I ordered a nail flange window from American Craftsman 50 series via home depot. I am trying to frame my rough opening ahead of time. But I am very confused as to the dimensions. I assumed when I bought the window that I could frame my rough opening 36in x 52in, but the rough opening size is actually listed as 35.875 in. x 51.75 in. Does the rough opening have to be exact or can it be a little larger (36in instead of 35.875, or 52 instead of 51.75) I know the rough opening is supposed to be larger than the actual size but I don't know just how much bigger the opening can be compared to the RO listed in the specs. Is it supposed to be exact? Also the specs listed 36 x 52 as the "Standard Size, and a "Nominal Size of 3/0in x 4/4in. What do these measurements mean?

Actual Size: 35.375 in. x 51.25 in.
Nominal Size: 3/0 in. x 4/4 in.
Standard Size: 36 in. x 52 in.
RO: 35.875 in. x 51.75 in.

 

3-0 nominal means 3 ft 0 inches, or 36".

4-4 nominal means 4 ft 4 inches, or 52"

 

So do I frame the opening using the Rough Opening size (35.875 in. x 51.75 in.) or nominal size? I really want to have nominal size.

 

You will shim it in place, so go with the 36 x 52.

 

I ignore suggested rough opening sizes and add 3/4" - 1" to the actual size. I like to use full shims around the perimeter, which leaves 3/8"-1/2" of space for foam insulation. I hate it when the space is 1/4" or less.

 

Agreed. With too small a gap, you can't get the nozzle of the foam applicator in. Nothing will be harmed by using the nominal size, or a little more as Brant stated.