The manufacturer will list the appropriate rough opening in their catalog for each window size and that is what you should give the builder in a window schedule or give him the model number and let him find the R. O. Don't add anything to a frame size and tell him it is the R. O.; it's not always that simple. Some manufacturers provide 1/4" on the sides and the top and nothing on the bottom, some provide 1/2" on the sides and top and some actually subtract a fraction at the bottom if the sill nosing hangs over the sill framing. Use the R. O. from the manufacturer's catalog and let the framer do the rest.

Exterior doors usually rest on the subfloor or a slab but unless it is a glass door sized similar to windows, you should only need to tell the framers the center line of the door and the nominal frame/door size and they can figure the rest out.

one step further swampwhiz..
if you are using a housewrap product..
a lot of builders still cut an X in the housewrap
and then install windows. this is incorrect.
cut should actually be an inverted Y and the top
flap of housewrap goes over the top flange of the window.
in the X install flaps are folded into the RO and
water enters windows as houswrap directs it to flow this way. Most window mfg's have excellent install recomendations..the problem is getting the proper person
to read them and execute them!!
educating yourself on correct install and flashing would
put you a step ahead of the game. water intrusion is a huge issue that costs big dollars to remediate later.
Does anyone have that link to Grace's website?
excellent info there!
best of luck.

The rough opening size is what the window/door manufacturer gives you. I actually have the salesman write it on the drawing.

The height of the door is your calculation. I find it works best to allow for a half-inch above the finished floor height. At least, in our climate where it is customary to have a heavy rug at the entrances to absorb snow, mud, etc on entry. If you don't allow for that, you eventually wreck the doorsweeps.

If the windows are too tight of a fit in the rough opening, there will be no room to squirt some foam insulation. It's better to have a gap large enough to insulate. A 1/8" gap is too small to insulate, but large enough to let a lot of air infiltrate. I prefer around a 1/4" gap so I can get enough foam in there.

OK macv, so you are saying to just give the rough opening size, and let the framer get to those dimensions exactly.

I wonder though, that exact is an impossibility, as there is always a small error (e.g., 1/32" or 1/16" ??) I am wondering if the dimension is too small by this amount that the window will not fit - IOW, that I would need to specify 1/16" larger just to make sure that absolutely, the rough opening is big enough.

"As I understand it, there is to be a 1/4" nailing strip installed on each side, for a total of 1/2" for each dimension, which is why the rough dimension is quoted as 1/2" larger."

That is not the reason for the rough opening being 1/2" larger. You need to look at a window. The nailing strip is actually a 1 1/2" +/- flange that overlaps the rough opening and is nailed to the sheathing so the gap between the window frame and the framing does not have to be exact or perfect. The framer will treat the specified R.O. as a minimum and try to make it as square and level as possible. A slightly larger opening is not a problem.

You only need to tell the framer the R.O.'s and the only reason to modify them is if you wanted to use a sill pan or another kind of sill flashing that required more space. A long span over a set of windows might require a larger gap at the head for future deflection.

Adding to the specified manufacturers R.O.'s can result in redundant increases in the final opening size so such increases are better shown in a note in the window schedule with an asterisk at the R.O. for the appropriate window.

Many designers have gotten into the habit of showing a frame size on the elevation drawings. That is OK for vinyl and fiberglass windows and perhaps clad wood casements and awning windows but for many double-hung windows the frame size is not an even dimension (windows sizes are a function of standard glass sizes) and varies a lot between manufacturers. The solution is to pick a manufacturer and put everything into a window schedule. If the manufacturer is changed all of the R.O.'s will have to be changed.

You only need to tell the framer the R.O.'s

I don't tell them anything but to follow what's written on the drawings.

swampwiz All the ROs I've seen were certainly not designated in 1/16" (yet alone 1/32" increments). I haven't seen a micrometer on site yet.

"I wonder though, that exact is an impossibility, as there is always a small error (e.g., 1/32" or 1/16" ??) I am wondering if the dimension is too small by this amount that the window will not fit - IOW, that I would need to specify 1/16" larger just to make sure that absolutely, the rough opening is big enough."

The RO size already contains allowances for shimming and squaring,

There is no reason to add more,or worry about RO sizes being to the nearest 1/16 inch.

It is a ROUGH OPENING, not finish work.

OK, it looks like a small error in the rough opening will be OK, so I will only tell the framer the rough opening specs.

Now, why would I want a sill pan or flashing? It is a good idea?

You should start by looking at the installation instructions for your windows and for the air/weather barrier you have chosen for your particular wall assembly design. Talking in generalizations can be a waste of time and effort since every building is different to some extent. WR Grace (Vycor) and Tyvek (StraightFlash & FlexWrap) have some good instructions and videos online.

Where is the project located?

hot humid climate

heavy rainfall?

About 60" per year