A lot depends on what type of framing is present.

If the roof is all trusses it may not be designed for any significant loading.

If it is a framed roof you can actually measure the ceiling joists and span and determine what load it might be able to carry.

Framed roofs usually have some margin since there are defined sizes for the lumber used and you must use the 'next larger' size unless you exactly hit on a size based on loading.

Keep storage over garage.

To install flooring over
the living space you would compress the insulation and
you need what you have in texas!

I like to screw the plywood decking down in case
you ever need access to anything below.

Size of framing is important, because of weight/span.

I don't think anyone starts putting stuff in the attic
with the idea of storing a lot of stuff..but a lot
of stuff sure finds its way there over the years.

Always be careful working in the attic, Make sure there is enough light to see where you rest you tools and put your feet.
oh,,,and cut plywood in half two 8x2' pieces so that it
goes easily into the attic. I have to cut into 4x2 because
it is heavy,,but your big guy wouldn't have that issuel

best of luck.

I used Infinite Attic.

http://www.infiniteattic.com/index.html

Worked awesome. But I have trusses in my attic, so I had to use something like that.
Plus, I have to be very careful with what I put up there.
Trusses arent designed to handle load on them....only there to hold up roof essentially.

good luck.
mm

Here is a link that might be useful: Infinite Attic

Try a product called AtticDek. They are hard plastic and can hold up to 300lbs each! Easy to install and will not rot or warp like plywood. They are expensive though. Expect $100 for 10 24x16 Squares.

http://www.atticdek.com/