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I am replacing a flat carport roof with PBR metal sheets over 5/8" inch plywood sheathing with underlayment. I know 1/2 inch is strong enough, but I was thinking more of the nailing. Anyway... it's installed.

I plan to install Tyvek 200 underlayment using 3/4" cap nails, and 1-1/2" cap nails where the joists are.

For the PBR I have the same, 1-1/2" metal to wood screws for the joists, and 3/4 for the metal to metal ribs, so I don't see any protrusions there unless I don't hit a joist which is possible.

Since it's a carport, of course when you look up at the bottom, you will see any screws protruding out. There will be quite a few. I guess I could live with them if I had to but I don't really want to.

Question: Is it OK to cut off (either with my cutters or rotary tool) the tips of any protruding screws or nails? Will it hurt the "holding power?"

FYI i am in Los Angeles. We don't get hurricanes etc. I looked at the high wind history for the last 20 years and it never got above 35 MPH.

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    The cap nails are rather pointless. Use a staple hammer to tack the underlayment in place. The roofing will hold it down. – isherwood Mar 31 at 18:10
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The portion of a screw sticking out into space doesn't have anything to hold onto, so it's not doing any good. Once the wood has been compromised enough for the threads to pull out, additional threads will just pull through the larger hole.

A nail, OTOH, will continue to provide friction resisting pull-out for its entire length, as the hole generally isn't enlarged much due to "natural" causes pulling it out (as opposed to being broken up during intentional removal). This can be proved by trying to pull a 3" nail - it takes nearly as much effort to get the last inch out as it does the first inch.

This is a long winded way of saying "go to town cutting them off".

Also, if you take the time to measure where the joists are and snap chalk lines on your roofing, you'll have about a 99.99% chance of getting every screw into a joist, so you won't have anything to worry about in the first place. Getting the screws right up front will probably take less time than cutting off the ends later anyway.

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  • yes makes sense that is the plan. thanks – eaglerockdude Apr 1 at 18:30
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After chatting with my contractor buddy, I just decided to use normal 1.5 cap nails only where the joists are, which are 16 OC. Any other places that need it I will just use Tack 1/2 Staples.I will also tape the edges down with my fantastic 3M flashing tape to the drip edge I will have nailed down. I think that will do the trick, and not have to cut any nail heads off. It will have metal PBR on top of it anyway in a few weeks. I don't see it going anywhere.

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