Double up sheathing for garage cedar shingle siding
Hi, I’ve been trying to find information on line about my situation and can’t seem to dig anything up so I’m coming here to see if I can get some advice. I’m putting up Alaskan Yellow Cedar shingles on our house and garage as a retro fit to existing T1-11 that has been on since the build around 1980. The main house went fine but now that we’re moving to the garage I’m not happy with the sheathing on the garage. The house had what I would call standard T1-11 but the garage is sheathed in 3/8″ think ‘fake’ T1-11 in that it has the T1-11 profile/design but the wood thickness isn’t like the house and the grooves are more like dents. It’s just thin. The garage is unfinished. Anyhow, from what I can find, 3/8″ is really not structurally able to hold the cedar shingles so I’m looking to either replace the sheathing or double it up. The question I really have is that the garage is painted and I’m concerned about putting new bare wood sheathing over the painted surface and sandwiching in the painted side. I think this could cause breathing issues and ultimately be a mold/mildew/rot issue. But, it’s just speculation that it’s a problem as I don’t really know. So, I’m trying to determine if I need to rip off the 3/8″ old painted siding and replace with thinker sheathing. Then side per normal. Or other options… any recommendations or expertise on if I have a valid concern. We are Pacific Northwest for region and the garage is detached from the house.
Replies
Is your garage insulated? Is it heated?
My first inclination is to say dont worry about the paint it will act as a vapor barrier and the second layer will dry to the outside. I think the most important detail with shingles is either to have them fully coated on all six sides or allow a way for the backside of the shingle to dry out. Rains screen construction is best but Cedar breather or Benjamin Obdyke type house wraps account for this.
If you were super worried you call always put an air gap between your two layers of plywood but I think that would be overkill.
Thanks for the reply Steve. The garage is not insulated, nor heated. I do have the Benj. Obd. rainscreen slated for the West and South sides as I did this on the house too for the same sides. The North and East just don't get wet. The North never, and the East very seldom.
For anyone who may stumble into this thread I'll try and provide a clean ending. I found this other thread on the GBA site that satisfies as an answer that it's ok to sheath over what I have. https://www.greenbuildingadvisor.com/question/downsides-to-zip-r-sheathing-over-existing-board-sheathing.
Per Martin Holladay (GBA Editor) he states "Your plan is fine. Installing new OSB sheathing over old board sheathing is a routine approach when installing new siding on an older house."