Cedar Breather under standing seam roof
Has anyone ever heard of, or actually done, this type of installation? We’re building a timber frame enclosed with panels (uerathane with OSB ext. skin) and have been advised to put the cedar breather between the 30# felt or ice and water shield and the metal itself. This is vented at soffits and ridge. Supposedly this will prevent condensation. It seems to me that the condensation will be worse if there’s an air space, as the temp. of the roof and the temp. of the metal would be the same if one is in direct contact with the other. With the airspace, I’d think the roof would be cooler, and the moisture content of the air would be about the same as ambient.
Am I being stupid, or is this one of those totally counterintuitive things, or is this a CYA move on the part of the roofer?
Replies
On our timber frame with SIP roof panels - OSB urethane OSB we used 30 pound felt and then 1 x 4 southern yellow pine "nailer" on top of the felt. The metal roof was then screwed to the yellow pine nailers which are set on 24 inch centers. The roof space "vents" via the ribs to the ridge which has a vent type (local fabrication) ridge cap. It has been in place only two years but we have had no problems. Roof is a 8 / 12. Our roofer felt very strongly about using the yellow pine nailers because they did not like the screw holdling ablity of the OSB -- not because of venting or seperation of the metal from the surface. They used 1 1/2 inch screws applied on the flat that go through both the yellow pine and the OSB. I would think that cedar would be way too soft to have much screw holding power. It terms of water damage -- between the pine and the cedar -- you got problems if you have much water under your metal roof anyway -- but if you are really worried about this you can get treated yellow pine nailers -- at least here in western NC. At this point I am very happy with this roof.
Deblacksmith
Cedar breather is a membrane product about 1/4" thick that promotes air flow. I think it's made by the same company that makes "house slicker" house wrap.
We've built several outbuildings and put the metal agri roof over 2 X 4 purlins.
What holds the yellow pine nailers to the OSB?
Bing, I missed the fact that the cedar breather was a material -- not wood cedar nailers.
The yellow pine is attached to the OSB with galvanized ring shank nails. Of course the screws for the roof also go through both the yellow pine and the OSB. I think the screw holding here that is important is that the yellow pine holds "each" screw well -- and that the nailer is held by multiable screws and the ring shank nails. Our roofer here -- and they do a lot of metal roofs really likes using the yellow pine nailers.
Deblacksmith
Metal is no different than wood - if it is in continuous contact with moisture, it will corrode just as wood will rot. This is true for all types of coated sheet products but particularly so for coated steel material. Installers are warned not to stack roof panels so that moisture can be trapped - "white rust" can form very quickly on galvanize and galvalume prior to installation. Hence installing metal roof panels on nailers allows for air flow to prevent premature corrosion.
Likewise for cedar shake roofing - it should be installed on nailers to allow for air flow. Unfortunately, too many roofers just lay them down on felt and walk along shooting them in resulting in a roof destined for premature failure.
Just guessing here, but I imagine the need for air space would be different with a vent-rib type exposed fastener roof, and a true standing seam roof that the poster mentioned. the whole standing seam roof will lay flat to the underlayment, the exposed fastener roof wil likely have ribs every 10-12" which serve to vent it already. Am I off base?
When in doubltr contact the manufacturer for thier detail.
Being a roofer in the great white north (Michigan)doing both risidenital and commercial I yet to see a metal roof system rot from the inside out, most premanufactored panels have or should have a kynar coating applied to the the material before it is coated.Sounds kike your roofer is trying to bring the cost of the job up by using a high end material which by the way was intened to used in that appilacation, its intened use is to promote drying of cedar shakes from the bottom side not to prevent condisation.even if the metal roof system is applied on battens at some time in the the year the dew point will form on the bottom side.I would agree with the other posters and contact the manufactor and install it to their specs if you them to stand behind their product
You definitely don't need any Cedar breather.
What everyone in this discussion has been talking about is the roofing.
That has nothing to do with the moisture transmission through the underlying material.
The SIP roof structure is a vapour bar in and of itself. _IF_ it is installed correctly with al seams sealed, there is no way that any moisture is going to migrate through it to get anywhere near the backside of either a standing seam, a ribbed metal, or a shake roof. The only moisture the back side of that material will ever see is what leaks beacause of poor roofing so if he needs it for a CYA, look for another roofer.
But even if he is a good roofer, he apparantly knows nothing about the other systems of the house. He needs to focus on keeping the water out.
Excellence is its own reward!
Thanks for the input, everyone. I am trying to keep the budget from exploding here, and the cedar breather seemed to be overkill. Not that I'm against overkill when the lifespan or quality can be enhanced (hence my avid reading of Fine Homebuilding), but when it's unnecessary....