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Just to pick people's brains on this one:
I'm building a small addition which will have plywood sheathing on roof and metal roofing. To ensure waterproof I'm thinking of tar paper too, since there's plywood, but here's the question. You can't just screw the roofing to the plywood, for strength and looks reasons (all those screws coming through), but if you lay down purlins, you ends up with water dams across your tar paper. Any thoughts on this?
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Screw the roofing to the plywood. What do you need purlins for if you have plywood?
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I've seen roofing attached to purlins over plywood sheathed in plastic (it's definitely a technique, I can only speculate as to why) and wondered the exact same thing.
In this case the purlins appeared to be 1x2 or similar, not "structural".
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quote:
Originally posted by Rob Tucker
Screw the roofing to the plywood. What do you need purlins for if you have plywood?
Guy I'm doing it for just wants an exposed rafter ceiling. He's mostly doing it so he doesn't have to look at the corrugated roofing, but seeing screws coming through the plywood would look even worse. Also, is screwing roofing to plywood considered strong enough?
My personal preference would be to just use 2x4 purlins and then cover the ceiling with some cheap paneling. One other consideration is noise. I've "heard" that plywood sheathing under metal is a lot quieter in rain.
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quote:
Originally posted by kalakoa
I've seen roofing attached to purlins over plywood sheathed in plastic (it's definitely a technique, I can only speculate as to why) and wondered the exact same thing.
In this case the purlins appeared to be 1x2 or similar, not "structural".
That is exactly the case in the house I'm living in now, and it's definitely confusing. Why would you put purlins on top of tar paper or plastic when they're just going to trap water? And sure enough, at each corner of the hip roof (where rain got under the caps) I've had to cut away rotting plywood and replace the fascia boards.
Seems to me if you go with plywood and tar paper you should screw directly to the plywood, as Rob suggests.
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I agree as well as attaching the panels straight to the plywood, with fasteners through the roofing felt. Purlins are generally used in tile roofing to create a "ledge" formthe tile to hang, and create the "stacked" , or shingle look. The roofing felt actually seals around your fastener when it goes through with the tars impregnated into the felt. In this respect, do not use gray colored, as it has dried out, but instead the deep black, almost glistening roofing felt, and if still nervous, go for the heavier, 30 pound felt underneath.
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quote:
Originally posted by Rob Tucker
Screw the roofing to the plywood. What do you need purlins for if you have plywood?
I think the screws have to be at least 1" into wood to meet hurricane codes.
Not only that, it would be pretty easy to strip out 1/2" plywood.
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I believe what Rob meant was to screw ithrough the plywood, centering your nailing line over the trusses and blocking. Other roofing (composition) nails through the 1/2 inch ply with about 1/2 inch poking through. This method keeps the fastener from "stripping" out.
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