How to Measure a Rough Opening for New Construction Windows

Last week on #TwoCentsTuesday we asked whether a window supplier has to deduct ¾ of an inch from a window measurement to determine the rough opening. 

Most of our readers said this was false, and they’re right! We typically take ¾ of an inch, but sometimes suppliers will take ½ an inch, or even an inch. They’ll usually make the window smaller than the rough opening so that there’s space to make it plumb, level, and square during installation.

Supply Only Windows

If you’re ordering your windows for a supply only project, here is what you need to know about specifying them. You don’t need to include a deduction, as most window suppliers will make your windows slightly smaller than the rough opening anyway.

In new construction generally, the rough opening sizes are sent to the estimator. We would then deduct 3/4" from the RO size to determine the window size needed.

Example: Contractor asks for a 4' (48") x 3' (36") window, we would quote a 47.25" x 35.25" window. So they don't need to include the aforementioned deduction, but they would need to specify the rough opening and the install detail they will use. The install detail may require its own deduction which will allow for shimming, levelling, and insulating. So an install detail is a method chosen to install the window and this will determine how much insulation and levelling etc is needed.

To be clear, the "rough opening" is the framed opening in the building into which the window would be installed, so you would always determine the window size by the dimension of the RO. It would not make sense to do it in reverse and try to determine the RO from the window size.

Trimmers/Jack studs and cripples are all interchangeable definitions for the same framing components.

How to measure a rough opening: Width (cripple to cripple) and Height (header to sill).

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