What to put under metal roofing?

[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.