Purlins or plywood?

Guest User
7/6/2013
I have an existing steel roof, presently just on purlins. It needs to be replaced due to leaks, and one contractor (the one I'd really like to hire) refuses to put the new metal roof on just purlins. He won't do it unless I put on plywood as a base, he also recommended ice and water underlayment. Do I need the plywood base? Would it help keep the screws from backing out? The roof I have now, the manufacturer wouldn't warranty unless it was on purlins and had the proper venting. I'm concerned that if the new roof is on plywood, it won't vent correctly. I have vents in the soffits and a ridge vent. It's a saltbox style home and I don't know the pitch of the roofs. The front is over an attic, with the insulation in the floor of the attic. The back roof is over a living area, so the insulation is just under the below the surface of the purlins. If I add the plywood, would it be best to just mount it on the purlins? I'd hate to have them tear them off and possibly lose venting.
Todd Miller
Isaiah Industries, Inc.
7/7/2013
Let me ask a couple of questions, please. What is the pitch of the roof? Is there any attic ventilation?
Eric Novotny
An informed customer is the Best Customer!
7/7/2013
You have the unique opportunity to permanently address the insulation and roof system in this case. I would suggest that you look into spray foam for the hot/insulated roof deck.
Guest User
7/7/2013
I don't know how to calculate the pitch. The front roof drops 4 feet in a 16 ft span. The back roof drops 12 ft in a 16 ft span. The attic does have ventilation from the soffit vents and ridge vent, the same with the back roof. There is 9 in. of fiberglass insulation between the ceiling and roof on the back and about 9 or 10 inches in the attic floor. I don't think I explained it well. I'm looking at replacing the roof. I don't have a problem with insulation or ventilation right now. I am asking if I have the contractor install plywood underlayment over the existing purlins, how would the new roof be ventilated? Do I HAVE to have plywood underlayment installed? He seemed to think so. Does that increase the life of the roof?
Todd Miller
Isaiah Industries, Inc.
7/7/2013
Normally there is no reason why you need ventilation above the decking, especially not if you have it below the decking. That is why I was asking. I think your contractor's plan to install decking and underlayment over the old purlins followed by your new roof is fine.
Guest User
7/13/2013
Thank you very much for your help.
Eric Novotny
An informed customer is the Best Customer!
7/15/2013
Good luck with the project and be sure to post some progress pictures.
Guest User
6/6/2017
Hi and aloha, I have a near flat i guess 24/1 pitch patio cover w 4x8 4' on center. I do not want to add purlins due.to the.proximity of transom windows above...would 3/4" ply cross laid be acceptable w mambrane pre metal? Thuanks, Michelle
Todd Miller
Isaiah Industries, Inc.
6/7/2017
There are mechanically seamed metal roofs that can be used on roof pitches as low as 0.25:12. Many of those can be installed on battens. It would be critical to use only a metal roof approved for a pitch such as yours.

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