For proper fabrication and fit we must know the existing roof curb dimensions when it comes to measuring your skylight. This can be best accomplished from the roof, by removing the skylight from the curb. The Outside Curb Dimension (OCD) is a critical piece of information for the ordering process. This will be the dimension of the unit that will be the base for the skylight. Compare this to a shoe box and the lid for the shoe box. The OCD would be the dimension of the shoe box that must fit inside the net frame dimension of the skylight or lid of the shoe box. 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).
skylight measuring guidelines