The most common varieties of softwood plywood come in three, five or seven plies with a metric dimension of 1.2 m × 2.4 m or the slightly larger imperial dimension of 4 feet × 8 feet. Plies vary in thickness from 1/10" through 1/6" depending on the panel thickness. Roofing can use the thinner 5/8-inch plywood. Subfloors are at least 3/4-inch depending on the distance between floor joists. Plywood for flooring applications is often tongue and grooved. The mating edge will have a "groove" notched into it to fit with the adjacent "tongue" that protrudes from the next board. This keeps the boards from slipping past each other providing a solid feeling floor when the joints do not lie over joists. Tongue & groove flooring plywood is typically 1" in thickness.
Sources--
http://en.wikipedia.org/wiki/Plywood
For a roof, plywood with a thickness of at least 5/8 inch is typically recommended to provide sufficient strength and support.
11/16 plywood is commonly used in construction projects for applications such as subflooring, roof decking, and wall sheathing. It is also suitable for making furniture, cabinets, and other interior finishes.
Roof decking, exterior sheathing.
Tar paper is used as a moisture barrier between the plywood decking and the roofing materials.
For a roof, plywood with a thickness of at least 5/8 inch is recommended to provide sufficient strength and support.
The best practices for using plywood for roof sheathing include ensuring the correct thickness and grade of plywood is used, properly securing the plywood to the roof framing with the appropriate fasteners, staggering the plywood seams to increase strength, and providing adequate ventilation to prevent moisture buildup.
For roofing, it is recommended to use exterior-grade plywood that is at least 5/8 inch thick. This type of plywood is designed to withstand weather elements and provide a sturdy base for the roof.
For roofing, it is recommended to use exterior-grade plywood that is specifically designed for outdoor use. This type of plywood is more durable and weather-resistant, making it suitable for protecting the roof from the elements.
1/2, 5/8, 3/4 depending on what it needs to do.
For 1/2 inch plywood, a 6 or 8 screw size is typically recommended.
If the seal is down and the plywood is going on roof trusses, the inspector can see it when he performs the framing inspection and verify that you used the proper plywood. Othewise, it doesn't matter very much.
For a subfloor, typically 3/4 inch thick plywood is recommended for stability and durability.