Rough opening vs. actual window sizes...

[Erin]

What's the rule of thumb for rough opening as compared to the actual size of a window?

How small of a gap would you NOT want? 
How about large?  Is there is a gap size that is just too much?

[MountainDon]

Window mfg supply RO measurements when ordering windows. I believe the RO was a half inch wider and taller for our windows. That was for windows with a flange mount.

How close to cut it? Depends on how skilled the builder is at placing the studs plumb vertical and the sill and header level.


I did the half inch and found I could have made it maybe 3/8 with no difficulty. But if something was off then too small a RO could cause problems.

I'd want enough of a gap to make it easy to get the spray can foam into place, but not so wide as to make the nails/screws too close to the wood edge.

[dug]

I built my rough openings 1/2 in. larger than the window as suggested by the manufacturer, which leaves 1/4 in gap all around. I was pretty careful but somehow some of the windows ended up being a snugger fit than I wanted. In retrospect 1 in. larger opening wouldn't have hurt a thing. Too big is better than the other way around.

[Erin]

Thanks guys.  Exactly the info I was looking for. 

[PEG688]

  If your talking vinyl windows what ever the stated size is , that the RO size. Pretty simple , a 3-0' x 5-0'  window gets a 36" x 60" RO.


  When you get into wood windows Anderson , Loewen , Marvin's etc the company provides the sizes for their window. 

 

[Erin]

Neither.  We're going with fiberglass... 

[PEG688]

Neither.  We're going with fiberglass... 

Same as vinyl the net RO size matches the size they say it is , they reduce the window by 1/2".   So if they say 3-0 X 5-0 , it's a 36"x 60" RO.