patio door questions?

Use a recriprocating saw (Sawzall) to cut out any rotten wood. Each end should have a double stud so you can remove one with a recriprocating saw and then rip a custom 2x4 with a table saw to replace it (or the rotten wood). Often times though, when a door is put in the opening, there is space between the frame of the door and the 2x4 studs. This is so you have a little room to level it 90 degrees perfectly on both sides. There could be some spacers or shims between the door frame and 2x4 studs. So there is probably a 72" opening. Carpenters make sure the frame is perfectly up and down. Then, they slide shims at the top and bottom to hold it, and then they drill the frame to the post. I'm not sure a sliding glass door would be done like this, but you can buy a bundle of shims at any Home Depot/ Lowes. Good luck.

Use some good clear silicone or rubberized caulk in places where you don't have to paint. It's stronger than paintable latex caulk and will last 35+ years. You can also spray any large gaps with a can of spray foam insulation. Use a wonder bar to remove the old moulding. You can find them at the Home Depot. It's a small carpenter's crowbar. Put a cloth/ small towel behind it so you won't leave marks on the wall. Hopefully, you can put the moulding back in.

If you are going to be replacing the door, then remove the inside trim and measure from your framing to obtain your rough in measurements. Give those measurements to your fabricator and you should be fine. I like to have a half an inch on both sides and top so I can make any and all adjustments to insure an easy install. I suggest you have two levels on hand a 2 ft and a 4 or 6 foot, a pack of shims to level and adjust your doors, and some spray foam insulation to finish the job off. If you are anything like me you will measure your new door prior to removing your old one as I have had 2 come to the job and were 3" too tall.

cheers

Try and get a quote from a builder instead of a double glazing installer...