One last comment "DO NOT USE OSB under the metal roof" period. OSB will not hold wood screws near as well as good old fashion plywood. 5/8 thick min. If you don't believe me just ask my neighbor. His entire metal roof has peeled back over the ridge twice in 10 years. Mine has never even rippled. It cost me 2500$ more than OSB but money well spent. OSB is fine for the side walls. Although I usally put good ole ply wood on the four corners for shear loads. [color:#CC0000]This I did not know. I'm not sure I totally agree with the analysis that the torque of the screw, as it sets the rubber gasket, is right at the same torque required to alter the integrity of the OSB structure, but I surely can see where plywood wood blow OSB away in structural integrity for the application. I have always been a plywood guy; just never knew that OSB was close enough to the failure curve to be reconsidered for sheathing on a metal roof. This does make sense to me.
I do a fair amount of repairs on homes to have seen a variety of failures on roofs. In every case, it's not the decking that's the problem, it's the fasteners that were used to hold down the roofing material. With shingles, roofers can save money, and/or make a bigger profit by using one inch nails to hold down the shingles. Those nails are not long enough to fully penetrate the decking material. It doesn't matter what the decking is, the nail wont hold unless it goes all the way through. The point must not be inside of the wood for the nail to hold.
The same is true with screws and metal roofs. The screws used to hold metal roofs come in different lengths and every time I see a problem on a metal roof, it's because of shortcuts that were taken in attaching the metal. Wood screws have to be long enough to go all the way through the decking material. If the threads have not gone all the way through and you cannot see threads on the underside of the decking, the screws are not at full strength.
There is also some discussion on where to put the screws. I put them on the flat of the panel so when tightened up, there is no gap between metal and wood. It is solid. Some manufacturers recommend screwing through the peaks of the metal. I've seen it on their websites, so I know it's true, but I don't agree with this. I buy from Mueller's, and they recommend screwing through the flats. .
I would really like to have looked at that roof that failed twice in ten years to see what they were doing wrong. I don't think it was the OSB, but in how the metal was attached to the OSB.
Eddie
