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As an Architect, I worked on two single family home projects years ago here in Minnesota. We used two different contractors and two different roofers, all very highly respected in this area. Both houses had flat roofs as the Architect I was working for at the time, Ralph Rapson, always wanted flat roofs. One roof was filled with 12" of fiber glass insulation in the 2x12 ceiling joist spaces, then stripped in both directions with wood nailers, then covered with a plywood deck and a BUR. The finished ceiling was sheet rock over a standard 6 mil poly vapor barrier. Metal wind-powered vents were installed in the roof deck to allow the entire roof area to be vented to the exterior.
The second house had the same 2x12" ceiling joists stuffed full of fiberglass insulation, with the 3/4" t&g plywood decking installed directly on top of the joists so there was absolutely no air space above the insulation. A BUR was installed as well. The interior of the exterior walls and ceiling was wrapped with two layers of 6 mil poly which was carefully taped everywhere. The second roof system was not vented, intentionally, in any way whatsoever.
The first roofer said that, of course, a roof has to be vented, especially a flat one here in Minnesota. The second roofer said that with his roof system (and the extra 6 mil poly vapor barrier very carefully installed by professionals), the dew point would be in the plys of the exterior grade plywood roof decking (because there was no dead air space above the insulation and open to the outside ambient temperatures) and, because moisture could not penetrate that decking, there was never any condensation.
Does anyone want to guess which roof failed, partially collapsed and had to be completed replaced?
Of course, it was the first roof, the one that had been vented. There was so much condensation held in the insulation, and above the vapor barrier, that it rotted out the joists; first the sheet rock dropped and then the joists gave way. The solution was to remove the entire roof and framing, re-frame, and foam the entire roof cavity full with no venting anywhere.
Since then I have been vary wary of vented roofs here in Minnesota where the outside air can get down to -30 and drive the dew point into the insulation if there is a vented air space open to the exterior. The logic of the second roofer seems to make lots of sense and his roof is fine after 30 years.
I mention these examples as I contemplate installing a steel roof system (my first)on my unheated (except for a few days in the winter when we will use a wood burning firebox) cabin in Northern Minnesota. I have 3/4" 1"x12" roof boards/decking as the exposed ceilings at an 7/12 pitch over 2x4 rafters with a king-post truss design. I am planning on installing 30 lb felt, then 2x4's running with the rake, eave to ridge, spaced every 4' with 4x8 sheets of rigid foam in between. Then I was going to lay down another set of 2x4's running parallel to the eave, spaced 4' oc, with 4x8 sheets of rigid foam in between. Then I was going to nail down the exposed-fastener steel roofing, using my horizontal 2x4's as nailers for the roofing. Does that sound like a good plan? I don't know how often the steel needs to be nailed in each direction and have seen no directions on other web sites yet. Thanks for ideas and sugestions and comments.
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Best approach I have seen used when it comes to insulated roof deck retrofits is well illustrated in this roofing pdf.
Take a look as it describes a very similar approach to your situation.
http://danperkinsroof.com/1108_JCL_Perkins_A.pdf
Eric Novotny An informed customer is the Best Customer!
8/10/2010
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Many thanks for the suggestion. What Dan is doing is just about the same thing I was going to try; good to see someone else doing it before I try it.
I am concerned about condensation on my roof (I was not planning on building a cold roof for my unheated cabin) because if the eave 2x4 nailers are set horizontally, and if water gets under the metal roof but above the deck, that nailer could act as a dam and accumulated moisture could cause it to rot.
Isaiah Industries, Inc.
10/10/2009
Or copy this URL: https://www.metalroofing.com/spirit/comment/4758/find/8/10/2010
Or copy this URL: https://www.metalroofing.com/spirit/comment/4759/find/An informed customer is the Best Customer!
8/10/2010
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Or copy this URL: https://www.metalroofing.com/spirit/comment/4761/find/An informed customer is the Best Customer!
8/14/2010
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