If you’re getting ready to replace an old skylight, taking the right measurements is the first and most important step. Ordering the wrong size can lead to leaks, air gaps, and installation headaches, so it’s worth taking a few extra minutes to measure correctly.
In this guide, Matt from Replacement Skylights walks you through how to measure for a VELUX curb mount skylight, explains what to look for, and shows you how to use the VELUX Skylight Size Chart to find the correct model for your roof.
Step 1: Identify Your Skylight Type
Before you pick up your tape measure, determine which kind of skylight you have. In the video, Matt demonstrates how to measure a curb mount skylight – the type that sits over top of a raised frame, or curb, built onto the roof.
This is different from a deck mount skylight, which attaches directly to the roof deck. If you see a framed “box” on the roof and the skylight is mounted over it, you have a curb mount unit.
VELUX makes both deck and curb mount skylights, but the measuring process is different for each. Getting this wrong can lead to ordering a skylight that simply won’t fit, so take a moment to verify your existing skylight type before you start measuring.
Step 2: Measure from the Outside of the Flashing
Once you know it’s a curb mount skylight, it’s time to measure. This step is where many homeowners go wrong. Do not measure the old skylight frame — that will not give you an accurate size.
The proper way to measure a curb mount skylight is to measure from the outside edge of the existing flashing, both width and length. These dimensions represent the outer size of your curb and determine which replacement model will fit correctly.
In the example shown in the video, the measurement is 26½ inches wide by 26½ inches long. Always list the width first, followed by the length (W x L).
A few tips:
- Measure to the nearest ⅛ inch.
- Double-check both directions to confirm the curb is square.
- Write your numbers down immediately to avoid confusion later.
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