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guuby
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# Posted: 1 Jan 2013 10:14pm
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Hello, I am currently building a small 16x16' cabin in southern Indiana. Roof is the next step and we are debating if we really need to put some sort of underlayment down or just screw metal directly to furring strips. It will be used mostly in the summer but we plan on putting in a wood stove so if want to stay a night or two in winter we could. The only heat (obviously) will be from the wood stove so my question is this...with a 16x16 cabin, how long would it take before sweating would occur? Or if we only plan to heat it for no longer than 24 hrs at a time maybe 2-3 times a winter (with a drying-out-the-air wood stove) will metal roof sweating ever be an issue?
Its not so much the $ its more the extra work involved. Its just me and my 65 yr old dad so we are trying to keep it as basic as possible. We understand that we should do it but since we wont be living in it and heating it all winter do we really need to do it.
Thanks so much...and any other tips or advice would be appreciated
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toyota_mdt_tech
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# Posted: 1 Jan 2013 11:41pm
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I sheeted my entire roof with 1/2" plywood (rafters at 16" OC) and covered it with 30 lb felt then laid the metal roof. The felt protects, but still breathes too.
If you have the metal exposed underside, you will get lots of sweat. I can not hear the rain on my roof either (though, I do live the sound of rain) but I do have a full ceiling that is insulated between myself and the roof.
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Gubby
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# Posted: 2 Jan 2013 12:03am
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I guess what I'm needing to know is how quickly will the metal start to sweat with no plywood or felt. Will it happen within 2 hrs of starting your stove? Does it take 24+ hrs before sweating starts?
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Martian
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# Posted: 2 Jan 2013 07:15am
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Its not the stove heat that will cause sweating. Metal roofs just sweat. If its freezing, they frost. Eventually, any wood in contact with the metal will rot.
Tom
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toyota_mdt_tech
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# Posted: 2 Jan 2013 09:02am
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Quoting: Gubby I guess what I'm needing to know is how quickly will the metal start to sweat with no plywood or felt. Will it happen within 2 hrs of starting your stove? Does it take 24+ hrs before sweating starts?
If its cool outside, warmer inside, sweat. Sheet the roof. The only time a person would do it without sheeting would be like a large open barn with plenty of air movement where temps inside and outside remain close to the same.
Sheet it, felt it or its going to give you nothing but trouble from here forward.
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Martian
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# Posted: 2 Jan 2013 09:09am
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Quoting: toyota_mdt_tech The only time a person would do it without sheeting would be like a large open barn with plenty of air movement where temps inside and outside remain close to the same.
Even then, metal sweats on humid days. One shed roof patio cover I built a couple of years ago sweats, and it is totally open.
Tom
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guuby
Member
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# Posted: 2 Jan 2013 08:12pm
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Sounds good, decking and felt it is. What size decking? 7/16 work? Thanks
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toyota_mdt_tech
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# Posted: 3 Jan 2013 12:13am - Edited by: toyota_mdt_tech
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I'd go 1/2" minimum. Stagger your sheeting like brick work, so all the joints do go straight through. (its code anyway) and get your "H" clips too. .
I did a ridge vent on the top of mine also. Plus the 2 gable end vents.
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rayyy
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# Posted: 3 Jan 2013 06:21am
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It will sweat and drip down on you all the while you are heating it.If you can insulate the ceiling above you leaving the steel roofing above that vented to the outside,you will be alright.
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oldgringo
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# Posted: 3 Jan 2013 06:57am - Edited by: oldgringo
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No one has mentioned it, but bare steel will turn your cabin into an oven in summer.
My next little project will utilize furring strips on top of osb decking so that hot air can move to the ridge and escape.
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Montanan
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# Posted: 3 Jan 2013 01:05pm
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Quoting: oldgringo My next little project will utilize furring strips on top of osb decking so that hot air can move to the ridge and escape.
That's essentially what we did, although our decking consists of vertical hand cut 2x6 pine boards. We put roofing felt on top of that, then laid furring strips horizontally, then put in foam insulation sheets and then the roof metal on top.
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guuby
Member
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# Posted: 10 Jan 2013 06:33pm
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oldgringo, Montanan, you guys read my mind, that was gonna be my next question. Montanan, to be clear, did you put down 4x8 foam sheets on top of furring strips or cut them to fit in between the furring strips and screwing metal directly to the wood? What thickness of foam did you use?
Thanks
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Deerfan
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# Posted: 10 Jan 2013 07:01pm
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I have a cabin in Canada. We screwed the metal on the furring strips. Insulated between the bottom of the trusses. Most important, put poly up on bottom side of trusses, then put up your ceiling. Make sure your attic is well vented. You will not have any sweating problems at all.
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guuby
Member
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# Posted: 10 Jan 2013 07:40pm
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So no OSB decking? What kind of insulation did you use? Is poly a plastic sheet? Did you put up fascia and soffit?
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tcmatt
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# Posted: 10 Jan 2013 08:34pm
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Here's what I did on mine...
rafters 2' on center
1/2 sheeting w/gap at the top for a ridge vent
ice and water shield over all sheeting
2x4 "furring" strips every 2' running horizontally
metal drilled to 2x4 furring strips
(see pictures)
then,
soffet vents
baffles on the inside
insulation on the inside
my friend did his this way...
rafters 2' on center
1/2 sheeting
2x4 furring strips running horizontally
1' foil-backed foam in between furring strips
metal drilled to strips
not saying either was the "right" way, but neither of us are having any problems.
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MtnDon
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# Posted: 10 Jan 2013 08:41pm
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IMO, it is poor practice to not first install sheathing and then a weather resistant barrier. Then the metal. When bare metal encounters moist air there is a very good chance of condensation occurring. Depending on a vapor barrier to prevent moisture from the cabin below entering the attic space is not foolproof, nor does it do anything about moisture in the exterior air. If moisture condenses on the underside of the metal it will drip into the insulation.
And plesase... metal roofs do not sweat. Moisture condenses from the air on the cool/cold metal. Living things may sweat, but not a metal roof. I'm funny/weird/strange when it comes to using the correct terminology.
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Martian
Member
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# Posted: 10 Jan 2013 10:54pm - Edited by: Martian
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Quoting: tcmatt my friend did his this way...rafters 2' on center1/2 sheeting2x4 furring strips running horizontally1' foil-backed foam in between furring stripsmetal drilled to strips
From my experience, your friend will be having problems in about 10 years. That's how long it'll take for the condensation to attack the 1/2" decking.
What I've seen happen is the condensation runs down the metal until it hits the strips where its screwed down. At that point, the water runs down the 2X4 onto the decking.
If he taped the edges of the foam with some kind of rubberized sealant over the top of the 2X4's, then it may take a few more years for trouble to show up.
You, only have to worry about your 2X4's rotting. They take longer.
I repair a lot of metal roofs. If you want to build the perfect roof, IMO, the layer under the metal will be flat and sealed.
Tom
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toyota_mdt_tech
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# Posted: 10 Jan 2013 11:29pm
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Quoting: oldgringo No one has mentioned it, but bare steel will turn your cabin into an oven in summer.
I have an attic above my insulated ceiling, it stays cool in the summer.
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creeky
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# Posted: 11 Jan 2013 11:54am
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in my shizzershack I put 1" foil faced poly iso insulation which I held to the 24" oc rafters with the nails holding the furring strips. I then attached the metal roof to the furring strips.
My understanding (I did this in November) is that the metal roof and the foil faced insulation will work as a venting system. As the sun heats the roof it will pull the cooler air from the roof edge and vent the warmer air out the vented roof ridge.
There's some good research being done on this system.
So far I can report only that, with the shack at 7 C, the north facing part of the roof was holding onto accumulated snow. It was -3 outside.
I guess i'll find out if the air movement is sufficient to keep moisture from rotting my furring strips.
I hated putting that roof up (not good with heights), so I really don't want to do it again in 10 years.
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Deerfan
Member
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# Posted: 11 Jan 2013 12:57pm
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No we did not use OSB. Fiberglass insulation between bottom of trusses. Yes poly is plastic sheeting. Yes we did soffit and fascia. Make sure where your wall and roof meet that there is a space between insulation and roof so attic will vent, eitherwise your vented soffit does no good. The ridge cap is also vented. We also did gable vents. Remember the most important thing is that your attic is vented properly and you will have no problem with moisture. You could even install whirly-birds on the roof. Our cabin had to be inspected by the township and this method is used alot. Also make sure your insulation goes all the way over your top-wall (top-plate) or you will get a frost line inside your cabin. We really like our metal roof, the snow slides off so nice when it builds up on the roof (when the weather warms up). Make sure you screw the metal roof down with wood-grip screws with the rubber gasket. We used Metal Sales steel and screws.
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guuby
Member
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# Posted: 12 Jan 2013 06:54pm
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thanks...just curious, but what are your (and everyone else) thoughts on possibly doing this
No OSB, attach 1" foam insulation sheets directly to furring strips, then put metal roof directly onto foam (screwing through foam to furring strips). Would putting felt paper on foam then metal help?
For safety reasons (plus wouldn't be able to walk on foam anyway) what if you cut the foam same size as metal roof sections and put them on at same time? At least rotting wouldnt be an issue.
Thoughts?
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MtnDon
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# Posted: 12 Jan 2013 07:33pm
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No. The first bad thing is that the screws attaching the metal go through the thickness of foam before hitting the wood. Foam has no lateral movement resistance at all. Any lateral force on the screws will have them wiggle-woggling larger and larger holes in the foam.
Even if the roof metal panels were rated as structural panels the cushion of the foam would destroy that.
Yes you are right; the roof would not be as strong. Why invite trouble?
You had a perfectly fine and strong idea with the structural roof sheathing (OSB or plywood), then the roofing paper weather resistant barrier and then the metal. Leave it like that and be happy you have a great roof.
If you have decided some insulation would be desirable add a second layer of osb on top of the foam insulation and secure that through the foam to the rafters or trusses. That is the way good insulated roofs are done when adding insulation on the underside is not wanted or not practical.
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MtnDon
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# Posted: 12 Jan 2013 07:49pm
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Open this, then scroll to the last image. It shows the roof deck on the rafters to be plank for the nice visual effect from the underside. But everything on top is how it should be done.
If you insulate you need enough foam on top to keep the temperature of the inside under surface warm above the dew point (inside). If not you can still get condensation on the interior in the coldest of outside days.
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Deerfan
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# Posted: 12 Jan 2013 10:33pm
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For our cabin we bought regular trusses, witch give use a flat ceiling inside the cabin. If your are using 2X10's for trusses and inside is gonna be vaulted, I would install roof the way MtnDon explains. I would for sure use plywood on roof and not OSB. I would definitely insulate or your gonna be sorry when you start your wood stove on those cold days.
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