One of the causes of cracking in Portland Cement Plaster is a lack of embedment of the lath in the scratch coat. The Standard, ASTM C-926 suggests that sufficient pressure should be exerted when spreading or applying the scratch coat so that the wire is encased by the plaster. This is known as embedment. Some lath products make it difficult to bed the wire and some make it very easy. But well embedded lath in a full scratch coat is the bedrock of a crack free plaster application.
What you are looking at in this picture is a full 7/8″ plaster panel sitting on top of paperback lath with a crack starting at the stiffener wire. This is one of the reasons the industry continues to emphasize installation of lath that is easy to bed.
This is also why you don’t see the use of paperback lath applied on top of wood or gypsum underlayment any more. The stiffener wire combined with woven wire’s ability to be flexible makes it perfect for open stud construction but not so positive when you’ve Oriented Strand Board or wood underlayment.
The stiffener wire is attached to the woven wire restricting the bounce of that wire when you are plastering a wall that has OSB or plywood installed under the lath. This is why, as underlayments have proliferated, contractors have gone back to installing paper first then the lath independently.
Woven wire when installed over OSB has some bounce and can be lifted off the surface of the wall to bed nicely as a plasterer pushes a trowel full of mud up the wall. Expanded metal lath with small openings is not as easy to bed the wire on vertical installations, but is perfect for bedding the wire on lids or horizontal ceiling installations.
I was invited to look at the renovation of a theater exterior that had thin brick installed over expanded metal lath because the brick kept popping off. The joints of the lath instead of being 1″, were almost a foot of overlap. The diamond holes did not match up. The plaster never bedded the lath. When I pushed on the brown coat it flexed and more brick fell to the sidewalk causing a very real problem for anyone walking by.
One of the reasons for the development of aggressive furring boots on woven wire products, grooved expanded metal lath and the newest product, welded 1″ wire, Structalath; is to promote easy embedment.
If you’ve questions about lath and how it’s installed, or what product to use on your project, call or email:
Bell Construction Consulting
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