Another Metal Roofing Question

[Beltzington]

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.

 

[mbohuntr]

Well... I put a galvanized roof on an old dairy barn over 20 years ago. laid it over and directly on both mineral paper as well as on shake shingles. No sign of rust or corrosion....

 

[MAN IN BLACK]

I would do it the way they offer you to.
1x4 on 16" centre nailed or screwed in the farm trusses.
Then screw your thin to the strapping 1x4.
The metal roof needs to breathe...air movement between your sheating and thin...corrosion will form on the under side, it won't be visible from the surface until too late.
The way I do it here on a 32x210' barn , 60x40 shed and my house.

 

[Rustyiron]

I've allways put it on (ag type) buildings directly on the purlins. About 6 years ago I had a shed roof below a much larget roof and I used sheathing for concerns of the "upper" snow coming off might damage the lower shed roof that had barely a 3/12 pitch. Tar paper & screwrd it to the osb, So far so good, but any poblems would take a long time to appear. Fingers crossed. I like Man in black's (above) procedure. I think that is what's called a "cold roof" as it's fully vented directly under the panels.

 

[Beltzington]

I would do it the way they offer you to.
1x4 on 16" centre nailed or screwed in the farm trusses.
Then screw your thin to the strapping 1x4.
The metal roof needs to breathe...air movement between your sheating and thin...corrosion will form on the under side, it won't be visible from the surface until too late.
The way I do it here on a 32x210' barn , 60x40 shed and my house.

I will be doing this myself it is a small roof, however I have been using materials I plan on using building my house to see if they meet advertising claims.

Agree air circulation is a proven method of removing moisture however help me understand where the air will circulate to and from when I have installed a grid of enclosed 1x4" cells across the entire roof? The top and sides will also be enclosed by a ridge cap and flashing. I appreciate your feedback just don't see the science in this concept.

 

[Billrog]

I see no problem screwing right to the shting. the only place that will get air is under the ribs and they'll be vented from bottom to top. Ridge cap doesn't block the ridges from venting.

 

[Sawyer Rob]

Where "I" live, all the pro's (tin installers) put down the 1x4's and screw the tin to them...

All the guys doing it on the cheep or that don't do a LOT of it, just screw it down!

There's NO way I would install, or would I let ANYONE install metal without the 1x4's on a roof of mine...

SR

 

[Beltzington]

Where "I" live, all the pro's (tin installers) put down the 1x4's and screw the tin to them...

All the guys doing it on the cheep or that don't do a LOT of it, just screw it down!

There's NO way I would install, or would I let ANYONE install metal without the 1x4's on a roof of mine...

SR

But why do the "pros" do this, is it the best method or just means they don't waste anytime having to be concerned of hitting the rafters when attaching the sheets, thus saving money?

Also suspending the thin metal between firing strips can allow the metal to bend or deform as opposed to a nice flat roof screwed directly to the sheathing.

The cost of installing the 1x4" is too small to factor into my decision, only looking for the best construction method.

 

[buckeyefarmer]

Commercial here put down grace membrane then stand seam on top, no 1x4s.

 

[Sawyer Rob]

So the metal with breathe, THEY say, both the roof and metal will last longer... Good installers won't install it without the 1x4's...

Perhaps it's done differently where they don't have much winter...

SR

 

[MF283]

I will be doing this myself it is a small roof, however I have been using materials I plan on using building my house to see if they meet advertising claims.

Agree air circulation is a proven method of removing moisture however help me understand where the air will circulate to and from when I have installed a grid of enclosed 1x4" cells across the entire roof? The top and sides will also be enclosed by a ridge cap and flashing. I appreciate your feedback just don't see the science in this concept.

It will allow more air through the ribs being it's not directly on the sheeting. 2X6 ON BARNS AROUND HERE & 1X4 on OSB as other's have stated.

Ronnie

 

[LD1]

Dont worry about the metal spanning between the 1x4 strips. The ribs add strength and keep the metal flat. The metal spans 24" no problems and will support someone walking on it and remain flat. Thats how its done when attaching to purlins and and no OSB.

 

[PILOON]

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.

 

[gsganzer]

Go to Mueller's website and download one of their manuals. They show installation details.

Mueller Installation Manuals

 

[gsganzer]

Go to Mueller's website and download one of their manuals. They show installation details.

Mueller Installation Manuals

 

[Deere Dude]

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.

I would think that moisture buildup would contact and corraled by the 1x4s and they would rot. I also would want this bubble wrap under the tin no matter how it was installed.

 

[tallyho8]

I have an unconventional way of installing a metal roof that many may not agree with, but it works for me and it works well.

My shingle roof was damaged during hurricane Katrina so I installed a metal roof over it. As 1X4s aren't strong enough to hold the metal down during severe windstorms, I used treated 2X4s purlins at 24" spacing screwed down with screws rated for treated wood and made sure the screws went into the rafters. I nailed 3/4" foam insulation sheets to the roof between the purlins and this left a 3/4" air space. I cut strips of felt paper which I stapled to the 2X4s so the metal would not touch the treated wood.

I screwed the panels down on the flats and I did not use the foam filler strips that they use on low pitch roofs so that I would have good air flow under the metal. An advantage to this method is that my attic is now at least 10 degrees cooler in the summer and the roof is so well insulated that you can not hear rain falling on the metal roof.

My neighbor had a contractor install his metal roof using 1X4s and the next hurricane 2 years later blew the metal off one side of his roof breaking all the 1X4s that were holding it down or pulling out the short screws that went through the 1X4s. My roof was not damaged at all as the 2X4s were strong enough to not break and the longer screws did not pull out. Attached is a photo of his home after the hurricane.

 

[deezler]

Dang, I'm not real thrilled to jump in here and see such disagreement. I am preparing to build my pole barn this year (hopefully) and wanted to put metal over sheathing to A) strengthen the roof assembly, B) insulate a bit for noise and temperature, C) prevent condensation dripping inside the barn, and D) make insulating the barn roof interior a bit more straightforward.

However now I don't know how to do it. drat! haha.

Question: Why is there moisture under the metal in the first place? Condensation? screw holes in the metal panels? other leaks?

 

[KennyG]

deezler, my opinion (which is worth the pixels invested here) is to forget the sheathing on a pole barn. If you think you need more strength, use 4 foot spacing on the trusses and cross brace them heavily under the top members. Install a ceiling under the trusses and you will have real insulation capability. Put on 26 ga. quality roofing per the manufacturers instructions and forget about it for the next 50 years.

 

[LD1]

Agree. Forget the sheething under the metal.

Its condensation that is what causes the issues. Look at ANYTHING metal outside. Look at your car and car windows on certain mornings. thats what the underside of the metal does.

Using osb to stop it from dripping on you....the moisture is still going to be there, only instead of dripping on you, it is saturating the OSB and it will fail sooner rather than later.

Leave the underside open with adequate ventilation, or use a vapor barrier.

As far as insulating, its best to insulate along the bottom chord of the truss as opposed to along the roof line. Which also keeps a smaller space to heat/cool as you arent climate controlling the "attic"