Join the MRA | Member Login
Find a Professional
Sign Up Log In Add New Topic

TOPIC: Installation

Metal roof over warped roof deck

Return to Installation

Return to Forum

I have a newly constructed house with closed cell spray foam applied to the underside of the OSB roof deck. My foam installer sprayed the foam to thick and it has pulled the OSB down between the trusses causing a wavy pattern. My question is can I remove my shingles and put a standing seam metal roof over this with purlins. If so, what size. I want to limit the additional height as much as possible. I am trying not to OSB over the whole roof or tear out the additional OSB and spray foam insulation. Also, how would I or do I need to create some sort of venting between the purlins. It is not warped evenly so not all areas are pulled down. Any other advice would be greatly appreciated. Thanks Kevin
Kevin Mauer

8/28/2013
This sounds like a reasonable approach. I'm not sure that removing the old shingles would be essential. The spacing and size of the purlins, along with fastening pattern, will come from your metal roofing manufacturer. This type of application is usually done without any vertically oriented ventilation. Todd Miller
Todd Miller
Isaiah Industries, Inc.

8/28/2013
you may want to consider one of the horizontal oriented panels that require a wood batten or has a built in batten. Many of the stone or granule coated panels require battens. Installing 1 x 2's directly over the trusses will allow over additional ventilation under the metal roof. Metro, Gerard, Decra and ATAS are at least four companies that manufacture these panels. There maybe others that I am not aware of. Dick Bus
Dick Bus

8/28/2013
So if I understand correctly I do not need to worry about the dead air space under the metal roof created by the 1x4 solid purlins? I had one installer come out and was concerned and thought I would need an structural architect approve that type of installation since the air pockets between the purlins would not be vented. The metal roof design I prefer the looks of is standing seam.
Kevin Mauer

8/28/2013
Kevin, many many installations are done as you plan. Am I correct in understanding that you now have a conditioned and non-vented attic? Folks might be pointing to the need for ventilation based upon model building codes. However, again, what you're proposing is done on a regular basis. Is there more to your roof story that I may be missing? I see you're in Michigan. How are you as far as ice damming on the roof? Has that been an issue? Todd Miller
Todd Miller
Isaiah Industries, Inc.

8/28/2013
I do not think you need a structural engineer. Due to the light weight nature of metal roofing you should be ok. There are a lot of metal roofs that are installed direct to deck and do not have problems. However, there are studies that show that above sheathing ventilation will reduce the energy cost of a home. Take a look at this presentation. http://www.coolmetalroofing.org/elements/uploads/files/Above-Sheathing-Ventilation.PDF Dick Bus
Dick Bus

8/28/2013
The roof deck is sprayed with 5" of closed cell foam and is not vented. Early on we did have some ice issues but I had the foam installer come out and re-spray the junction between the ICF walls and the roof deck so hopefully that addressed the problem. Right now I have an insurance company who is proposing pulling the roof deck off including the OSB and foam and then re-sheeting with OSB spraying new foam and shingling the roof. They are covering the cost. I thought the metal roof was a better option until the installer warned against it. He does asphalt shingles as well so there might have been a bias. The insurance company is open to other options as well. I was hoping this option would be protective against any future ice build ups as well
Kevin Mauer

8/28/2013
It looks like you would shoot for a 3/4" gap between the purlins and the OSB. This would require a 1x4 running along the trusses and another perpendicular to that to secure the roofing to this adding 1 1/2" to the height of the roof. Am I reading that correctly?
Kevin Mauer

8/28/2013
I feel that the insulation is probably very helpful as far as avoiding ice dams. If you wanted you could run vertical battens and then horizontal. By cross-battening you will pick up some airflow which definitely will help with ice damming and also will help with summer time heat gain, as Dick pointed out. Todd Miller
Todd Miller
Isaiah Industries, Inc.

8/28/2013
How would this additional air flow impact the ability to withstand sheet winds, not uncommon in MI
Kevin Mauer

8/28/2013
You'd want to create a detail so that the air is coming in your soffits and then of course exhausting at the ridge. As long as your intake and exhaust vents are well balanced in terms of net free air flow, this should be wind resistant, Todd Miller
Todd Miller
Isaiah Industries, Inc.

8/28/2013
This is unrelated to the purling and above deck venting question, but did they apply the 5" in one lift of the CC SPF? One other possible design consideration would be the thermal bridging of the roof structure. It is obvious by the material choices (SPF, ICF, etc) that you have built and high efficiency home. While it certainly add to the expense, some rigid foam to the outside surface with the integrated above deck venting that the purlins and strapping creating would go a long way to creating a bulletproof and supremely efficient roof system. http://www.danperkinsroof.com/venting-and-insulation/ Eric Novotny
Eric Novotny
An informed customer is the Best Customer!

8/29/2013
The intitial spray was too thick. It ranged from an inch to almost 4" all in one lift. He later came back and resprayed to bring the average to 4". He also didn't cap the top ridge and foam extruded between the OSB boards at the peak and the contracted and pulled down the OSB at the top. It was also pulled down in the length of the trusses do to the contraction of the thick foam crushing the larger internal cells thus pulling the OSB down. Your Dan Perkins design looks interesting but I think it is overkill for what I am looking for and adds about 6 1/2" to the height of my roof. According to the cool roof link above I only need 3/4" gap to get the benefits of a vented roof. This would only require running 2 1x2 perpendicular and on top of each other which would add only 1 1/2". The additional labor and materials that would be required for Dans design would probably not have a reasonable payback for my situation since I already have a substantial amount of closed cell foam attached to the underside of the roof deck.
Guest User

8/29/2013

If you would like to reply to this thread, please log in. If you do not have an Ask the Experts forum user account, create one here.