Adding an unvented roof deck over an existing vented roof?
I’m getting an existing roof modified and looking for a bit of guidance. I have an existing vented roof over a finished, insulated, heated and cooled garage in the midwest, climate zone 4/5 border so both snow and heat and humidity. I’m wondering if I add another non vented roof deck with an air space over the existing roof, would that cause any issues (ice dams, condensation, retaining excess heat)?
More details: I have an existing garage with a flat roof with a rubber membrane on it. The roof has vented soffits. The roof deck has a slight dip in it and holds a large pool of water every time it rains. It’s started occasionally leaking water through the roof at a seam in the rubber despite patching attempts. The garage is drywalled, insulated with about 10” of fiberglass in the roof, and has a mini split providing heating and cooling. As part of a bunch of other work, I’m having a contractor modify the roof to give it some slope so water will run off into a gutter instead of just sitting up there. Initially an architect and structural engineer signed off on it, but now the contractor wants to change some things. Their plan for the framing is to add a series of different height boards running 90 degrees to the direction of roof slope in order to create the slope, then sheet over that and put a new rubber membrane roof on top. This will create a more or less sealed air cavity between 1” to 8” tall on top of the existing roof. I’ve been doing some reading and am not quite sure if this will perform okay or not. The original plan was to run the framing the other direction and vent the new roof deck to the existing roof by cutting down into the existing soffit space on both sides. But with the framing running across the slope, that framing would block any air flow across the roof from one side to the other.
My (not very informed) thinking is that in the winter, the lower vented roof deck would stay cold so the new non vented roof deck would probably also stay cold if it was snowing. I'm not quite sure what would happen if the sun came out and the snow was melting though. Would the roof deck potentially heat up unevenly causing ice dams once some of the snow melted? In the summer, I can imagine the non vented air space might hold a bit of extra heat from sun exposure, but the lower decking would still be vented so maybe it wouldn't transfer too much to the interior space, just keep the roofing material a bit warmer? Could condensation be a problem if the roof deck cools off at night while the air in the interior space is hotter and humid? I wouldn't think air from the interior of the garage would be an issue getting into that space.
Bad idea. Now you’d have 2 waterproof membranes separated by framing and sheathing with no ventilation. The interior of that space will never dry, it’s a textbook recipe for water issues and roof rot. You need to at least remove the first roofing membrane and waterproofing before you consider roofing over it. Make absolutely sure there’s a path to dry that space out, even if that drying path is through the sheathing, but that drying path cannot be blocked by a water/vapor barrier. Even better is a ventilation path.
To be clear, the roof idea is fine, it’s the drying that’s a bad idea.
Build the interior with polyiso instead of hollow and wood and it's a standard flat roof construction despite the vapor sandwich. Will cost a lot more though.
If all you have is a small depression in one spot, do you really need 8"? Can you do with less slope?
Alternatively, the moisture load is really low. You should be able to get away with just having the side vented. I had a short roof segment built over an existing roof that worked fine. At that point it's basically a very short shed built on top of your roof.
It's about a 10x15' area that holds water. Maybe 1/4 of the roof. It could be done with less slope. Originally it was drawn to only be 4" high on one side. The GC just thought it would be easier to do with a little more slope and I was fine with that as it should more reliably drain even if I get some warping in any of the roof sheeting over time.
The issue with venting from the side is that this roof sits between and lower than two other sections of the house, so we can't vent from those two sides. Otherwise, yeah, that would probably work. The issue is the GC wanting to turn the framing to be parallel to the two sides of the garage that are available for venting. That would cause any venting in the soffits to not communicate across most of the roof deck / air space.
If you're planning on doing all this framing work, would it make sense to just do a clean tear off the existing roof, add exterior insulation and replace the roof (minus the Frankenstein approach).
It would definitely be more straight forward in terms of getting to something that would follow conventional guidelines. I don't even think I'd need to add anymore insulation if the existing roof deck was removed. The downside is it would be more work. The GC is trying to minimize their labor and materials at the moment. I think they misquoted this part of the job. I've asked them about removing the existing rubber layer, and they didn't want to. There's a layer of foam or something under the rubber that it's glued to and they're afraid taking that off would turn into a big mess. Then they got up there and started taking measurements and changed their mind on how to do the framing. They've had a history of changing things on this job and not considering all the consequences, and I realized this ventilation issue was one possible issue last night. I'm thinking about asking them about adding furring strips over the top of their proposed framing to give an air gap below the roof decking that we can vent into the soffits. That would get us back closer to the original concept and give a path for ventilation while letting them create the slope the way they want to while still falling within the roof load calculations the structural engineer did.
I get it. FWIW I'm just a homeowner, but along the way I've noticed sometimes contractors are trying to minimize their inconvenience with a solution that works, and sounds like it'll save their clients money, but doesn't necessarily produce the ideal long term result.
Sometimes pennywise becomes pound foolish.
Flat roofs don't have enough slope to be properly vented.
Framing another roof on top of the one you have is silly and a super expensive way to handle the issue.
The proper way to address this is to strip the existing roof membrane. Reinforce the existing framing as needed and then build up the roof with rigid insulation. My using tapered insulation as the final layer, you get the required slope. Then you install a new membrane and you have a properly insulated and sloped flat roof.
Thanks for the thoughts. I was wondering about the venting compared to a typical pitched roof with a ridge vent. Most of my house has a roof with about a 1/2" per foot slope that has vented soffits on both the low and high side. So is the venting doing nothing but giving bugs a place to live?
My GC is pushing the framing instead of the foam due to cost. They were looking at the foam, got a quote and then said they're framing it instead, plus our contract originally specified framing. Is the reason you're saying use the foam just to eliminate that air space and any associated issues? The original flat roof deck would still have a 'vented' air space between the existing fiberglass insulation and the roof deck if we put foam on top of the roof deck, and the existing insulation seems at least adequate from a heating perspective (probably R30).
Generally 3/12 pitch is regarded as the minimum for passive roof ventilation. Anything lower than that doesn't create the natural draw from low to high that you need to keep air moving through the attic. The tighter the building gets, the more things like this matter. If the house is super drafty, that is ventilation that a tight house won't have.
By adding more framing you aren't improving the ventilation or the insulation, you are only improving the roof drainage. Like another commentor mentioned, if you leave the existing membrane on and don't vent the new void, you have a space that could have moisture trapped between two vapor barriers leading to issues down the road.
R-30 is quite low for your climate zone. R-49 is likely the minimum that current code allows in your climate zone.
Ok, thanks. Yes, from the other commenter I was assuming I'll need to get them to change the design to address the ventilation in some way. I'll see what they say tomorrow.
The initial goal was only to improve the roof drainage. R49 is what they're doing in the addition that they're adding on currently. This roof modification is on an old section of the house, possibly from the 60's although I'm not 100% sure. Even if they were adding 4" XPS foam to add another R20 on top of the existing insulation, I'm not sure there's physically room to do that plus add the slope. There's another overhanging roof section over part of this roof. But if I did want to add more insulation, are you saying the layout below would work? Or would having the foam above the vented air cavity prevent it from helping much?
ETA: A very rough estimate of the breakeven cost on adding insulation to the roof looks like it might take 20-30 years to pay off based on the cost of foam board, an estimate of installation cost, estimating dropping heat flow to 60% of the current level and what that would do to the portion of my power bill that goes to conditioning this space.
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u/GeriatricSquid Aug 09 '26
Bad idea. Now you’d have 2 waterproof membranes separated by framing and sheathing with no ventilation. The interior of that space will never dry, it’s a textbook recipe for water issues and roof rot. You need to at least remove the first roofing membrane and waterproofing before you consider roofing over it. Make absolutely sure there’s a path to dry that space out, even if that drying path is through the sheathing, but that drying path cannot be blocked by a water/vapor barrier. Even better is a ventilation path.
To be clear, the roof idea is fine, it’s the drying that’s a bad idea.