Backside venting for wood shingle roofs?
I’m an architect specializing in historic preservation projets. We’re going to be re-roofing some wood shingle roofs on some historic barns and outbuildings in Long Beach, CA. The buildings need seismic upgrading, so we’re also considering adding plywood sheathing to the roof on top of the existing skip sheathing.
We’re concerned about the loss of ventilation to the backside of the singles. Conventional wisdom is that this shortens the shingle life. Do you see that in the field? How much shorter is shingle life on a solid substrate?
If we want to ventilate the backside of the singles, any tips on how to efficiently do this on top of plywood sheathing? Putting skip sheathing nailer boards on top of the plywood only creates a sealed air space below the shingles. A double layer of perpendicular 1x skip sheathing (boards going up the roof at 16″ o.c. under boards across the roof at the normal skip sheathing spacing) would create a place where air could flow, but we’d need to add eave and ridge vents to make that actually work. Of course we’re not real anxious to change the eave and ridge detailing on a historic property.
Any suggestions and input would be greately appreciated.
Andrew
Replies
The names escape me now, but there are heavy "mesh" products that are designed for this application (think a super industrial-grade dish scrubbing pad). They are installed over the solid sheathing, and the shingles are then applied over the mesh, so the airspace is preserved. The shingle manufacturer should be able to provide a list of recommended products.
Edit to add a link:
http://www.benjaminobdyke.com/html/products/cedar.html
Never used this, but this is representative of what I'm talking about.
Edited 5/8/2006 2:51 pm by torn
Yes, I was thinking about that type of product too. American Wick Drain has a similar product.
In my area wood roofs are very common and have been putting them on my projects for 20+ years.
The problems I have are not from ventilation, but the cedar deteriorating from the top side.
On one job, we put an addition on, put the wood shingles directly on papered plywood, the remainder of the house we re roofed over existing skip sheathing.
It is now 14 years later- it all looks the same. The plywood section, the skipsheathed section,- it all needs replacement.
The shingles when installed were 3/8" at the butt, they are all now down to about an 1/8".
My house is handsplits on skip sheathing, 18 years old, if you walk on them they crumble like burnt toast.
Some say the cause of the fast deterioration is acid rain, some say that the cedar trees are harvested to quickly, I don't really know.
As a historic preservation, you will have to use wood,but in my pesimistic veiw the sheathing /ventilation doesn't really matter because it won't last very long anyway.
Thanks for your observations. Part of my inquiry is whether the longevity issue is an "old wives tale." As a natural product, wood shingles are subject to a wide variety of variables that will drasticly effects their life. Wood quality, local climate, orientation to the sun, overhanging trees, lichen and moss growth, etc. I wonder if many of the anecdotes of shorter life on solid sheathing aren't really due to other factors that are discounted or unrecognized.I have wood shingle siding on my house in Richmond, CA. after 15 years, the wood erosion of the shingles on the south side is definitely noticable, maybe 1/8". On the north side, the shingles look brand new.
Interesting thread. I installed Eastern White Cedar shingles over CedarBreather on my roof last autumn. It will be interesting to see how they do here in upstate N.Y. I recently checked a barn roof I did 15 years ago; still looked good, but I didn't inspect it closely. On that roof I used lath applied over the the original wide plank sheathing to give an airspace, which I left open at the eaves.
Is there a product that can be applied to the cedar that will protect it from UV, and keep it from getting too dry and brittle? In an article I read years ago, the author suggested Thompson's Water Seal, but since then I've heard that product is useless. True? False?
Allen
The best protection from UV would be an opaque coating, i.e. paint. Assuming that's not the look you want, I'd look for a clear sealing product that emphasized UV blockers in its formula. Just like the clear sunscreens we wear, however, I wouldn't expect any product to protect very long. Can you significantly extend the life of wood shingles with yearly or bi-yearly applications of a UV blocking sealer? Don't know, but I'd love to hear if anyone's tried that route.Another issue to consider for longevity is that biological growths (lichen, moss, etc.) live on your shingles because they are a food source and will be slowly eated away by their action. So the ideal sealing product will have a biocide in it too. Many years ago, I worked for an architect who was involved in a lawsuit where the formula for Thompson's Water Seal was disclosed in court. If I remember right, I think it was just a thinned wax. Nothing fancy. Makes the water bead up on the surface, but not very long lasting. Of course it's highly likely that the formula has changed or been improved since that time (which was at least 20 years ago.)
I think you're correct about the wax formula. There was a discussion here a year or so ago about deck treatments; I'll have to search the archives. Whatever I use, I'd assume it would be an annual application. The shingles weathered (i.e. UV damage, I imagine) quite quickly to a lovely gray. Ideally, that look can be maintained for another 20 years.
Allen
Back to the venting issue ... I was happy with the CedarBreather product, though a bit pricey. In my installation I created a ridge vent by shingling a couple inches short of the ridge, then lapping a stip of the CedarBreather over the top course of shingles before attaching the ridge board on top of that.
On the very first course, the CedarBreather holds the shingles a quarter inch off the deck surface, and that quarter-inch is exposed. In theory, air can enter at that point and exit at the ridge line. There's a CedarBreather ridge vent product available, too, that I believe functions similarly.
Allen
some thoughts on your predicament and your project ( and it sounds like you've given all good consideration )start out by saying that the biggest detriment to the efficacy of cedar roofs in today's world is the general lack of pitch in roof structure - the climate of a roof structure changes dramatically both in heat retention and dampness when a roof is sloped at 14 / 12 vs. more common 6 / 12 or less today - couple that w/ a more closed system utilizing sheet goods and it becomes less favorablegrowing up in southern Cal w/ a cedar shake roof I think spores and the like are the least of your worriescan't help but think the combo of cedarbreather, open eaves and free flow venting at the ridge would be the best case scenario for your situation / cupolas at ridge w/no glass in sashes or ridges set off the roof 2" or so then shadowed say 2 courses on side then ridge cap rids barn of odors & breathes
that last ridge treatment first noticed in Sun Valley probably 15 yrs ago
hope you're dealing w/ a 12 /12 or steeper & send pics John