TOPIC: Is A Metal Roof Right For My House
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I maybe over thinking this, but I started re-researching metal roofs, and the one main thing I'm concerned with is condensation..
From what I understand, you get condensation forming and possibly trapped on the bottom of any metal roof when the outside is 35 or lower, and worse if the space below is heated.. I watched a video of a barn raining inside that got my attention! Imagine the water damage that could slowly occur..
I like the durability and the price of a AG metal, but wonder what it would take to address the issues. I don't want to talk about hidden fastener standing seam now.
I think I figured out how to prevent screws from backing out by using a new epdm flat top serrated wafer screw I found..
But regarding the condensation, I think the only solution would be by adding vertical battens over the existing trusses for a air path before the horizontal battens, and using a foil faced bubble wrap type condensation barrier over the battens before the metal.
But isn't the main issue how to prevent heat and moisture from escaping the house and reaching the bottom of the cold metal? In most cases it's a re-roof, and installing a vapor barrier is unrealistic. Along with common static mushroom vents cut into the roof that would need to be patched and replaced with a ridge vent. Is the foil face bubble wrap condensation barrier enough?
Are there any other better steps I can take to minimize the condensation? In my case, I only have static vents close to the peak and no overhang or soffit vents. So I could get fancy and extend the 2x4 vertical battens out and over the edge to make a overhang and soffit. But this is allot of expense just to take care of condensation..
Mike, I am glad you are doing your research. Condensation does occur when warm moist air hits a cold surface. The exact point at which that happens is a combination of humidity and temperature -- dew point. If your home has a vented attic and a solid deck on the roof, then that is what I call "typical construction" and metal roofs are installed on that sort of construction day in and day out without issue. My own metal roof on top of a structure like that was installed in 1996. So, that then begs the question ... does your home have good attic ventilation (meaning intake and exhaust vents of adequate quantity) and is there a solid deck on the roof? If not, let's talk further.
Thank you Mr Miller..
My home has static mushroom vents close to the peak (which my luck, would be located under each panel seam), but no overhang or soffit vents. I wonder how it can work with no low intake, but I haven't noticed ice dams..
I wouldn't have asked this question unless I hadn't found a few reports of issues with condensation including battens rotting out, condensation dripping from under a porch roof and the barn raining inside video. But these probably weren't "typical construction".. One was a sips roof. Another one the home owner insulated and drywalled the roof joists etc..
So what your saying, if the attic has "typical construction", one can install a metal roof right over the underlayment without additional ventilation or condensation barrier, and no condensation will form or be trapped?
Can we get some pictures of the attic? Is the roof deck solid?
Mike, to answer your question, yes, that is done all the time. However, in your case, I feel you need to add intake vents. I am concerned that yours could be a situation where, without intake vents, a metal roof direct against the deck could become the straw that breaks the camels back by dropping the roof deck temperature enough to hit dew point and cause condensation to occur. Adding intake vents to create air flow though will keep that moisture (which originates inside the structure) from accumulating. If it doesn't accumulate, it doesn't condense.
I 100% agree Mr Miller.. Its a mobile home with shingles and Ive been told "don't mess with the ventilation the mobile home engineers designed." I read one topic here where the home owner removed the cardboard under the sheathing and then later realized, after it was raining inside his home, it was the condensation barrier..
Since I cannot install a soffit overhang and low intake ventilation air (not enough free air space due to shallow rafter tails and top plate) , my only metal option is for a cool metal roof like houstoncoolmetalroofs.com design, but I imagine they didn't come up with the idea. I cant find allot of forum talk regarding Houston cool roofs. Maybe I should be searching "Texas cool roof"? They claim houstoncoolmetalroofs do not need "typical construction" meaning the attic can be fully insulated because of the radiant barrier on top facing the metal, and the large ventilation air space..
Is it a well thought out and proven design?
Thanks for the additional information. In your case, you could accomplish what is being done by Houston by removing the existing shingles to get rid of the weight, installing a suitable underlayment followed by a radiant barrier, then install vertical battens as sleepers along each truss, then horizontal battens, and then the metal roof. The resulting vertically oriented air chambers would be vented with intake air at the bottom and exhaust at the top. Make sure that the metal roof panels and the ridge vent you choose are all approved for this sort of application over battens and at the approximate 2:12 pitch of your roof.
My roofer and I might just do sleepers and a metal roof because we decided to postpone the roof until next spring as the repairs have held tight in the last week of rain. I'm pretty sure its not a 2:12 pitch Mr Miller. I figured 4:12 because it had shingles.. But I have not measured.
What might happen is my roofer will remove old shingles and install a underlayment designed to withstand UV for 6 months. That will give me time to install the 2x4 or 2x6 sleepers and battens.. But what I'm wondering if how to safely add more R-value.. Should I remove a row of sheathing and blow fiberglass into the truss space? Or just reduce the sleeper air space with 2" of polyiso between them, or install polyiso before the sleepers, maybe laying them on the 4" side to distribute weight and reduce thermal conductivity? Not sure how I would locate the existing trusses to screw into them with a layer of polyiso or underlayment covering the nails though..
And then what to do with the old static vents? Cover them and cut a 4" ridge vent? I'm pretty sure the static vents didn't do much to vent the attic without soffit vents, but more to allow moisture to escape..
Thanks so much. I am sorry for the mistake on the roof pitch; photo perspectives can be misleading sometimes. Yes, if you add the ridge vent as planned, you will cover over the other vents. I like the idea of installing the poly iso between your sleepers. Yes, you still get some thermal conduction through the lumber but at least everything is solid and you know it's solid. If you leave underlayment exposed that long, make sure it is fastened with plastic cap nails. Nail the horizontal laps at least every 4" if not closer. Lap the underlayment down over all roof edges. Be sure to use a high quality synthetic.
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