What to put under metal roofing?

[CalG]

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.

"Shouldn't get" ?? Are you saying the OP didn't observe what he stated.

 

[rScotty]

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

Yes, I know some old carpenters will differentiate between stringers and furring strips. But this is a case when either will work. We are after a little bit of strength, some leveling, and getting the air space right.

After reading some of the replies on this thread I understand more of why so many roofs drip and rot.
There's no one answer. I'd recommend folks get local advice for a roof coverng technique that works in their climate.

 

[nyone]

"Shouldn't get" ?? Are you saying the OP didn't observe what he stated.

I'm saying something dosnt sound right or the previous shed is enclosed with no ventilation.

I recently took down a open firewood shed that was 20yrs old. No rotting perlins and I live in the northeast where its wet and temps can swing 70* in 24hrs.

 

[paulsharvey]

Well, with an area with cool-dry nights, and hot humid mornings, I could see it. Moisture would condense all of the cool metal roof, even the bottom side. I dont think it would be "raining" condensation, but there would be moisture. No idea how much of a temp swing or humidity would be needed.

 

[deezler]

"Shouldn't get" ?? Are you saying the OP didn't observe what he stated.

He said he observed that condensation inside a "tractor shed" which I assumed to mean enclosed or at least 3 walls, etc.

An open roof structure is not going to rain condensation, the breeze is all you need to equalize the air conditions (similar to how another poster said his neighbor cured his inside rain with a pair of ceiling fans.

 

[woolyAcres]

When I built my garage (which has both an enclosed and potentially heated if I turn it on section) and a lean-to off the back which is open on 3 sides, I used DripShield by MetalSales. I haven't had any problems. Nothing additional to install. Just paid for it upfront and installed sheets as normal. I don't have trouble with condensation in the lean-to or the vented attic above the enclosed portion.

I'm in a northern/heating climate. We get humid days in summer with cool nights.

https://www.metalsales.us.com/wp-content/uploads/2020/01/Drip_Shield_Flyer_MetalSales.pdf

 

[PhysAssist]

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.

I beg to differ, our woodshed has never been even close to fully enclosed, but when we had snow on the roof, on a day where the air temps were warmer than freezing, the cold roof condensed moisture from the warmer air and it rained in the woof shed.

Conversely, when we had our pole barn constructed, we specified 1/2" insulation with a vapor barrier which was to be installed with the vapor barrier facing the inside of the barn. Initially, they did a terrible job of sealing the seams with tape, so we had to have them come back and remove/replace the damaged sheets and re-tape the seams.

Despite wild fluctuations in weather, including warm air temps and copious snow pack on the roof, we've never had any condensation issues since it was built in 12/2012.

Of course, the standard, YMMV disclaimer applies...

 

[PhysAssist]

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.

Please see my last post...

 

[Mike1955]

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.


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Well I have an idea although not a suggestion. What about this spray insulation kinda new on the market. I can’t think of the name but the installer cost shouldn’t be anything much. I know it be great just don’t know the price

 

[paulsharvey]

Well I have an idea although not a suggestion. What about this spray insulation kinda new on the market. I can’t think of the name but the installer cost shouldn’t be anything much. I know it be great just don’t know the price

Mobilization would be atleast $1500.