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 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.
