Another Metal Roofing Question

[EddieWalker]

I am amazed, not really, on the disagreement amongst installers on the best way to install metal roofing panels on top of sheathing. So what does TBN folks have to say. Existing construction not changing so not allot of value in this thread to discuss why it is better to not use sheathing under metal roofs in the first place.

1/2" ZIP OSB sheathing, metal panel approved 1st layer moisture barrier.

I was going to just screw the panels to the roof, hitting the 24" OC rafters if needed, however this method seems to cause a great amount of gas buildup if you google on the internet. Their preferred solution is to screw 1x4" strips to the sheathing to keep the metal off the roof which they believe will prevent moisture damage from condensation. Thinking this through, a problem of mine, any condensation will still end up on the sheathing so I do not see how this is a solution any better or worse than screwing it directly to the sheathing. Actual experience greatly appreciated, opinions will be tolerated.

First you have to decide why you are using metal for the roof. The most common reason is that it's the cheapest roof you can create when using purlins across trusses or rafters. It is fast, easy and it will last a very long time. If you are just wanting a roof over something to keep the rain off of it, then this is the best way to go based on price.

If you want a metal roof over a house, it's very common to use decking under it. It's not required, but it does give you that extra amount of strength, and another barrier to keep water out. The decking has to be either ZIP system with tape, or you have to put felt paper over it. Moisture will form under the metal and it needs a way to either drain out, or just sit until the air flow dries it out. I had a job once where I added a metal roof over a deck that I built and attacked it so that it went under the existing metal on the roof. Water flowed from the roof to the deck like it was supposed to, but every morning, and after a rain, water would drip out under the roof where the deck started. It took some time with a hose on the roof, but I figured out the gaskets on the screws where leaking and allowing water under the metal that flowed down over the top of the felt paper on top of the decking.

I'm curious what you mean about gas build up? There should never be any type of gas in an attic, or anywhere in a house that isn't properly contained.

 

[Rustyiron]

Eddie, what came to my mind :confused2: was that he was talking about (maybe) the sheathing off gassing or some type of gas created by having the moisture reacting with the osb in the confined space ??

 

[Beltzington]

You folks need to get out more the gas I was referring to is the verbal flatulent expelled by the internet police when someone suggests a method or process they are not familiar with. Again with the millions of metal roofs being installed the differences in techniques and opinions are staggering.

Much of this thread has covered the same areas bought up on other sites, I did just find a metal roofing forum that provides much more specific reasons when and why.
To oversimplify, as Eddie mentioned metal roofs on sheds and barns typical applied on purlins with no sheathing for greater cost savings. Residential construction use sheathing for a variety of reasons, with or without purlins (manufacturing installation instructions support this, thanks GSGANZER.)

Condensation can be prevented, as many of you suggested, by keeping warm air off of cold metal either by air circulation or insulating and sealing.

So I am going to do one half my roof with purlins and half without and report back in 10 years.
Thanks for all the advice!

Metal Roofing Installation & Materials ? Ask-The- Experts ? MetalRoofing.com Page 1

 

[Beltzington]

I did a demolition of a tin roof* that had been laid directly on plywood.
Roof was about 8 years old.
That roofing was so rusted from condensation that it had started to develop pin holes in some areas and the plywood underlay was useless as well being advanced rotting.
*This was a galvanized metal.

My wood storage shed is galvanized as well and in the spring you need to wear a raincoat as the condensation simply drips off like a rain storm. That is due to cold snow on the roof and warm humid spring air. However since the shed roof is open air it drys and not rust.

Metal in direct contact with wood simply sits in damp wet evvironment and rusts away.

PILOON - what was the pitch of this roof? Is it possible this was caused by standing water (leaks) not condensation?

 

[PILOON]

PILOON - what was the pitch of this roof? Is it possible this was caused by standing water (leaks) not condensation?

While not steep, the pitch was not the problem IMO. (about 3/12 I'd guess), possibly shade did not help but I doubt it.)
It was a straight run with about 12 ft per side and trees sheltered any side driven rain. All fasteners were on top of the ridges so leakage would not be an issue.
(I have done my fair share of tin roofs so while not world's expert I have some experience.)

My biggest project was 2 60 X 200 farm structures that stand proudly 20 years later with no problems. Roof sheets were , like 32 ft long per side and all screwed with , believe it or not, 9 volt battery drills. We pre drilled all sheets on the ground (5 at a time) and used vice grips and rope to haul the sheets into position. Helpers helped from buckling the sheets.
Pre drilling (using a template) resulted in the prettiest screw pattern but made attachment a breeze while up there at frigid winter temps.