No-overhang eave detail
Hey all!
We’re in the final planning stages of our house and I’m creating the details for the plan set. I’m NOT an architect, but a former contractor. I’ve tried to immerse/educate myself in the forums, details library, podcasts and articles. I have not yet seen a detail or article with exactly what I’m trying to build and would love some input.
First, some context:
– Climate zone 3C (Santa Cruz Mountains California), mild climate with relatively low humidity (averages about 68-70%), about 36″ of rainfall yearly
– Temps average 46 low and 75 high throughout the year
– Were shooting for no overhangs both for aesthetics but also for fire prevention. We’re in a high wildfire zone and I’m doing anything I can to reduce fire risk (un-vented crawlspace, un-vented attic or only gable-end vents, stucco siding, metal roof, etc.).
I’ve done my best to combine all the details that I’ve seen in the GBA library to create this detail. The gable ends will be a similar detail, just without the gutters.
Thank you!
Patrick
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Replies
Pat,
Looks good. A couple of comments on the drip-edge.
The typical profile for metal roofs extends out beyond the wall, and the roofing panels extend the same distance (usually 1 1/2" ). That makes whether the vertical leg is inside or outside of the gutter much less important. Many gutters have support straps that preclude running the leg inside them.
Thanks, Malcom! I had not thought about the gutter supports and their interaction with the drip edge. I'll have to give this a little more thought but perhaps, as you said, if the top of the drip edge protrudes over the gutter the vertical portion is not as critical.
Not a fan of no overhangs, puts too much water into the cladding. I think you are on the right track on the walls, but I would read through here to make sure:
https://buildingscience.com/documents/insights/bsi-029-stucco-woes-the-perfect-storm
For the roof, you don't need spray foam. You can do all fluffy with a diffusion vent at the ridge (you can use a strip gypsum sheathing as a non combustible vent). Since it looks like flat ceiling, assuming no air handler up there, the insulation can also go on the floor with a diffusion vent.
If you do want fully unvented, you still only need about 1.5" of closed cell spray foam for condensation control, the rest can be fluffy. The unvented assembly needs to be all in the rafters though.
Akos, thanks for the reply! I have read a number of articles and threads and am certainly approaching the no overhangs detail with some skepticism. I read through the article you sent, thank you!
- I plan to use plywood and not OSB
- I plan to use a drainage mat
- I definitely will not be using any kind of interior vapor barrier
- I plan to spec a relatively vapor-open WRB
As far as I know, stucco failure in this area is not prevalent, which gives me some optimism with my proposed detail.
The attic will have air handlers (both ductless mini split units as well as a few ducted units for larger areas). The attic will also house the ERV units.
That's good to know about the thin layer of spray foam on the underside of the roof. I had a feeling that was possible, but wasn't certain.
I'm not super familiar with the pros and cons of a fully unvented attic, but I do want to minimize openings into the attic for fire reasons. Can you explain what "The unvented assembly needs to be all in the rafters though." Meaning the air and thermal control layers both need to be in the rafters? Would a 1.5" layer of closed cell spray foam and then mineral wool (flash and batt) be sufficient to create the unvented assembly, or is there more to it than that?
Finally, perhaps could you help me understand the overhang issue a bit more. I've done some research but most of what I find doesn't really have an explanation for why an overhang matters, only that it does (or does not, depending on the article). From what I have read it seems that wind-driven rains are the biggest concerns. However, with a typical 16" overhang and even a mild wind, the eave only protects the top few feet of a wall. I'm just wondering how the top few feet of the wall being dry has such a big impact on the wall as a whole.
Thanks!
I have more of an anecdote, colder climate reasonable precipitation. I walk by this garage in a laneway all the time and always wondered why it was bulging so bad (the picture doesn't do it justice, it looks like the brick will fall out soon). Of course there was a lot of neglect over the 100 years, but many around with similar age and care and don't have issues.
The other day, I walked by during a rain storm. No gutters on the garage so the water gets dumped into the laneway which is also common enough. However on this one, most of the water comes down in one section and you get a lot of splash back right where the big bulge is. So don't mess with water, in the long run it always wins.
I'm not saying zero overhangs can't work. They are a common detail in the certain parts of the US on old houses and seems to hold up. You do have to take extra care to make sure to keep water off the siding and your window flashing details have to be perfect especially on the gable ends.
In zone 3 you have two options for unvented attic.
-fluffy insulation either in the rafters or the attic floor with a diffusion vent
-closed cell spray foam and fluffy in the rafters with no diffusion vent.
For the diffusion vent you can read here (see Fig6 for fire areas):
https://buildingscience.com/documents/guides-and-manuals/gm-2101-guide-building-conditioned-unvented-attics-and-unconditioned
For the hybrid spray foam (table 1):
https://buildingscience.com/documents/building-science-insights-newsletters/bsi-100-hybrid-assemblies
Akos, thanks for the additional information! That wall is definitely in rough shape! I'll read through those articles.