How to Measure for a VELUX Curb Mounted Skylight Replacement

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.

Even a small measurement error can throw off the fit of your skylight, so accuracy matters.

Step 3: Check the Height of Your Existing Curb

Next, you’ll need to measure the height of your curb. This ensures your new skylight will have enough clearance for proper installation and weatherproof flashing.

VELUX recommends a minimum curb height of 2 inches. Anything shorter can lead to drainage problems or make it difficult to install the new flashing kit.

In Matt’s example, the curb is 3½ inches high, which is perfect. If yours is too short, it’s easy to add an additional layer of wood to raise it to the correct height.

✅ Pro Tip: Check your local building code – in some areas, minimum curb height may vary depending on roof pitch and material.

Step 4: Compare Your Measurements to the VELUX Size Chart

Once you have your curb measurements, the next step is matching them to the correct skylight size using the official VELUX Skylight Size Guide. The key measurement to look for is the Maximum Skylight Clearance.

This number tells you the largest curb dimension a skylight model can fit over. Your existing curb must be the same size or smaller than the maximum clearance listed. If your curb is too large, the skylight simply won’t fit. If it’s too small, the skylight won’t sit properly and won’t flash correctly.

Real Examples of How This Works

Let’s say you measure your curb and get:

• 50 ½” x 50 ½” — This is perfect for any VELUX 4646 model (FCM, VCM, VCE, VCS).
  • The 4646’s maximum clearance is 50 ⅝” x 50 ⅝”, so your curb fits comfortably within that limit.

But if your curb measures:

• 51″ x 51″ — This is too large for a 4646 skylight.
  • Even being ⅜” over the clearance prevents the skylight from sitting properly on the curb.

On the other end of the spectrum, the curb can also be too small:

• 48″ x 48″ — This is too small for a standard 4646 model.
  • The skylight would not have proper bearing on the curb, and the flashing would not install correctly.

What if Your Curb Is Slightly Too Small?

If your curb is a little undersized, you don’t have to rebuild it – you can block it out.

For example:

• Your curb measures 47″ x 47″ (just shy of what’s needed).
• You can “wrap” the curb with standard 2x lumber (which has a 1½” nominal thickness).
• Adding that all the way around increases your curb dimensions by 3 inches, giving you a 50″ x 50″ curb.
• This now fits the 4646 model perfectly.

This is a simple, common solution and helps avoid costly curb reconstruction.

Custom Skylight Sizes

If your curb doesn’t match any standard VELUX size — or if you’re replacing a non-VELUX skylight with unusual dimensions – VELUX also offers custom-size skylights.

Here are the custom-size rules:

• Smallest dimension: 18 ¾”
• Largest dimension: 76 ½”
• BUT both dimensions cannot exceed: 50 ¾” x 50 ¾” at the same time

So, for example:

• A custom skylight 30″ x 70″? ✓ Yes, that’s allowed.
• A custom skylight 60″ x 60″? ✗ Not possible — both dimensions exceed the 50 ¾” limit.

Important to Note About Custom Skylights

• Custom skylights are fixed only — no venting custom sizes are available.
• Shades are now available for custom-size skylights, which was not the case historically.
• Custom units typically take longer to manufacture and ship, so plan ahead if you need one.

Need Pricing on a Custom Size?

We’re happy to provide quotes or help verify your measurements:

📞 Call us: 1-800-900-0759
📧 Email: support@replacementskylights.com

Step 5: Choose Between Fixed and Venting Skylights

Once you know your skylight type and curb size, the next step is choosing between a fixed and a venting model. VELUX offers several options in both categories, available for curb mount and deck mount installations.

Fixed Skylights

A fixed skylight provides abundant natural light without introducing airflow. It’s ideal for spaces where ventilation isn’t needed—such as hallways, stairwells, closets, or any room that could use extra sunlight.

VELUX offers two main fixed skylight types:

• FCM (fixed curb mount) skylights: Designed to fit over an existing curb.
• FS (fixed deck mount) skylights: Installs directly to the roof deck without a curb.

Both are energy-efficient, low-profile, and built to last, providing bright, natural light and reliable weather protection.

Venting Skylights

A venting skylight opens to let in fresh air and release heat and humidity, making it a great choice for kitchens, bathrooms, or living areas.

VELUX venting skylights come in several configurations:

• Curb Mount Models:
  • VCM Series – Manual venting, curb mount.
  • VCS Series – Solar-powered venting, curb mount (includes a solar shade and remote).
  • VCE Series – Electric venting, curb mount.
• Deck Mount Models:
  • VS Series – Manual venting, deck mount.
  • VSS Series – Solar-powered venting, deck mount.
  • VSE Series – Electric venting, deck mount.

💡 Bonus Feature: All VELUX Solar Venting Skylights (VCS and VSS series) include a white, solar-powered, room-darkening shade and a remote control at no additional cost. This gives you effortless light and ventilation control with zero wiring required—making them one of the most energy-efficient skylight options available.

Step 6: Double-Check Before Ordering

Before you finalize your order, take one last look at your measurements and confirm your model using the VELUX size chart. This quick step ensures you’re choosing the exact skylight that will fit over your existing curb.

If you’re unsure or your curb dimensions don’t perfectly match one of the standard sizes, contact our team. We’re happy to help you verify your measurements or identify the right flashing kit for your setup.

Step 7: Order Your Skylight with Confidence

At Replacement Skylights, we make it simple to order genuine VELUX replacement skylights online. You’ll find every standard size, easy-to-follow installation resources, and free shipping to your home.

Whether you’re replacing a leaky skylight, upgrading to solar-powered ventilation, or simply refreshing your home’s natural light, we’re here to make the process easy. Our experts can also help you choose accessories like VELUX replacement flashing kits, shades, and mounting hardware to complete your installation.

Final Thoughts

Measuring for a replacement skylight doesn’t have to be complicated. By confirming your skylight type, measuring from the outside of the flashing, checking your curb height, and matching your numbers against VELUX Curb Mount Size Chart, you’ll know exactly which model will fit your home. Taking the time to measure correctly ensures a watertight seal, better energy efficiency, and a skylight that performs beautifully for years to come.

If you’re still unsure about your measurements, need help choosing between fixed and venting options, or want pricing for custom-size VELUX skylights, we’re here to help.

Whether you’re replacing a leaking unit, upgrading to solar venting, or exploring custom sizes, we’re just a call or email away.

📞 Phone: 1-800-900-0759
📧 Email: support@replacementskylights.com

You can also visit ReplacementSkylights.com to browse all standard sizes, compare models, and order online with free shipping. Let us help you get the right skylight the first time — brightening your home safely, efficiently, and effortlessly.