What to put under metal roofing?

[N80]

I am building a post and beam, open carport down at my cabin. It is shown below. I will use 2x4 (actual) purlins. It will have a 5v crimp metal roofing.

My much larger tractor shed that i built about 15 years ago has the same metal roof set up; metal on purlins. When the conditions are right water condenses on the underside of the roof and it practically rains under there. This is causing the purlins to be moldy and they'll eventually rot (although I'll be tearing it down before long to make room for a shop).

Anyway, I'd prefer not to have this condensation problem. My initial thought was to use 1" thick boards as 'sheathing'. I have a sawmill and have plenty of it. So the sheathing would sit on the purlins, then tar paper on top of sheathing then the metal roof. Some folks have suggested that there would still be a condensation problem and the 1" pine boards would rot.

I've thought about OSB sheathing but it would be ugly from the underside and probably also rot if the boards would.

So I'm looking at the insulating stuff used in this capacity. Usually white from what I've seen. Sometimes silver. I don't know how much it costs or how to install it. I don't know how well it would hold up over time either. I've seen barns and sheds with it hanging down in tatters and I don't want that.

I am looking for any thoughts or recommendations on this issue.

 

[Fuddyduddy1952]

Right before Covid I had a 38x64 horse stable built. They used this "double bubble" insulation before screwing down steel roof. Same under a shed overhang which is fantastic to me, weatherproof and reflective.

 

[Fuddyduddy1952]

To install is easy, just cut length you want on ground, take it up and use hammer tacker staple it on. It's clean & lightweight to work with.

 

[Hay Dude]

How about T&G PVC planks?
Attractive on underside, won’t rot.

 

[5030]

Had the contractor do what HD posted. Just had the side shed on the big barn roofed.

 

[caver]

My cabin plans call for 17mil Epilay Protectite Platinum roof underlayment. (It's included in my kit) I told them I wanted to use some type of metal roof. The roofing will be attached to osb that is glued to 10" styro panels (like SIPS) and they call it a nailbase panel. Platinum Synthetic Roofing Underlayment

 

[JJT]

How about a pair of ceiling fans? A neighbor's pole barn rained like you said, installed a pair of ceiling fans to move the air and that cured the indoor rain issue.

 

[grsthegreat]

The fabric insulated underlayment gets eaten up by birds pecking at insects and squirrels looking for nesting materials. Ask me how i know. I had a company tear off my barn roof, remove that nasty rolled insulation, install cdx plywood, tar paper and reinstall metal roof using longer, thicker screws. The tar paper will channel any moisture. When they use metal roofing on houses, they do the same thing.

 

[Jstpssng]

^^^^
Exactly!
I had a shed with metal roofing. When I moved here I stored a lot of things in it and lost anything which wasn't plastic.

I'm looking at the same problem and like you, have a sawmill and plenty of trees. Put your boards lengthwise up the roof so that any moisture will run down them; they used to build barns that way with no outside covering. Then put furring strips lengthwise to hold the metal screws. That will create an air gap and should eliminate any mildew problems.

I plan to use asphalt shingles instead.

 

[CalG]

30# felt

 

[paulsharvey]

I'll go against the grain on this; its a post and beam Car port, it's not going to be dried in (atleast in my mind, car port means just a roof and posts). It's never going to be dry, and I would just use PT above ground 2x4s as the purlions. If you're worried about the pressure treated wood corroding the metal, you could lay a layer of Sole Seal thing foam directly over the purlion to provide separation between the roofing metal and the purlion. It's cheap, comes in long rolls, and is easily stapled on top of the purlion before the metal is attached. If you already have milled the purlions, or just prefer to not use the PT 2x4s, you could still staple (or button cap nail) the sole seal to the top.

Also, without having walls, it's not like there is the temp difference between inside/outside air like in a barn, that leads to large amounts of condensation.

Edit: ignore the part about PT 2x4s, I didn't realize the frame was already built.
.

 

[milkman636]

My cabin plans call for 17mil Epilay Protectite Platinum roof underlayment. (It's included in my kit) I told them I wanted to use some type of metal roof. The roofing will be attached to osb that is glued to 10" styro panels (like SIPS) and they call it a nailbase panel. Platinum Synthetic Roofing Underlayment

I used epilay under a metal roof last year over osb sheathing. My purlins run on 2’ centers between the trusses. The building is fully enclosed and has closed cell foam to keep everything dry and tight. The Epilay was my most cost effective option for synthetic membranes when I did mine last year.

 

[buckeyefarmer]

My cabin plans call for 17mil Epilay Protectite Platinum roof underlayment. (It's included in my kit) I told them I wanted to use some type of metal roof. The roofing will be attached to osb that is glued to 10" styro panels (like SIPS) and they call it a nailbase panel. Platinum Synthetic Roofing Underlayment

Can you share more about your cabin plans and your styrofoam panels?
I’m planning a cabin with metal roof.

 

[rScotty]

It's probably different for every climate and every type of steel roof. Here we cover the rafters with ply and then tar paper, put stringers down on that, and fasten the metal roof to the stringers. The stringers are laid so that there is a continuous air space between the roof and the steel all the way from the eaves to the peak. Special edging makes the gap vey small at the eves to prevent insects and such. But up at the peak, the cap row is always set up on ridge stringers so that it forms a longitudinal chamber with an umbrella type cap. That way the air can circulate from the eaves up to the space under the cap and then out easily.
Formed end caps with screens at both ends ends of the peak is used to keep critters out. Getting the peak cap right is a critical part of the job. Sometimes it needs raised vent structures on the peak - you've probably seen vents on the roofs of barns. That's why.

The whole thing depends on building it so that air can circulate under the steel and get out the top.

That's what works for us here in a relatively cold and dry climate. Your local roofers will know what works for your area. From what I'm hearing, you might want to hire a local contractor to put the steel on your roof. Don't go for cheap; get a real roofer with a serious attitude and lots of experience. It is worth the money.

 

[hslogger]

I think you carport is a dandy. SOME day some one might want to enclose it. Maybe you, even though at this time, no.
Bite the bullet. Lay plywood down then thirty pound felt and then your metal. All of these vapor whatknots in rolls sooner or later get birded or bugged. They look like heck and hang and are PIA. The plywood will dampen the noise when it rains hard or hails. Mark my word. I have a 48 by 60 barn that began as a twelve by thirty six implement shed. When I built the shed I went cheap and put down stringers, vapor paper and metal. Stoopid. The bugs and birds had a field day forever, and even a deaf man hated the hail and hard rain coming down. Long story short, when I added on to it four times, I finally went back to that section, tore the roof off. Put plywood and 30 lb felt and then metal back on top. Did it right the second time. That's my story and I'm stickin to it. Annnnnd, I wouldn't put anything but Snap lok metal roofing down vs old time nail or screw up the wazoo.

 

[goeduck]

Bite the bullet. Lay plywood down then thirty pound felt and then your metal.

^^ Agree. Cry once.

 

[two_bit_score]

Agree. Cry once.

Sometimes a hard lesson to learn. But. Once learned it’s stays with you.

 

[mikester]

How about a pair of ceiling fans? A neighbor's pole barn rained like you said, installed a pair of ceiling fans to move the air and that cured the indoor rain issue.

Depends on your climate. Won't work in colder climates like here in southern ontario with cold spring nights and warmer humid days.

Spray foam insulation is an option that works but is expensive.

 

[Hay Dude]

It's probably different for every climate and every type of steel roof. Here we cover the rafters with ply and then tar paper, put stringers down on that, and fasten the metal roof to the stringers. The stringers are laid so that there is a continuous air space between the roof and the steel all the way from the eaves to the peak. Special edging makes the gap vey small at the eves to prevent insects and such. But up at the peak, the cap row is always set up on ridge stringers so that it forms a longitudinal chamber with an umbrella type cap. That way the air can circulate from the eaves up to the space under the cap and then out easily.
Formed end caps with screens at both ends ends of the peak is used to keep critters out. Getting the peak cap right is a critical part of the job. Sometimes it needs raised vent structures on the peak - you've probably seen vents on the roofs of barns. That's why.

The whole thing depends on building it so that air can circulate under the steel and get out the top.

That's what works for us here in a relatively cold and dry climate. Your local roofers will know what works for your area. From what I'm hearing, you might want to hire a local contractor to put the steel on your roof. Don't go for cheap; get a real roofer with a serious attitude and lots of experience. It is worth the money.

We call those furring strips. Very effective for creating air space and adds a layer of strength to the roof structure.

 

[nyone]

30# felt

It's not 1980 any more. There far better synthetic underlayments than that and arnt as expensive as even OSB.

If its an open car port you shouldnt get any condensation on the underside of the roof as there should be no temperature difference and that's what makes the condensation.