Metal roofing choice help

Jim Hauenstein
4/10/2014
I'm getting ready to re-roof an existing shingled roof (1-layer only) on my sun room addition with a new metal roof and I plan to do the job myself. The roof is a non-vented cathedral ceiling design with a 3/12 pitch. The roof layout is just a simple single slope design that is attached to a vertical wall of the the main house. I'm trying to decide between a snap-lock standing seam or a 3' wide raised rib panels with exposed fasteners. Another question is whether to install the metal roof directly over the shingles or to use a 1x4 or 2x4 batten system under the metal roof and try to vent the new metal roof above the old roof surface. My cathedral style roof system is not well insulated and my sun room is cold in the winter and hot in the summer, so I'm hoping to improve this issue with this new metal roof.
Todd Miller
Isaiah Industries, Inc.
4/10/2014
It sounds like a good plan. Thanks for thinking metallically! :-) A couple of thoughts ... make sure obviously that the panel you choose is suitable for the roof pitch. Next ... metal roofs that have a lot of direct contact with the surface beneath them ( as compared to an airgap formed by strapping or by the formation of the metal itself) can drop the temperature of the roof deck enough to sometimes create condensation issues on the underside which did not exist before. I am concerned that your situation could lead to that.
Jim Hauenstein
4/10/2014
"metal roofs that have a lot of direct contact with the surface beneath them ( as compared to an airgap formed by strapping or by the formation of the metal itself) can drop the temperature of the roof deck enough to sometimes create condensation issues on the underside which did not exist before. I am concerned that your situation could lead to that." Funny you should mention that, I actually need to determine whether the condensation issue you wrote about already exists in my Cathedral ceiling system. I recently read an article on the proper ways to insulate a cathedral ceiling and found that the way I did it was incorrect, since it can lead to the vary condensation issue you wrote about. I currently have 2x6 rafters with R19 kraft faced fiberglass insulation covered by 1/2" drywall on the underside of my roof deck. After I complete my re-roofing project I plan to tare down my ceiling and see if condensation was occurred on the under side of my roof decking. I plan to install ~ 3" of closed cell spray foam onto the underside of the decking to totally air seal the roof area and then added fiberglass insulation just beneath the foam and then add a 1" foam board to under side of the rafters before re-drywalling the ceiling. This should give me about an R-40 once done, but the sun-room will be much warmer. So you are recommending that I install strapping on top of my shingled roof prior to installing my metal roof, do I need to lay down any kind of tar paper or roof membrane under the strapping? My roof panels will only be about 14' long at the most, so I'm thinking I should be able to go with a metal roof product that uses exposed fasteners without any metal expansion issues. Correct?
Todd Miller
Isaiah Industries, Inc.
4/10/2014
Thanks for your response. Yes, I was kind of reading between the lines and figuring that condensation might already be a concern. I like the foam idea. In fact, by doing that, you will probably be okay without putting the roofing up on strapping. The fasteners and the holes are obviously the weak spots of any through-fastened roofing system. However, you are in a situation where it shouldn't post a major risk. My interpretation of the International Building Code (*not all people interpret it the same way) calls for underlayment in this situatiuoin before the strapping or roofing are installed.
Jim Hauenstein
4/11/2014
I contacted my roofing supply store to inquire about some standing seam metal roofing quotes and during our discussion I asked about the use of strapping above my roof to support the standing seam roofing. He told me the 26ga standing seam roofing I was considering needed to be fully supported by the roof deck and not to use the strapping method. He did say some installers used horizontal 3/4" thick strapping and filled the voids between the strapping with 3/4" foam board to give the metal panels full support. This approach eliminated the vented metal roof design I was after with the strapping method. As you may recall from yesterdays posts, I was considering pulling down the underside ceiling and fiberglass insulation and then spray the underside of the roof deck with a closed cell spray foam and that still may happen, but that makes the project a lot more difficult and more costly. I'm now wondering about trying to tackle this under insulation issue from the top side while installing the new metal roof. My idea is to remove the old roofing and then cover the roof deck with a synthetic membrane/underlayment. Then install 1.5" polyiso foamboard insulation followed by a second layer of Nailable polyiso insulation board to make up the new roof deck surface for the standing seam metal roofing. The nailable polyiso insulation board is also sold in a venting configuration if I want to vent the upper roof system. Would I also need to cover the top nailable polyiso insulation board with a synthetic membrane/underlayment before installing the metal roofing? Do I really need to worry about condensation issues and upper roof venting below the metal roofing if the metal roof is separated from original wood roof decking by 3" of foam insulation and underlayment layer?
Todd Miller
Isaiah Industries, Inc.
4/11/2014
That should relieve the condensation concerns. I would encourage you though to reach out to the company manufacturing your roofing and make sure that what you're doing is okay with them and in keeping with testing and approvals which they have on their product.

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