Zip R – accounting for difference in thicknesses
We are in the process of building a detached garage with a mother-in-law suite above it. For the apartment portion have decided to go with ZipR12 sheathing, leaving a thickness difference of 2 inches between the ZipR and the plywood for the garage below it. I was originally planning on making up the difference in the furring strips for the rain screen gap (ripping a 2×3 to 2”). The alternative would be to let in the 2 inches on the second story framing so that the plywood and outer face of the ZipR were on the same plane. My primary goal is to make air sealing between the garage and apartment as straight forward as possible while using the ZipR to increase the r-value of the wall. So my question is whether the reduction in bearing of the second story wall is of any concern and / or if there are pluses or minuses to either approach beyond what I am thinking? Both stories will be 2 by 6 stud walls.
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Replies
You'll probably be OK. Bearing surface for joists and headers is usually supposed to be 3" minimum, which is why jack studs get doubled up. There are some exceptions but this all depends on the load parameters of the floor (pounds per square foot allowed for in the design, and the span of the joist which concentrates the load on the bearing surface on the ends).
This is a quick answer for an architect or engineer working on your project. If you're doing it all DIY and you have structural concerns, it's worth having someone review your plans first and also possibly check the foundation which ultimately has to carry all of the load that is being transferred down by your framing.
Bill
Peter,
I'd opt for setting the second floor framing in.
'Puffed out' sheathing at the upper level sacrifices a bit of shear strength, not bearing capacity. If your geometry is simple and your fenestrations aren't excessively large, I wouldn't lose sleep over it. Follow zip mfr. installation instructions for fastening. Zip flashing tape comes in wider rolls that should handle the 2" change in plane without issue. I would hesitate on the alternative to offset framing from level to level simply to avoid plumbing drain route conflicts.
You'll have to deal with the wider rainscreen bottom gap. Aluminum or fiberglass window bug screening wrapped in the furring can work.
Don’t use aluminum screening here, it is too prone to rotting out. Fiberglass is easily damaged by critters. Best is stainless steel mesh or the perforated strips Malcolm frequently recommends.
Bill
Thanks Bill,
Saved me from repeating my speal.
Point taken on the fiberglass, but I'd be surprised to see aluminum mesh "rotting out" unless it's in direct contact with dissimilar metals. Would you mind providing a link to your spiel?
Jason,
Once rain-screens were adopted for all residential projects here in BC after being required in our building code, builders quite soon changed from using insect screen to perforated metal flashings. The problem isn't so much the deterioration of the screening, it's mechanical damage (often caused by weeds or grass growing up into the cavity and tearing the screen when removed) , and that the location means repairs or replacement is very involved or impossible.
Perforated J flashing make strapping easier too, as they provide a channel to set the base into as you are installing it.
Since they are used so much here they are commonly available at most lumberyards. I use perforated stock a lot. Not just for rain-screens, but also for soffit vents.
Aluminum tends to rot away when in contact with soil in many areas (depending on the soil chemistry), and the detritus that tends to accumulate in these places acts like soil whenever it is moist. This is one reason why aluminum is not permitted to be used for underground ground conductors. It’s not an issue of dissimilar metals contacting the screen, it’s moist materials that react with the aluminum. If the gap never accumulated any detritus and/or doesn’t get wet, you only have to worry about physical damage. In my experience though, detritus always accumulates and can off be moist enough at times to be a problem.
The fine gauge wires in typical aluminum screen don’t be last very long when exposed to this. The heavier gauge aluminum used in the flashing strips is MUCH more durable. Stainless steel costs more but lasts forever.
Bill
Thanks for sharing. The perforated J metal looks to be a tougher product and a good idea. I still wouldn't rule out the aluminum screening if a suitable size of that flashing isn't available. Design landscape control measures for the weeds and of course keep the soil clear. Alkalinity will certainly affect uncoated aluminum and quickly so at thin gauges.