I'm building a new cabin and plan on using metal for the roof and looking for some advice on prepping the roof before install. It's a ranch home with 9 pitch attic trusses and wondering what the better way to frame the roof is with the roofline being insulated and the hot/cold temp change I'm worried about moisture. Would it be better to sheet the roof with say 5/8" osb and lay ice and water down and screw the metal right to the osb or should I just sheet the roof with 7/16 or 1/2" and ice and water that and then install 2x material perlins to create an air gap between the metal and roof? What do you experts do? Thank you

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Patrapper, 5/8” plywood for the roof deck, not osb. Ice and water on bottom and in valleys if you have any. And roof paper the rest. It’s like tyvek for the roof. I like the kind with the spider on it, scorpion maybe. That’s how we do it.

In the past six years I've built two buildings with metal roofs. The first was my new wood shop. I stick-framed the rafters and sheathed them with 1/2 CDX plywood. I put 30lb felt over that and then the metal panels. The only issue I has was that when I decided to put snow guards on I had to use specialized glue because they want a minimum of 1.5" fasteners and I only had 1/2". I didn't want to add blocking between the rafters because I had already insulated and finished the ceiling. The glue has held through six winters so far.

The second building is a summer kitchen. I went with traditional 2 x 4 perlins over the rafters. Since the building will be heated I was worried about condensation issues, you can have condensation issues with steel roofs even without heat in the building. I bought 2" Dow polystyrene panels, ripped them to width on my table saw and fit them tightly into the rafter cavities under the perlins. I caulked the edges where needed. There will be faced fiberglass insulation in the cavities under the polystyrene.

I figure the polystyrene and fiberglass will create a thermal break and the polystyrene will drain any moisture into the soffit area.

I'm a general contractor but I'm not an expert on steel roofing, I don't do that much of it.

Frame the roof and perlin it.
Now go inside and put tongue and groove boards on the inside with no gaps or hole
Industrial spray foam the roof side until it's above the perlins
Trim all the foam above the perlins and install the metal roofing
I would contact a company in your area that does the spray foam
It's not a cheap roof but you'll have no problems with condensation

Did it seem like the 1/2" plywood had enough holding power? That's a good point about the snow guards I didn't think about. I'm a carpenter by trade but like you I really don't get into metal roofing

I've had no issues with holding power on the shorter screws. They are approved by the manufacturer as long as you adhere to the specified screw pattern.

If you go that route and want snow guards, add a row or two to the rafter cavities in the proper locations. it would probably be easier to do that from outside before you sheath. But doing it from inside before you insulate would work too.

I wasn't going to use snow guards until the first good snow dumped an avalanche of snow from the roof onto my freshly plowed driveway. That's why I didn't frame for them.

I was skeptical of the glue-on snow guards at first. I used SnoBlox/SnoJax brand and their recommended Surebond SB190 Everseal and followed the installation instructions. I was pretty sure I was going to be picking up snow guards the following spring but so far, so good.

They are rated at a high shear strength but it's still significantly less than mechanically fastened guards.

did roofing for 35 years and just recently changed professions as a bad knee doesnt like being on a roof anymore so roof decking should be min1/2" ply do not use osb it will workbut you are looking at issues if any sort of moisture happens use ice and water in the valleys and roof edges as for roof underlayment synethic felt is not reccomended under steel roofs unless it is designed for steel which most are not i would use tyvek or old school tar paper to cover the roof decking you could strap your roof also but i would reccomomend strapping every foot instead of every two(you can still screw every 2')

now for all you new roofers out there synethic felt is just garabage it is like putting a tarp on your roof condensation can not escape from it and will saturate the roof deck and eventually rot the roof yes i know the roofing companys reccommend it and they say it breathes well i have made almost all of the companies synethic felt at a factory i worked at in the winter if you dont believe me place a tarp over a car and let it sit the car will rust and be wet whenever the tarp is removed if it keeps water out it cant let water evaporate in the late 70s they tried something similar and all the roof decks rotted the reason shingle companies recomend roof underlayment is because they all have had class action laws suits agaisnt them in the past 25 yrs because of poor shingles water should never get too the under layment to start as forpeople saying tar paper sucks to this day you can strip a roof and its still good it gets wet and dries its just not safe to walk on

I agree with you about the synthetic felt.

And insulation.

I built a skinning shed/workshop,insulated it,and put on a tin roof.
I put strapping directly over the rafters and put fibreglas batt insulation between the rafters.There is an airspace about 2 inches between the tin roofing and the insulation that can vent.
It gets cold here in winter and I heat it with a woodstove and havent had any problems with condensation.
I did not use any vapour barrier and it hasnt been a problem.
Dont know if that would work on a home sized structure.

In NE Wisc the only time I’ve had condensation on the underside of my steel roof is if I run a propane heater. Even then it’s just frost that forms. It evaporates by the next day. It has never dripped in over 25 years.

Sheeted my garage with 5/8" osb, then ti paper & 2 rows of non coarse ice guard at bottom. No issues with the roof or snow guards through heavy wind events or several winters.

Doesn’t mater what you use plywood, perllins , OSB, etc. . But put some sort of vapor barrier on top of it and then metal roofing. Otherwise you will have condensation dripping.

Yes, you will. It must not be a thing in less humid climates like the northwestern US. But here on the east coast condensation inside unheated buildings with exposed steel roofs is common. Anybody that says it ain't hasn't been paying attention.

I've yet to see condensation under my lean to but it's wide open 3 sides and gets plenty of air movement.