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Attic ventilation with metal roof installed over a shingle roof

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If a shingled roof is strapped covered w/corregated metal roofing, should 1st deck be ported to allow lost heat from the attic to be vented to ridge via the strapping between roofs?
asked in House Roof by rene-major (120 points)
edited by darekrudy
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1 Answer

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Hi Rene-Major,
If I understood correctly your description: you have a metal roof secured to wooden strips installed over an existing shingle roof. During installation any upper attic / roof vents were blocked or eliminated (assuming there were some vents) and currently there’s no attic ventilation.
You are asking if there should be any ports created in the decking surface so they improve attic ventilation by allowing air to escape from the attic - under the metal roof / between the straps – through the metal roof ridge vent, is this correct?
If so, I wouldn’t do that / make such ventilation ports in any areas below the ridge line. I don’t know your geographical (climate) location, attic condition, insulation, etc. but any openings in the decking surface material could allow potential condensation from the underside of your metal roof to leak into the attic. 
The ventilation should be direct, from the attic to exterior, which might be difficult to install (flashing). The only way this could work is if you can cut a ventilation channel along the ridge / ridge board. That way the attic air would be forced directly into the ridge vent. In case you have a gable / cross gable roof, you might be able to install a vent or vents on the side walls. 
answered by darekrudy (21,730 points)
So you're saying that for this reason, one should never find or install a metal roof [corregated or otherwise] with no waterproof deck underneath. Simple strapping over rafters will not suffice as there is no protection from possble condensation from the from a [poorly or even properly] vented metal roof?


No, I’m not saying that, there could be many scenarios; I just said that I wouldn’t make holes in your decking except for a slot along the ridge.
As I mentioned in my answer there are several condition that might cause / increase / decrease possibility of condensation on the underside of a metal roof panels (beginning from your climate). Once the metal roof panel’s temperature drops down below the dew point temperature for the relative humidity of the air underneath (either in your attic or between the decking and the panel) you will get condensation.
A shingle roof or waterproofed decking material installed underneath vented metal roof gives you extra protection in case of any condensation buildup. If this metal roof is already vented (has a space underneath allowing air passage between the bottom and the ridge) and you make holes here and there in the decking surface underneath, you’ll be disturbing that air flow. Depending on your climate, the attic’s air temperature, and humidity this could result in condensation buildup.
The decking acts like a buffer which gives you a little more room / allows for a less perfect attic ventilation (you still need it in unconditioned attics). 
Metal roofs can and are installed over the open rafters. In that case you have to make sure to maintain the attic’s temperature at the same level / as close as possible to the outside temperature. This is often difficult to achieve, especially in cold climates, and sometimes results in condensation buildup.