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

outdoorsmylife

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.

Tscarborough

Why do you tape your joints in ductwork?

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outdoorsmylife

Just wondering if it wil cost me in the long run not to do a scratch coat.

bert0168

Hey, I'm a carpenter and I was wondering if it would cost me in the long run if I sized my own ductwork and heating system?

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Quote:

Originally Posted by CCCo. Yep, you guessed, sold by a fancy white guy, installed by the brown army.

lukachuki

No...you will be fine...in fact i wouldn't even use lathe. I just wouldn't suggest you do this job for your father in law.

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Stone

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.

Tscarborough

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.

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stonelayer

I hate to dispute your word here, but the method of attachment is what it is all about on drystack or grouted stone.

Tscarborough

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.

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Stone

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.

Tscarborough

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).

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jomama

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!

stonelayer

It all has to do with the installer. Here in Ky we scratch wether it be block or lathe it doesnt matter.

Tscarborough

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.

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MasonryPro

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

Tscarborough

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.

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MasonryPro

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

consolidatedbui

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

RZB

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.

rock man

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....