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TOPIC: Roof Pitch

New construction

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Hi - I've read through most of the messages and I want to make sure I've got this right. Our new home is all 10/12 pitch. All cathedral rafters are 2"x10" with rolled R-30 between each. Metal will be vetical ribbed galvenized with external screws but brand is not yet known. Planned assembly from inside-out is: sheetrock, rafter and R-30, 1/2" OSB decking, 15 or 30 weight tarpaper, 2"x4" strapping, and the metal. We're in the hot, humid south so cooling and condensation are my big concerns. Does this assembly sound correct? Shouldn't the space between the decking and sheetrock be ventilated with soffit and ridge vents? Also - what is the difference between an air barrier and a moister barrier? One more - Do I benefit from adding ridged insulation sheets between the decking and the metal or could that cause condensation problems? Thanks
Guest User

5/20/2002
You are very correct in taking a close look at this issue. And, please keep in mind that this is not a "metal roof" issue. Regardless of what type of roof you use, building codes dictate an airspace in cathedral ceiling systems and, in your case, the appropriate placement is beneath the OSB. I believe code requires 1.5 inches but I'd try to get at least 2 inches or more. This airspace should be ventilated utilizing, preferably, soffit and ridge vent combination. This airspaces guards against moisture buildup (caused by moisture created inside the home)and will also help reduce your cooling loads. Based upon your construction, I do not see the need for rigid insulation beneath the metal roofing. It absolutely would not eliminate the need for ventilation. Furthermore, if you were to do such a thing, you would need to check with the metal roofing manufacturer. There are varying opinions between metal roof systems and manufacturers as to the appropriateness / usefulness of such insulation. Generally, though, I do not feel it would serve any benefit. Todd Miller
Todd Miller
Isaiah Industries, Inc.

5/20/2002
we are also building a new home with a gable roof. we are still looking at various metal roofs, however it appears that the standing seem is what we'll end up with. our major concern is sound proofing or at least adding some type of additional underlayment as the master bedroom is the top level. also the city building codes require class A for fire code. any thoughts.
Guest User

8/8/2002
Most consumers are very surprised by the quietness of metal roofs. Many metal roof styles have built-in airspaces and even optional insulation boards which can make them even quieter. You'll need to see what is required for a Class A under whatever building code is in place in your area. In some cases, you might need a special underlayment or other material. One material which is often used to get a Class A is a product called Dens-Deck. I believe it is made by Georgia-Pacific. It would have some sound-insulating qualities as well. Todd Miller
Todd Miller
Isaiah Industries, Inc.

8/11/2002