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TOPIC: Standing Seam roof

OSB sheathing Question

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I'm considering metal roofing as a replacement for the current 20 year old composition roof on my workshop. Before I get too excited about metal I have one big question. Does 7/16 OSB sheathing have adaquate holding power for screw fastners? The metal roofing I'm thinking about would be a standing seam style. The roof is about 900 square feet, a simple gable style, 6/12 pitch. The old shingles can be removed. The job is located in central Ohio. Ron
Guest User

8/20/2003
The question of uplift resistance is much more complex than just fasteners and substrate. You need to make sure that bother the fasteners and the substrate are approve dby the roofing manufacturer as being suitable for their product. In many cases, though, yes, 7/16" OSB will be adequate. Todd Miller
Todd Miller
Isaiah Industries, Inc.

8/24/2003
I'm building a whole new roof-rafters and all. With the sky rocketing price of OSB, I was considering putting on a metal roof if I didn't have to sheat the roof. I'd attach the metal to 2x4's running perpendicular on top of the rafters. Can this be done? I heard that condensation could be a problem. I live in a very wet climate-Oregon.
Guest User

9/4/2003
In order to do this successfully in your climate, you will need two things: 1) A roof system which is designed for application over purlins rather than decking. 2) Strong ventilation to ensure that misture is not trapped in the attic. If that moisture is trapped, it will condense on the backside of the roofing whenever dewpoint is reached. Todd Miller
Todd Miller
Isaiah Industries, Inc.

9/4/2003
All roofs condensate especially in your climate. The new building code looks at the entire roof assembly. That is an air barrier to the inside to stop air laeks with the resulting heat loss which will add condesation as the air cools. Next is insulation. This holds the heat to the inside obviously reducing heating and cooling costs. Then it is ventilation one of the most important factors. This helps reduce the heat cycle pressure in the attic or under the roof and clears our any condensation. Basically allowing the roof assembly to breathe. The code requires 1sf of freee air for every 150sf of ceiling area and if vented at the ridge and eaves equally, this can be dropped in half to 1/300. If your are putting in a vaulted ceiling you need a minimum of 1" of vented air space over the insulation under the roof covering. A moisture barrier is then required under the roof covering. The code calls for a minimum of #30 felt paper however this will not span open rafters. The purpose of the moisture barrier is to firstly breathe and collect any condensation on the top surface and directing it down and out at the eaves. It also acts as a secondary membrane in steep sloped (hydrokinetic) roofs in severe weather. Last comes the roof covering. Plywood sheathing can serve a number of purposes in building. First it generally acts as roof bracing for the structure. If you omitt it then you must istalla other framing bracing. If the sheathing is elliminated then you will have to upgrade your moinsture barrier (underlayment) to a reinforced material similar to a TriFlex 30 that will span the rafters. If you leave out the sheathing then you bwill require more attention to detail on workmanship and the underlayment. The lower the roof slope the more you will rely on the underlayment as a secondary membrane. Then you must consider the metal roof system. Are you buying a system designed to be installed this way specifically on a residence. Too many people buy an agricultural metal panel with lower standards on finishes and base metal, then buy the thinnest steel possible in a low profile and expect it to perform beyond its designed characteristics. Also look at the wind requirements for fastening strength required. Without sheathing the roof covering is required to take the wind uplift. You then may be required to space the strappings closer. Do your homework and investigate the systems, call the manufacturer and get their advice. The more complex a roof layout the more you will rely on workmanship so choose a contractor carefully. Remember you have decided to put a lifetime roof on your home so do itight and do it once. Good Luck.
Allan Reid
Dura-Loc Roofing Systems, Inc.

9/4/2003
Bide by Al's advice -- much more thorough and well thought-out than my slightly earlier response Todd Miller
Todd Miller
Isaiah Industries, Inc.

9/4/2003