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Mitigating Risk of Moisture Problems for Low Slope Cathedral Ceilings w/ Roof Replacement

wa_treehouse | Posted in General Questions on

Hey All,

We’ve got a low slope roof (~2:12) over cathedral ceilings throughout the entire house. It was shingled (uh-oh) by a previous owner and we want to replace it ASAP as it has started to leak. We believe (still need a core sample, but roofers will only take one once you’ve signed a contract so we’re a bit of chicken and the egg) that the sandwich is as follows:¬†

Composite Shingles
1/2″ Plywood decking
Air gap
R19 fiberglass in a 2×8 joist cavity
Cedar plank interior ceiling
Interior space

The roof is a series of separate planes. Two monoslope/sheds with “bird block” vents on both low and high ends and one gable with a ridge vent up high and soffit vents down low. There is a small amount of convolution in the shed roofs where we have vents at the low side and none at the high. This is a guess, of course, as I cannot see the framing.

I am 100% aware that this does not meet code in terms of insulation and I have read the article on how to properly detail a cathedral ceiling linked here:

My issue is this: I simply cannot afford to move everything out of my house, take down every cedar ceiling (and hopefully, by some miracle, keep it ordered and undamaged so we can put it back up), scab on additional framing, insulate properly, and then put everything back together. The other option for doing it by the book  seems to be tearing off all the plywood and insulating inside and out but I am having a very hard time finding a roof contractor who will even have that conversation with me and given how busy roofers are right now and how high the bids are coming in I am very afraid of the numbers if I could.

I feel like I may just have to accept that I will not be able to do things according to best practices, so my best course of action is to mitigate risk and this is where I could use some help from the community. We’re considering just about any option for our roof except metal since prices are absolutely wild right now.

I’ve been told a lot of different things by roofing estimators over the last 2 weeks from “Don’t let anyone convince you to use PVC/TPO. Your walls will sweat,” to “We vent our PVC/TPO installs with breather vents every 5 squares so there will be no moisture issues,” to “Your house was likely built with a torch down roof and we recommend putting one back on to reduce the risk of moisture issues.” Honestly the amount of contradictory information has got my head spinning and I’m starting to feel like I’m buying a used car. This is not my area of expertise and I don’t know what to believe. My understanding is that PVC/TPO/Mod bit all act as vapor and moisture barriers and that there really may be very little difference between them as far as long term moisture issues. Is this right? Is there some option I am not seeing to reduce my risk of having moisture problems down the road that A) I might be able to find a contractor/roofer willing to do and B) won’t put me, literally or financially, out of my house?

Thanks for any guidance,

GBA Prime

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  1. Expert Member


    None of the various options - asphalt shingles, torch on, etc. - allow any useful drying through them. So one is a good as another in that regard.

    Any new roof done correctly will stop the leaks. Switching the type of roofing won't cause any moisture problems, nor will it fix any that the roof presently has. If things are okay except for the leaks, you will be fine just addressing that problem with any material you want suitable for a 2/12 roof

    1. wa_treehouse | | #2

      Thanks Malcolm. We do have some evidence of condensation (the cedar planks have darkened around the end grain) but the ceiling is 40 years old. So as long as we aren't going to make the problem *worse* by choosing one material over another, I think we can live with it. My dream is to one day take it all down and throw baffles, 9 inches of rockwool, and a layer of foamboard up.

  2. Expert Member
    PETER Engle | | #3

    You may need some local expertise, but it is very important that you make sure that you are observing roof leaks and not condensation dripping back down. The easiest low-tech way to verify is when you observe the leaks. If they always occur during or shortly after heavy rains, its probably a leak. If they ever happen on cold, clear days, they are probably related to condensation. The roof you describe (regardless of covering) can be very prone to condensation and fixing the roof won't fix the problem.

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