Mortar, Bonding and Keys to Success
In this video, Chris Hines, Technical Manager of Boral Stone Products, provides a few tips related to achieving a good bond.
He begins by talking about how the manufactured stone installation system evolved. 50+ years ago it began as a stucco system. A stucco system is two layers of WRB, lath, a scratch coat, a brown coat and a finish coat, with a total thickness of about 7/8 of an inch. When we install our system we’re trying to achieve the same thing. Many of the systems and materials are the same. We’re installing two layers of WRB. We’re installing a self-furred lath. We’re installing a 1/2″ thick scratch coat. We’re installing setting bed mortar in place of the stucco brown coat. And we’re installing our stone place of the stucco finish coat.
He asks us to visualize him troweling a brown coat onto a stucco wall. The wall will be completely covered with that brown coat. That’s your goal with the mortar setting bed, and there may be a time when it’s desirable to trowel the mortar on the wall instead of back buttering. He covers method A, in which the stone is back buttered, and method B, where there is a thin back butter coat and the balance is troweled onto the wall itself.
There are other things you can do to improve the bond: wetting the scratch coat or wetting the back of the stone. You can also remove any loose material from the back of the stone. There are bonding agents available that may help you. If you use a bonding agent, make sure that it’s compatible with the mortar product that you’re using and that it meets ASTM C1384.
Next, Chris talks about sealing. This is not designed as a barrier system. It’s not intended to keep water out. It’s supposed to get wet. In fact, manufactured stone can have up to 18% water absorption, and then it dries right back out. It works the same way as a stucco system. If you seal it, you slow down the intake of water, but you also slow down the release of water. If you do use a sealer, you want to use a xylene breathable sealant, and there are some cautions that go with that. He addresses some of the downsides of slowing down drying. Chris then recommends that if you are going to use a sealer that you follow the product’s instructions to the letter, because a simple mistake in the application of a sealer can change it from breathable to non-breathable very easily.
There are applications where sealing makes sense. If you have a chimney, and you want to clean soot off of it, that’s an understandable application for a sealer. If you live in an area that has a very colourful soil, and you want to be able to wash it off if it splashes on the stone, that’s another decent application for a sealer. Another would be a restaurant food preparation area.