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I'm looking to Vent an enclosed rafter space where ceilings are applied directly to the underside of roof rafters. This situation occurs under a flat roof deck. How do I vent this condition? Do i need to provide ventilation at every rafter? How can i avoid doing that and get adequate ventilation???? hmmmm...

Thanks,

nick
 

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I think the easiest way to ventilate a flat roof is with a venting base sheet, but do not think it is warrantable on a residence.
 

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My understanding is that convection venting will not work under flat decks. I always believed that a slope of at least 1:12 was needed to get the air flowing. When I used to do flat work, the only time we used vented details was over lightweight concrete decks. If the building had high humidity, we mopped a vapor barrier to the deck, under the insulation.
Jim
 

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How long does a new lightwieht structural concrete deck have to cure before mopping to it?
 

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If I remember correctly, 7 days from pour for Zonolite decks. I think Perlite and foam are the same. Remember, we are not "venting" warm, moist air from inside the building, just the residual moisture from the concrete roof deck. These types of decks are usually on a vented (slotted) steel deck.
Jim
 
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