Vapor barrier – unvented low slope roof – drainage
First off, hi all and thank you for the years of insights that we’ve already incorporated into our build plans.
Looking for some thoughts on a low slope (1:12) unvented roof. Climate sone 5 (Great Lakes), 1776 sq feet, walk out basement, total house renovation (foundation up).
As of now, the plan is –
EPDM membrane
cover board
8″ polyiso continuous insulation
MFM Peel and Seal (using for the purpose of a vapor barrier, lowest perm option I can find at <0.01 perms)
3/4″ advantech roof deck
16″ TJI joists with two sistered 2x6s for overhangs
Was going to do outriggers for overhangs, but wanted to keep the main structural components below deck and that would significantly reduce the amount of continuous insulation. Would rather be meticulous in detailing, insulating around and air sealing 2x6s.
My main question, is regarding the vapor barrier. Is it warranted to provided some sort of backup drainage solution at the low side of the perimeter board in the event that moisture makes its way into the buildup? Or by penetrating vapor plane, am I inviting moisture in? The drainage solution could be as simple as some 1/2″ – 3/4″ holes at the lower perimeter board, ergo draining in the soffit? If there is no drain, isn’t the possibility of a swimming pool possible if/when the epdm leaks? Looking to keep water away from the main structure and kick out even if the main water barrier were to leak down the line.
Attached is the following
1 – Roof structure
2 – Roof structure with Advantech
3 – Roof structure with vapor barrier (inside of perimeter boards will also get covered with membrane and sealed to deck)
4 – Possible drain locations
Thank you for your thoughts.
J Lukas
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Replies
J.Lukas,
A lot like rain-screens on walls, the chance of water accumulating in your roof assembly in a large enough volume to need drainage is very remote. It would necessitate a fair sized roof leak will cause damage to your sheathing, and wouldn't be cured by drainage holes. The much more likely (but still remote) scenario is that you get persistent small amounts of moisture accumulating, which there isn't much you can do about.
Thanks for the thought Malcom,
Right, totally agree that the possibility is remote as long as primary defense (EPDM) is maintained and monitored. The vapor barrier is actually also water barrier that is usually used without thought to its vapor impermeability - https://static1.squarespace.com/static/57d715c0bebafbf5c3eaf44b/t/64da86de448e4f639b9e8f89/1692042974661/TDS+-+Peel+%26+Seal+2023-08.pdf
so the hope is the advantech sheathing won't have exposure to moisture, sits within the thermal envelope, eliminating condensation risk and is able to dry to the interior in a worst case scenario.
That being said, the tendency of water will still be down, so why not give it an opportunity for 'OUT' as well? Similar to how you wouldn't want to seal the bottom of a rain screen assembly?
I get that if there were a significant leak, the likelihood of having to tear into the epdm, cover board, insulation still exists, but perhaps it could be limited to that side of the vapor barrier/secondary water barrier and leave the structural components intact?
Adding a cross section to show buildup a with a bit more clarity.
JLukas,
I would stop the foam just past the exterior walls and leave the soffits empty. You can then provide vent holes that will allow any moisture to drain or diffuse.
Attaching pdf of the above images.
JLucas-
I used a similar roof build-up for my addition. I installed a GAF VB butyl product over the roof sheathing prior to installing the polyiso. I ran it under the roof deck edge blocking on the lower edge due to concerns about making a "swimming pool" in the event that there was ever a leak. I too debated installing a few drainage holes at the base of the blocking on the lower edge but decided against it. I figured that I could add holes later if desired.
I'm not an expert but I think your roof plan looks solid.
As an aside: I did all of the roof pre-work myself but had the EPDM installed by a contractor. Without going into detail, it was not installed to specs and was done with very poor workmanship. If you're subbing out the EPDM, I would encourage you to be very diligent with your contractor selection and expectations/details.
Best of luck with your project.
Malcom - Yes that was a thought as well. Ends up being a similar assembly, albeit pushed back towards the wall (vapor barrier, drainage holes or not, perimeter board - with the coverboard and epdm spanning another soffit cavity). Haven't figured out whether omitting the foam adds much benefit as the coverboard (haven't figured out what material we're using) would have to be sufficiently supported long term across the ~4ft overhangs and blocked up 8". I guess I was thinking just continuing the foam would be the simplest and most uniform, while I could move any foam defects to the perimeter where they would not interfere with thermal continuity.
Steve - Thanks. Good to hear. Yours looks great and I remember reading your posts while developing the plan.
Yes, you are right, drainage could always be added from below at a later date. I did look into GAFs SA product as well as other SA Vapor barriers, but ultimately decided against the perm rating for our use - above bedrooms, kitchen, bathroom, etcetera.
Did you end up bringing the vapor barrier up around the top and sides of the perimeter nail boards or just left them open as pictured? Seems inconclusive whether its necessary or not. Most details I've seen bring them up, but I think that's largely because the roofs involve scuppers, integrated gutters, and/or parapets that sit above the walls.
JLukas-
I installed the SA vapor barrier under the edge blocking. I originally had plans to wrap it up the interior side of the blocking but it made the install easier just to install it flat over the roof deck. It's sticky stuff and not tolerant of screw-ups. It's a very "tight" assembly so I'm not very concerned about any significant air entry from the perimeter causing internal condensation. In my case, it's for an addition so I did extend the SA up the headwall where the roof joins the gable wall.
I know your question is more specific to the drainage holes but I am a little curious about your 4ft overhangs. I can't quite see some of the rake details in your pics, but that is quite a cantilever. What's the snow load in your area? Also, it seems like the vertical perimeter blocking could potentially pose challenges with installation and keeping it aligned for fascia. Suffice it to say- I'm a DIY guy, not a pro.