Nailing plywood to concrete slab on grade - what length nails?

For nailing 1/2" or 5/8" plywood subfloor onto a concrete slab (on grade), what length of concrete nails would one recommend for use with a power load nailer?
I'm thinking 1" may be a little short; maybe 1-1/4" would be right (which would allow close to 3/4" nail into the concrete).
There won't be many upward forces on the floor so a minimum nail is best in this case.
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In my experience that is not enough. The nail will break the surface apart and you will not have 3/4" in solid. 2".
M Hamlin

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Really? I got my estimate from observing the nails that were shot into the carpet nailing strips around the edge of the room. These little strips had nails no more than 3/4" long and were only sunk into the concrete between 3/8" and 1/2". While this may not be strong enough to hold down plywood, I noticed they were indeed quite solidly in the concrete and took some solid blows with a hammer to remove them.
Becuase of this I was figuring a 1/2" penetration into the concrete was sufficient to cause a solid grip so the extra 1/4" depth would be enough to hold the plywood, since there's not much force being exerted on it.
Could there be something different about how these strip nails were set?
Of course, I don't know how well 1-1/4" nails would hold over time so I will consider using longer.
Thanks! :-)

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On Wed, 13 Aug 2003 01:38:26 -0400, "Stellijer"

I think the PP is right. Short nails will work loose over time. There is going to be a lot of strain/force on that plywood as the moisture comes and goes. I'd use PT plywood too.

Make sure the tool is flat and level. Still, concrete hardness or a stone will affect penetration. Use a 2 lb sledge to drive the tall ones in.

Make sure the top section is down flat against the bottom section an not sprung up at all.
Bob
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