What Is The Purpose Of A Scratch Coat If Im Using Metal Lath Why Do It?

rock man

just came from a job of 600+sq.ft we are estimating to replace.. the cultured stone was applied over cement backer board .the stone is cracked over 65%of wall area from top to bottom,at the seams.the seams were not meshed and board was fastened using roofing nails.still dont think it would have mattered in the long term( differential momement) of substrates.best advice would be to stick to manufactors guidelines and accepted proven trade practice,saving money is good but doing it twice always costs more in the long run.hope this helps

CR2

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!

Tscarborough

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.

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brystone

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)

Felix King

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...

CJKarl

Interesting point. If you apply CS are you a laborer or a mason? Guess it depends on who is paying you.

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turbomason

ever try to get supplier support without following manufacture guidelines?

tkle

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