Metal roof over sagging decking

Guest User
7/6/2017
We are wanting to put metal roofing over our existing shingled roof the problem I am seeing with our existing roof is that there are a few places where the decking sags up to 1 1/4" between the rafters, I was thinking about running purlins across the roof to attach the metal roofing to. I am also wondering should I put the underlayment under the purlins or on top of them?
Todd Miller
Isaiah Industries, Inc.
7/6/2017
In this case, the underlayment would go beneath the purlins. Yes, what you're proposing can be done but not all metal roofs can be installed over purlins. You would need to choose a product that can be installed on purlins. The purlin spacing would then be specified by the manufacturer. I hope this helps.
John Cervenka
7/6/2017
That's what I was thinking that it had too, the metal we have chosen can be installed on purlins @10 inches on center. My understanding for the use of synthetic felt is to not scratch the metal and because of the heat properties transferred from the metal to the underlayment and regular felt won't hold up to the heat. But since there will be a 3/4" air gap between the metal and felt would regular felt work or do I still need to use synthetic udl underlayment?
Todd Miller
Isaiah Industries, Inc.
7/6/2017
I would still recommend the underlayment. Although not all people interpret the code the same as I do, I feel it is required by the International Building Code. Additionally, if you should have any water occasionally get underneath the panels, the underlayment will be a great safeguard -- and inexpensive.
Guest User
7/6/2017
I understand using the felt is a good idea my question is will regular 30# felt be ok to use or do I still need to use a synthetic udl underlayment?
Todd Miller
Isaiah Industries, Inc.
7/6/2017
30 pound felt will meet code requirements ... and I would think it will be acceptable to your roofing manufacturer though you should confirm that with them.
Guest User
7/6/2017
Thanks for the information. I will check with them.
Guest User
9/14/2017
I have a question but my sign in won't allow me to do it so I hope its ok to ask from here. Can you install a metal roof on a wet roof after it has been raining all night but not raining now?
Eric Novotny
An informed customer is the Best Customer!
9/14/2017
Amy...that post has been answered. Please see here: https://www.metalroofing.com/v2/forums/index.cfm?action=mboard.members.viewmessages&ForumTopicID=9220&ForumCategoryID=62

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