How to Measure for a Curb Mount Skylight

It is important to know the dimensions for your replacement skylight so the correct size can be ordered to fit the existing curb.
When ordering or describing a skylight, the measurements are always expressed as width (dimension horizontally across the roof) x length (dimension vertically up the roof).

The Outside Curb Dimension is the overall outside dimension size (OD) of the finished, built-up curb. To determine a correctly sized replacement skylight, also include the necessary roofing materials (such as?) and sheet metal flashing. Skylights are sized slightly larger than the OD of the curb in order to fit over the top like a box cover.