subfloor attachments

i have a 5/8th inch osb board and a 5/8 inch plywood subfloor in my upstairs bedroom. i am going to screw it in. what size screws are the best. i found some 2 3/4 inch screws at the hardware store that are exactly the same as some subfloor screws that were only 1 1/4 inch. wondering if the 2 3/4 inch screws are to long.
so the floor is 1 1/4 inch and the screws would be 1 1/2 longer.... is that to much?
thanks
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larrykwiat wrote:

the rule of thumb I use is that a fastener should be at least twice as long as the material you are fastening is thick, and it should penetrate the wood framing by no less than 1 inch. by that logic 2 1/2" screws would be the minimum, and there is no harm in using some slightly longer.
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I am not an expert but shouldn't you be using under layment flooring nails?
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