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WRB behind brick or stone - One layer or Two?

Mortar droppings, protrusions, bridgings and smears contaminate housewrap. Best practice stucco prep calls for 2 layers of WRB, a 'sacrificial' layer that the stucco touches and the layer that drains water.
Is it necessary / does it make sense to install a double layer of WRB behind masonry cladding or does the airspace and minimal mortar contact make a double layer unnecessary?

Asked by Mike Guertin
Posted Aug 17, 2009 5:27 PM ET


6 Answers

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I've gone to carefully installing and taping tyvek, generally wrinkle-wrap style, (incorporating it into the header wraps we set between the sill seal and the mudsill and between the top plate and the upper top plate) and then covering it with a sacrificial layer of 15# felt. The felt is pretty cheap so the main cost is the extra trip around the house with the slap tacker. But that extra trip doubles as a quality control trip for the Tyvek install so I think it's a pretty good value. On the big stucco job we just did we ran Tyvek and then a double wrap of 15# felt.

"Install the best waterproofing you can afford, and then don't let it get wet."

Answered by Michael Chandler
Posted Aug 17, 2009 8:13 PM ET


If you are leaving an air space between the cladding and the WRB (as is typically done with brick veneer) your risks aren't too high. However, if you are adhering a product like manufactured stone, without an air space, your risks start to skyrocket.

There have been massive failures of adhered manufactured stone over OSB. These jobs are so risky that I tell anyone who asks that adhered manufactured stone should only be installed over rigid foam sheathing.

Stucco over OSB is almost as risky as adhered manufactured stone over OSB. These sidings really need an air space (for example, a three-dimensional drainage mat) plus several layers of WRB.

Answered by Martin Holladay
Posted Aug 18, 2009 5:12 AM ET



I was pretty cautious about going into the stucco siding and did a lot of research and comparison between traditional hard coat stucco and the BASF synthetic stucco approach. I was able to get some great help from a builder in Fort Worth Texas (Mike Wood) and the Jacksonville Florida building inspections department which (I think) led me to a durable and economical solution.

Hard coat stucco lathe needs to be fastened 6" OC each way, a lot of potential for leaks there and it needs to have good drainage behind it as well as good redundant drainage at windows, flashing and weep screeds. After taping the Tyvek we installed our weep screeds so the weep holes were an inch below the top of the ICF slab edge and two inches above all step flashings etc. we ran over all of that with two layers of 15# tar paper and then ran "Spider Lathe" with the EPDM nailing gaskets running vertical so they would create drainage channels every six inches from roof to weep screed. We covered that with two layers of hard coat type S stucco with a sponge finish and then went over that with two layers of elastomeric paint and protected that with 37" roof overhangs.

I think many of the failures we are seeing are due to metal lathe run horizontal and nailed 6" oc with no gasketing and no drainage channel so the Spider Lathe ends up adding a lot to the system esp when applied over two layers of tar paper over the wrinkle wrap. At that point the layers and the puckering of the tar paper when the wet stucco hits it do create an air space between the tar paper layers and the Tyvek. When the first coat of stucco is going on the lathe is very rubbery and flexible and it doesn't develop any stiffness until the stucco sets up. It's pretty clear that there are air pockets behind the system.

Still, only time will tell, we install the best waterproofing we can afford and then do all we can to keep it dry. adding a layer or two of foam between the stucco system and the OSB undoubtedly would have been a great addition but we already had 12" thick double framed walls filled with spider fiberglass and we're in a hurricane zone so I chose to go for the more economical and impact resistant solution of 100% OSB directly behind the cladding.

By the way, the cost of upgrading from 2x6 to 12" double wall was less than $4,000 for the entire house, it's a great solution and easy to implement, next time I'll go with 10" walls though, I think the big deal is that 2 1/2" break between the walls. We're getting Truvion in to do on-line data monitoring n the HVAC and solar and selected plug loads so we should get some good data out of this one.

Answered by Michael Chandler
Posted Aug 18, 2009 8:45 AM ET


I was wondering if any of you boys have tried using Benjamin Obdyke's Stone & Stucco Home Slicker behind stucco, natural stone, or cultured stone claddings. It seems like a great system, eliminated the need for 2 layers of 15# felt.

Answered by Brett Moyer
Posted Aug 18, 2009 1:06 PM ET


James - I have played with Obdyke's stuff when it was called MortAirVent (still called that by the company they partnered with to put it out - Advanced Building Products). Used it beneath synthetic stone. I liked the results. Like anything new, you have to work the bugs out of your application procedure. Also keep in mind there are going to be intake and exhaust venting details you'll likely run up against that aren't detailed - yet.

Here's what I found:
You have to get used to the sponginess when attaching your wire over Stone and Stucco. I used pneumatic staples and they pulled in way too deep until I backed off the pressure and reset the nose. Ended up leaving the crowns proud and hand tapping where I wanted. Because of the air space, the wire moved a little when we applied thinset. I typically like apply one layer of thinset and then stone right away but found it was actually faster to apply two coats. A quick scratch of quick setting to fill in the wire stabliized the whole surface, then I used regular thinset to work up the stone. Gave a lot more control and could stack more stones up than we could with a single coat over the mesh.

Answered by Mike Guertin
Posted Aug 18, 2009 1:31 PM ET


I love the Obdyke stuff but it is just so durn expensive. I think the Spider Lathe run vertically gets a lot of the advantage of the Obdyke at a lot less cost with the addition of gasketing on the nail penetrations and nonmetallic lathe and it's so much easier to install from a labor perspective than the metal lathe over the Obdyke.

Where we had 20' high walls we just sent one guy up the ladder with onwe end of a 20' roll, stapled it off 6" OC EW and then came back and cut out the expansion joint to expose the steel with a razor knife. I use it on chimneys a lot where we don't even use corner bead or weep screed. just staple it on around the corners and stucco down onto a 2x4 laid over the step and head flashings to expose the bottom of the spider lathe. (over wrinkle wrap and two layers of stucco)

I'm sold (obviously) no more cutting up my wrists and elbows on wire lathe or worrying about staple penetrations through the WRB for me. Obdyke would be a great product if the price dropped 50% though.

Answered by Michael Chandler
Posted Aug 18, 2009 6:30 PM ET

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