California WUI siding question
I ordered my gorgeous siding and it arrived on Friday. I am using Shakertown Dolly Varden which is a plywood backed clear Cedar T&G product. I am in a California WUI (High Fire Severity) area so I need to meet the standard on my cladding. The Dolly Varden markets that it is WUI approved but I missed the note that it is only for a horizontal application. I want to install vertically.
I talked to tech support and the vertical application with increased nailing is not a problem. Then the local rep asked the dirty question about WUI and my heart sank. He said that some people have gotten local approval by their building official and that is what I will try on Monday. Here is my written argument.
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Based on the fact that this is approved for WUI in a soffit and and for horizontal application, logic would point
to the material itself being the barrier, not the direction of board. I will be doubling the nailing pattern to 8″
which will increase the holding power over their reccomended 12″ o.c. for veritical applications. Also, I have
Advantech 7/16″ Zip sheathing. This is a high density OSB that is superior to either standard OSB or plywood.
Additionally, due to the amount of horizontal blocking in the house for nailing straps, we will be able to have direct
nailing to framing approx. 32″ o.c.
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I am looking for any input on what I might add (or remove) to that statement.
I have heard there is a provision (I am needing to research it further) for a layer of 5/8″ GWB below the siding to achieve the WUI rating. If I do that, I am concerned about moisture and the GWB at that part of the assembly. My wall is made up of 7/16″ Zip sheathing and then a layer of Benjamin Obdyke Home Slicker for a rain screen. I am keeping the top and bottom of the siding open to facilitate air and water flow through the rain screen zone.
If I use the GWB would I want the rain screen layer on top of the GWB? Would I want another layer of WRB on top of the GWB?
Instead of GWB, I am also considering a roof underlayment product which I may be able to keep thinner so I don’t have as large of a jamb extension or L shaped casing at my windows.
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Replies
Maybe Magnesium Oxide board will be my huckleberry? Have any of you used this stuff yet? What thickness would I need to match the one hour rating of 5/8" Type X?
I would look to resources developed by Steven Quarles for either California Extension Service or the Fortified Home project at IBHS. Oh shoot, looks as though Steve has retired; he was the WUI man...but still check out each organization's resources.
https://nature.berkeley.edu/news/2011/08/steve-quarles-home-fire-protection-expert-retires-uc
Peter
Thanks Peter. I will take a look at that. Hopefully my building official will give me a go ahead tomorrow.
Is a rain screen approved in a WUI zone? Does it need a special vent like the attic to keep embers from getting blown in and starting the house on fire.
Was the rain screen approved in plan check?
Thanks Tim. Really good question. It prompted me to drop off more sample material to my building official today. The WUI guidelines call for 1/8" max metal screen for vents. I wonder if my top and bottom openings (which I was going to screen off for insects) would draw air the same way an attic vent would. Does that flow of air come from the pressure differential and would there be enough of a pressure differential at that layer of the assembly?
我认为这个问题是风吹余烬。Driven by the wind the fire makes and the atmospheric wind. The Slicker is plastic of some kind. Perhaps get a sample and see how easily it starts to burn. See what the insurance people have done to simulate wild fires & houses.
https://disastersafety.org/wildfire/ibhs-wildfire-research/
Alan,
I don't think your first paragraph argument holds much weight, because you are talking about approval to install a material (siding) that was noted to be installed in the horizontal position only. Not that every inspector knows very material, but they could ask you or look it up themselves.
See 2015 IRC R104.11 Alternative materials, design and methods of construction and equipment.
...shall be approved where the building official finds that the proposed design is satisfactory and complies with the intent of the provisions of this code..., for the purpose intended, and not less than the equivalent of that prescribed in this code.
You may be ok, but you may want to ask before you install it.
Alan, I want to follow up on the Dolly Varden siding not the fire rating issues.
How has is held up? Where you pleased with install and how did you finish the product? I’m about to spend a fair amount on some and reviews are sparse.