Your installer will measure the skylight to determine the replacement size you need. However, if you plan to take on the task yourself, here are the measurements you need to record:
Measure the width and the length along the side of the skylight on the roof where the shingles meet the flashing. Measure the height of the skylight or the curb. Inside your home, measure the width and length of the drywall around the skylight’s opening closest to the skylight. Consider taking pictures of your tape measure showing the lengths, widths and heights while you’re measuring. Share them with your contractor or roofer to verify the accuracy of your measurements.