Two Basic Lengths
There are two different staple lengths that you might use when installing 3/4-inch hardwood floors. The length of staples you use depends on the subflooring and other materials underneath the floor. The length also depends on the floor's structure -- if the floor is supported by wood joists, you have to use longer staples than if there are no joists underneath the floor.
Use 1 3/4-inch staples when installing hardwood floors over one layer of plywood and a concrete slab. These shorter staples are better for this type of flooring because they won't penetrate the plywood. If you use longer staples, they may go through the plywood and damage it as well as scratch the concrete. If you use nails rather than staples, use the same length nail for the same reason.
You need longer staples if the floor is supported by wood joists or by more than one layer of plywood. The staples must be long enough to securely attach the floor to the subfloor without damaging anything underneath the subfloor. In general, use staples that are 2 inches long for this type of flooring. The longer staples give extra support to the floor and won't damage concrete because of the extra layers the staples have to go through.
How to Staple
After you set the first layer of flooring, nail brads into the floor, spaced 8 inches apart. Then "toenail" 1 1/2-inch brads by attaching them at 45-degree angles every 4 inches and place the next layer of flooring. Once you've installed all layers of flooring, use a pneumatic staple gun to staple the top layer of flooring to the subflooring.