Metal roof construction option?

Swamphunter · February 01, 2012, 01:29:32 AM

This is the style of cabin that we will be building in the spring. It will be 20' wide by 24' deep. I have a couple of questions about construction of the cabin and my first one is about the roof. The cabin will be insulated as I live in the very northern part of NY State, so with that being said here is my question.

I've seen some metal roof construction where they put plywood sheathing down prior to putting the metal on, and I've seen where they do NOT put plywood down first. What are the advantages and disadvantages to both?

alex trent · February 01, 2012, 01:41:35 PM

Noise is something to consider. Rain on a tin roof is cool...until the 4th hour (or the 50th time) it happens. Heavy rain can be deafening.

Lost of tin here. Several ways to deaden sound.

Styrofoam below the tin and then Plycem or plywood or boards below that....makes a big difference..

Bottom line..plywood more expensive but quieter and the foamcore is cheap so that adds more.

Likely get some good insulation value too.

That is a neat looking cabin to model after.

February 01, 2012, 02:17:02 PM

We sheathed before having the metal roof installed, partly because we knew there'd be a delay of weeks/months before getting it on. One option we considered, as suggested by a foam insulation installer, was to do just the metal and then spray foam underneath. Of course, what you save in plywood is more than made up for in insulation costs.

Alan Gage · February 01, 2012, 02:32:58 PM

Plywood adds a lot of strength. Some steel is rated to be installed over roofs with no sheathing or bracing. Some isn't.

Alan

MountainDon · February 01, 2012, 02:42:02 PM

I am a believer of sheathing the rafters with OSB or plywood before any finish roofing goes on. The OSB or plywood sheathing gives the roof structure great strength against racking from wind caused movement. It goes down more quickly than metal roofing itself and with the weather resistive layer (tar paper or whatever...) it makes the roof dried in sooner, IMO. Mainly I like the strength part. I believe this becomes more and more important as the roof pitch rises; steep pitches act more like a wall when strong winds blow against them.

When metal is applied over sheathing there is no problem with moisture condensing on the underside of the metal. metal roofing over horizontal purlins will have moisture condensation and dripping. There are ways around that. Spray on foam of sufficient thickness is a good method and in some jurisdictions is required to pass inspection.

As Alan mentioned, not all metal panels are rated as structural so in that case they must be installed over a properly sheathed structure for strength. Some folks have not followed that and as far as we know they've not had any issues. Not to say something could not happen further down the road.

Noise was mentioned. Our roofs are metal over sheathing and they are silent inside. I would not like trying to get to sleep with the noise of rain on metal. Not soothing to me. Others may disagree and that's fine.

I have trouble seeing advantages to not using OSB or plywood sheathing, but I guess maybe that's my bias showing.

February 01, 2012, 04:59:11 PM

I'm an advocate for sheathing under the tin, for the reasons mentioned above. I've been in tin-roof barns where it practically rained under certain weather conditions. If you don't sheathe, you almost have to spray foam on the under side of the tin to control condensation.

Then the argument progresses to laying the tin directly on the sheathing and vapor barrier, or put it on purlins. I was always told to 'double strap', putting furring strips parallel to the rafters, then crossing them sideways with purlins to fasten the tin to. This gives the condensate somewhere to go, and a means to drain. It's a lot of extra work, and I've never seen a definitive answer on whether it actually helps anything. It does make it easy to scamper around on the roof, though, with a built-in 'ladder'.

mnboatman · February 01, 2012, 05:06:24 PM

I did the:

Rafter,
OSB
Paper,
Steel

Method on my shed. It worked better for the intermittent construction schedule I had.

Swamphunter · February 01, 2012, 06:31:59 PM

OK... OSB it is.... I was leaning that way, but wanted to check. It may be a year or so before I get the roof insulated, and I definitely don't want condensation dripping inside. So, with heat inside, minus insulated ceiling, I should be ok with just OSB with paper then metal?

Thanks all!

NM_Shooter · February 02, 2012, 10:27:01 AM

Are you going to have an insulated roof?

My neighbor made a snowmobile trip to our property last year, and stayed in his lightly insulated cabin. The cabin had been cold soaked for awhile, and the temperature was 5 degrees. When he woke up, there was frost all over the inside of his cabin, especially the roof. You'd be surprised how much moisture you make just by breathing.

February 02, 2012, 12:41:32 PM

Quote from: NM_Shooter on February 02, 2012, 10:27:01 AM
Are you going to have an insulated roof?

Yes the roof will be insulated... eventually. Because the cabin is a pay as I go project, other than the initial build, the interior will be completed either late this year, or over the course of the following summer.

Alan Gage · February 02, 2012, 05:06:51 PM

Cathedral or flat ceiling?

Alan

Swamphunter · February 03, 2012, 06:20:02 AM

Quote from: Alan Gage on February 02, 2012, 05:06:51 PM
Cathedral or flat ceiling?

Alan

Cathedral

Yankeesouth · February 03, 2012, 12:10:16 PM

"I did the:

•Rafter,
OSB
Paper,
Steel
"

When I do my roof this is the process I am going to follow. I second the option. To me it just makes sense. BTW.....OSB not plywood....$$$$$

RIjake · February 03, 2012, 01:43:08 PM

http://www.rooftopguard.com/en/home.html

This is the product that I used under my steel roof and over the OSB. A 5'x200' roll cost me $152.00. It was great to work with and on and is totally waterproof.

Just my $.02.

February 04, 2012, 01:55:03 PM

I used something very similar on mine. I was going to go with tar paper since I figured I'd have the roofing up in no time. Guy at our building center talked me into synthetic. Said he recommends it with steel roofs mainly because it as a longer life span than tar paper (to match the metals life span). It was still pretty cheap stuff so it seemed like a good idea.

Then the delays started. It probably took me 3 weeks from the point I put down the synthetic underlayment until I started putting up roofing. In the mean time we had some good rains and wind. The house stayed dry and the underlayment held up fine (rated for 6 months exposure). I had some pieces of tar paper up in a couple areas covering seams and after only a few days in the sun they started to get brittle and tore ever time the wind blew hard.

Plus the synthetic comes in 5' rolls so it goes up quicker. It's lighter too.

Alan

MushCreek · February 05, 2012, 09:13:40 AM

Another (more expensive) option is Zip sheathing. The same company makes Advantech subfloor, and I'm impressed with it. My barn loft, which as it turns out is waterproof, held standing water for over a week, and it didn't hurt the Advantech at all. Zip sheathing is water proof, and properly sealed with tape, requires no other barrier. Since I'll be working alone (slowly) the Zip sheathing will prevent me from worrying during the inevitable rains.