/4" notch trowel horizontally. Dry scratchcoats suck the moisture out of the mud fast & allow grout joints to set at alot faster pace, which works better for us. I've only done drystack once, & it was inside. I hated it, as it seemed alot slower. I don't think I'd be willing to do alot of it my climate, as this winter alone we probaly had 50+ frost cycles.
It's funny you guy's bring up Taco Bell. We hve one with drystack a few miles awy, probably the worst installation of CS i've seen in my life. Somehow, though after 2-3 winters, its not falling off! :laughing:
It's funny you guy's bring up Taco Bell. We hve one with drystack a few miles awy, probably the worst installation of CS i've seen in my life. Somehow, though after 2-3 winters, its not falling off! :laughing:
It all has to do with the installer. Here in Ky we scratch wether it be block or lathe it doesnt matter.
We only scratch what we can get done without letting mortar set hard on wall before setting stone. This application ties everything to the lathe.
Should it be on block - it allows moisture in block and gives more set time. Using these procedures we still allow .5% fall off due to laborers just not getting them stuck good. In the end before final walk, all stones are stuck and job is complete.
Should it be on block - it allows moisture in block and gives more set time. Using these procedures we still allow .5% fall off due to laborers just not getting them stuck good. In the end before final walk, all stones are stuck and job is complete.
Another reason to do a scratchcoat/browncoat is for rigidity. I visited a job a couple of months ago where about 5000 SqFt of natural thin stone popped off. The stone was wetset in the scratchcoat over denseboard, and the substrate was simply too flexible to maintain adhesion (the back of the stone was too slick as well). It was all knocked off, browncoated and the stone was reattached with a highly modified thinset.
Using Mortar Scratch Coats
It seems every mason has an opinion on whether or not to use a scratch coat. Cultured Stone specifications call for it, but more often than not, scratch coats create a cold joint - that is, it creates a mortar set that doesn't fully bond with the mortar applied to the stone.
When a job is done without a scratch coat, all the mortar is mixed within two hours or less, making the bond much stronger, as the mortar on the stone bonds with the mortar that was just applied to the metal lathe an hour or so earlier. Temperature also plays a big part in how much time you have to work with your scratch coat.
Mastercraft Masonry
Professional Masonry Since 1974
Oregon / Washington
It seems every mason has an opinion on whether or not to use a scratch coat. Cultured Stone specifications call for it, but more often than not, scratch coats create a cold joint - that is, it creates a mortar set that doesn't fully bond with the mortar applied to the stone.
When a job is done without a scratch coat, all the mortar is mixed within two hours or less, making the bond much stronger, as the mortar on the stone bonds with the mortar that was just applied to the metal lathe an hour or so earlier. Temperature also plays a big part in how much time you have to work with your scratch coat.
Mastercraft Masonry
Professional Masonry Since 1974
Oregon / Washington
If the only issue were adhesion, then wet set would be the way to go. Depending upon the wall system, it is not, and stucco systems do not have issues with cold joints like structural concrete does.
The experienced Masonry Contractor will assess each situation, as each is unique, and use the one suited to the best outcome. Thin Stone definitely can cause undue stress for a mason, given its slick bonding side. It seems as though that surface needs to be much rougher to get a solid bond, not to mention the dust that permeates the bonding surface during cutting.
Yes, a scratch coat would definitely have its place there. It is much easier to install to a scratch coat, as the masonry material then bonds quicker, although a quick bond isn't necessarily the best bond.
Mastercraft Masonry
Professional Masonry Since 1974
Oregon / Washington
Yes, a scratch coat would definitely have its place there. It is much easier to install to a scratch coat, as the masonry material then bonds quicker, although a quick bond isn't necessarily the best bond.
Mastercraft Masonry
Professional Masonry Since 1974
Oregon / Washington
i am doing a 300lf x 4' tall wrap around a home... i proposed to use a 1/2" cemtious board or equal.
i thought the backer board would be a labor saver and we are so busy i thought we could attach cement board super fast... and start installing immediately.. i figure also that the board would cost about .60 sf to install backer.... i would love to hear from anyone that has used this method...
serious comments only please
i thought the backer board would be a labor saver and we are so busy i thought we could attach cement board super fast... and start installing immediately.. i figure also that the board would cost about .60 sf to install backer.... i would love to hear from anyone that has used this method...
serious comments only please
I've seen a product from Keene building products called rain screen that acts like your lath and has an air space behind it to prevent moisture problems. I haven't used it myself but I've heard guys have had success with it. It was even on an episode of Holmes on Holmes, they had done a 4 ft band around a house with CS laid on rhino board of cement board, and the whole thing peeled off in sheets. The guys used penny nails to hold the stuff on though, so that may have been the cause. The stuff looks pretty cool on tv and nothing on tv can ever be false, it just cant happen.
In my early years in this trade we did cultured stone directly over the lath,it wasnt long however till customers were calling complaining of stones falling off the wall.We have used scratch coats for over twenty years now and have not recieved any complaints of falling rocks.
The reason l feel its needed is the fact a scratch coat will wick the moisture out of the mortar quicker and therefore hold the stone secure,faster as more weight and stone is added to the stack.
We always use a scarifier rather than a stiff broom to apply the scratch it digs deeper and does a lot better job in keying one to another.hope this helps....
The reason l feel its needed is the fact a scratch coat will wick the moisture out of the mortar quicker and therefore hold the stone secure,faster as more weight and stone is added to the stack.
We always use a scarifier rather than a stiff broom to apply the scratch it digs deeper and does a lot better job in keying one to another.hope this helps....
We do scratch coat for winter time applications and don't use it on summer time ones, never had a single problem with any of them, cement board is OK for cultured brick (light material and need of chalk lines), but for cultured stone is not really good nor quicker(stone is heavier and tends to slide down).
Regards!
Regards!
Roofing nails are not an acceptable means of fastening cement board, and it does not really matter what you do if the framing moves the adhered veneer will detach at flex points.
mesh,scratch, dampen just prior setting stone,dampen soon after stone sets, use proper mortar mix type S with bonding agent, dampen completed area
approx 2 days. also cover with white plastic (approx 3 days) if in extreme hot exposure, seal with siloxane at completion of project. here in pa my business were done with thin stone by end october.(exterior work) these extra steps take time and more material cost $ however do right figure in cost. last thing anyone wants is call that stone fell off not to mention what it does to your reputation ! i follow these steps for over 20 years not one call back yet. like my saying goes for all of us masons (may look easy but its not, if your not a mason dont touch the trowell !! )
yours in masonry Bryan J Mabius (mabiusmasonry.net)
approx 2 days. also cover with white plastic (approx 3 days) if in extreme hot exposure, seal with siloxane at completion of project. here in pa my business were done with thin stone by end october.(exterior work) these extra steps take time and more material cost $ however do right figure in cost. last thing anyone wants is call that stone fell off not to mention what it does to your reputation ! i follow these steps for over 20 years not one call back yet. like my saying goes for all of us masons (may look easy but its not, if your not a mason dont touch the trowell !! )
yours in masonry Bryan J Mabius (mabiusmasonry.net)
My thoughts on the matter
When using metal lath, it is generally a good idea to do a scratch coat so that the mortar will bond through the diamond spacing in the lath. Just mudding the veneer rock and pressing it into the lath without a scratch coat is almost never adequate since it doesn't squish itself into those tiny holes in the back. The rock and the mortar will typically fall off. I prefer Stucco wire, instead.
Another thing is that if you are doing a scratch coat that you run over it with a raker so that there are notches for fresh mortar to bond to. Personally, if I am "forced" to do a scratch coat, I don't let it dry. Instead, I mud the back of the rock and stick it on to the wet mud from the scratch coat so that the mortar bond is homogenous. If the bond is a homogenous one, then it is considerably stronger since there is an actual chemical reaction going on and not "water drying out" of the mortar, which some people don't realize.
My personal preference is this: I prefer to use #17 gauge, inch and a half stucco wire (which is an equivalent to the lath if you are putting 15 lbs. or less per square foot) without a scratch coat on the wall. Instead, I do a scratch coat on the back of the stone so that the mortar will bond to the back of the rock more securely. The Stone veneer is typically VERY dry and just plopping mud on the back of a rock before sticking it to a wall is like briefly running water over a sponge that is so dry it is stiff (you need to get the sponge a little wet first before it will absorb more water). After scratch coating the back of the stone I plop mud on it and stick it on the bare wall and let it squish out around all sides. There is usually about a 1/4 to 1/2 inch of mud between the stone and the wall. Then I use the tip of my trowel to squish the excess against the wall so that there is about an inch scratchcoat bordering the stone. This ensures that you have tight joints if you are doing a drystack look and keeps the stone clean. Also, it ensures that there is a sheet of mud between the stone and the wall so that no moisture will get behind the stone and pop it off when it freezes.
This leaves the veneer secure to the building. It is even more secure than doing a scratch coat from my experiences of ripping off the stuff from mistakes and change orders. There was once a change order on some deck railing and I had to pry off some drystack chief joseph (harristone.com type stone located in Ogden Utah) and it took a 4 lbs. sledge hammer, two chisels as a wedge and another chisel to break the stone. It took about 2 minues of constant pounding to give myself some spacing so I could fit a chisel behind the rock to rip it out... Eventually, I had to get my wire snips to cut the wire because I was pulling the staples out and effectively pulling out the other stone. about 15 minutes to take out 1 square foot.
I have litterally climbed walls of drystack that I have done with veneer stone.
I have layed this stuff in places where it can get 12 inches of snow in a day and be in the negative temperature readings during winter. Stuff I have laid 10 years ago is still there.
In my honest opinion.... It is absolutely necessary to to put a scratch coat on lath so that the mortar bonds around the lath as strongly as it can...
When using metal lath, it is generally a good idea to do a scratch coat so that the mortar will bond through the diamond spacing in the lath. Just mudding the veneer rock and pressing it into the lath without a scratch coat is almost never adequate since it doesn't squish itself into those tiny holes in the back. The rock and the mortar will typically fall off. I prefer Stucco wire, instead.
Another thing is that if you are doing a scratch coat that you run over it with a raker so that there are notches for fresh mortar to bond to. Personally, if I am "forced" to do a scratch coat, I don't let it dry. Instead, I mud the back of the rock and stick it on to the wet mud from the scratch coat so that the mortar bond is homogenous. If the bond is a homogenous one, then it is considerably stronger since there is an actual chemical reaction going on and not "water drying out" of the mortar, which some people don't realize.
My personal preference is this: I prefer to use #17 gauge, inch and a half stucco wire (which is an equivalent to the lath if you are putting 15 lbs. or less per square foot) without a scratch coat on the wall. Instead, I do a scratch coat on the back of the stone so that the mortar will bond to the back of the rock more securely. The Stone veneer is typically VERY dry and just plopping mud on the back of a rock before sticking it to a wall is like briefly running water over a sponge that is so dry it is stiff (you need to get the sponge a little wet first before it will absorb more water). After scratch coating the back of the stone I plop mud on it and stick it on the bare wall and let it squish out around all sides. There is usually about a 1/4 to 1/2 inch of mud between the stone and the wall. Then I use the tip of my trowel to squish the excess against the wall so that there is about an inch scratchcoat bordering the stone. This ensures that you have tight joints if you are doing a drystack look and keeps the stone clean. Also, it ensures that there is a sheet of mud between the stone and the wall so that no moisture will get behind the stone and pop it off when it freezes.
This leaves the veneer secure to the building. It is even more secure than doing a scratch coat from my experiences of ripping off the stuff from mistakes and change orders. There was once a change order on some deck railing and I had to pry off some drystack chief joseph (harristone.com type stone located in Ogden Utah) and it took a 4 lbs. sledge hammer, two chisels as a wedge and another chisel to break the stone. It took about 2 minues of constant pounding to give myself some spacing so I could fit a chisel behind the rock to rip it out... Eventually, I had to get my wire snips to cut the wire because I was pulling the staples out and effectively pulling out the other stone. about 15 minutes to take out 1 square foot.
I have litterally climbed walls of drystack that I have done with veneer stone.
I have layed this stuff in places where it can get 12 inches of snow in a day and be in the negative temperature readings during winter. Stuff I have laid 10 years ago is still there.
In my honest opinion.... It is absolutely necessary to to put a scratch coat on lath so that the mortar bonds around the lath as strongly as it can...
Parging the wall and laying buttered stone as long as you don't allow it to pull out, I think is the strongest bond. You don't have a cold joint. Let the parge dry then it's a scratch coat. Not much difference if laid by an experienced mason. The integrity of a wet parge can be compromised and at a certain point in the curing you won't get a good bond. Strenghs andweakness to both
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