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Corrugated roof assembly

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Guest User
5/31/2007
We are replacing the corrugated metal roofing on an existing unheated machine shed in a northern climate. The original 1x8” board sheathing will remain, and will be covered with 1/2” plywood to strengthen the rafters, which are over-spanned. The original roof had felt paper under corrugated metal attached directly to the sheathing (no furring). My questions are: 1.Does the new corrugated metal require an underlay between it and the plywood? 2.Is it necessary to mount the new corrugated roofing on furring strips? 3.Do we do one or both or neither? 4.Would a water-repellent paint or applied coating on the plywood be a suitable substitute for underlay? Your help would be greatly appreciated!
Ken Buchinger
NCI Building Systems, Inc.
6/5/2007
You do not mention the roof pitch of your building. Some corrugated panels require a minimum pitch of 3:12, while others are as low as 1/2:12. Many corrugated roofs do not require an underlayment, if the minimum roof pitch requirement is met. Most corrugated panels can be installed directly to the plywood. Consult the panel manufacturer for minimum roof slope and underlayment requirements.
Guest User
6/5/2007
Thanks for the advice. The pitch is 8:12, and the panels are standard corrugated zinc-galvanized steel, so that should be OK. Still, is it worthwhile for "best practice" purposes to install strapping and/or underlayment for a more long-term roof? The roof area is approx. 13,000 sqft, so this has to last. I wonder if the ventilation afforded by strapping would make a difference? Attached is a photo of the building.
Guest User
6/5/2007
See attached
Ken Buchinger
NCI Building Systems, Inc.
6/6/2007
With a standard corrugated panel, it would be a good idea to install an underlayment. Also, it is best to lap the panels two corrugations as you are installing them, to minimize the possiblility of water intrusion at the panel side laps. Your first fastener should be past this two corrugation lap. By strapping, I assume you mean furring strips. Unless you were going to install a batten/counter batten system, which would allow air flow under the panels, you will not realize any additional ventilation.
Guest User
6/7/2007
That's just what I wanted to know. Thanks for your help!
Todd Miller
Isaiah Industries, Inc.
6/10/2007
You're welcome.
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