I've floated 2 layers of 3/8 CDX plywood over 6 mil poly, over concrete. I'm confident with this set-up. The plywood is screwed together and I have a 1/2 inch to 3/4 inch expansion gap between the plywood and walls.

I would appreciate help with these questions.

1. Do I need to cover plywood with #15 lb felt, before installing floor? Is the felt for sound? Do I need to overlap if installed?

2. I'm going to use 1-1/2 L-Nail 16 Gage Powercleats. The 2 layers of 3/8 inch plywood was the thickest I could go because of the sliding glass door.
The net height of the (2) 3/8 ply is .6875 inches, is this ok for the above listed fasteners?

3. I will leave an expansion gap around the hardwood floor of 3/8 to 1/2 inch. I'm in Southern California so we don't get much temp./humidity change. I can't figure out what to do where the hardwood meets the bottom of the vinyl sliding glass door. I'm tempted to install the hardwood right next to the sliding door bottom - but I know this is wrong. The sliding door bottom is 1/8 inch higher than the Hardwood floor, so I can't attached molding to cover the gap. I can leave a gap, but how do I fill it, with caulk? That seem like a bad idea. Is there a flexible product or material to put between the hardwood floor and the sliding glass door bottom that will look nice and expand and contract with the flooring?

Any help or suggestions would be greatly appreciated.

http://www.powernail.com/home/depthchart.htm

I'd use 15lb felt at a 45 overlapped.

If the wood is running toward the slider you don't need a gap. If not how about a couple runs of some stone there so you don't enter from outside with dirty or wet shoes.

Thanks for the chart and information, perfect.

The flooring will run parallel to the slider. I think if I leave a 1/4 inch gap it will be ok. We considered tile in front of the slider, but decided to only have hardwood floor in this room. It just looks so nice and finished if the floor was right next to the sliding glass door bottom frame.

Thanks again for your help.