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

[Stellijer]

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.

 

[MSH]

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

 

[Stellijer]

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!

 

['nuther Bob]

I tried shooting a couple 1-1/2" Remington nails that came with the nailer (I
didn't have 2"; just 1-1/2" and 2-1/2").

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.

I was having a lot of trouble using it. I tried 3 and 4 powerloads but found
they drove the nails rather unevenly. Sometimes much deeper than others.

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.

I also found it requires a rather severe blow with the hammer to get it to fire;
sometimes numerous blows (it was much easier when I tested it directly on
concerete)

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

Bob