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Structure Supporting Metal Roof for a 2/12 Shed Roof
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We are having a bit of a debate amongst our contractor, our architect and the company that supplied the timber frame and building materials package. The question is what is the proper support for the standing metal seam roof that will be installed. The house is of a rectilinear design - about 80 ft long - with a 2 on 12 pitched shed roof, which begins on one long length of the rectangle and rises to the other long length. There is a 2x4 tongue and grove douglas fir ceiling deck, with tar paper on top and a 10 inch structural insulation panel (10 inch of foam and OSB) - tar paper is also on top of the SIP. The house is in northern Pennsylvania. The architect's specs called for 2x4 cross-strapping on top of which the metal would be installed. (I interpret cross-strapping to be one direction and the another direction at right angles.) The timber frame/materials package supplier only supplied 2x4 strapping that would "cross" the long width of the house on which the metal would be attached. There would be no way to vent from the low end soffit up to the highest point of the roof. Instead, they claim that the resulting horizontal channels(left to right versus bottom to top) would adequately vent the roof of this long house sufficient to claim a cold roof design. Any heat and moisture that might accumulate in the airspace would be swept out by the wind. The builder does not believe that "horizontal venting" is practical and would prefer to run the 2x4 strapping from low end to the high of the roof and then install OSB on top and then install the roof. I just don't believe the timber frame/materials package supplier. It goes against elementary heat transfer mechanics, as I remember them from school. Also, the OSB is probably not a good choice because it does not hold screws as well as plywood. It seems as though the architect's specs (to which the two other parties agreed) is the right approach from what I have been able to piece together from your web site and various manufacturers' web sites. What is your take on our design and the arguements of the timber frame/building materials supplier? What would you recommend? Also, do we need a vapor barrier as part of this roof system? I would greatly appreciate your input. Kind regards, Peter
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First of all, I would suggest a vapor barrier directly behind the T & G ceiling. Next, I agree that the idea of venting horizontally is not practical and does not work well. Now, that is what "not" to do ... to find out what you "should" do, you need to check with the manufacturer of your roofing. My first flag is this -- is the roofing suitable for a 2:12 pitch? That is pretty low. Now, if you put down vertical strapping, you then have the option of either solid decking and underlayment or horizontal strapping before you apply the roofing. That question must be answered by the roofing manufacturer based upon how the particular metal panel you have was designed and tested to be installed.
Isaiah Industries, Inc.
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