When a low-slope commercial roof receives additional insulation, the finished roof surface becomes higher. This can create a problem around existing skylights: the skylight curb may no longer extend high enough above the new roof surface to provide adequate flashing height and water protection. In these situations, skylight curbs are typically raised or extended to maintain the required clearance above the finished roof system.
Why Does a Skylight Curb Need to Be Raised?
A skylight curb separates the skylight from the roof assembly and provides the vertical surface needed to install flashing. When insulation is added above the roof deck, the membrane or other finished roofing material moves upward. If the existing curb stays at its original elevation, less of the curb remains exposed above the roof.
This reduces available flashing height and can increase the risk of water intrusion. The National Roofing Contractors Association (NRCA) has traditionally recommended maintaining approximately 8 inches of base flashing height above the finished roof surface for skylight curb installations.
For this reason, increasing insulation thickness should be evaluated together with the existing skylight curb height rather than treating insulation and skylight work as separate items.
How Is an Existing Skylight Curb Raised?
The exact method depends on the skylight design, existing curb construction, roofing system, and amount of additional insulation. A common approach is to remove or temporarily detach the skylight, extend the existing curb, and reinstall the skylight at the new elevation.
For a wood curb, contractors may add properly sized wood framing or blocking above the existing curb. The extension is securely fastened to the existing structure and detailed so that the skylight frame remains properly supported.
For a manufactured metal curb, a compatible curb extension or replacement curb may be fabricated or supplied by the manufacturer. The new curb height is coordinated with the insulation thickness, roofing membrane, flashing requirements, and skylight dimensions.
Building Science guidance for adding rigid foam insulation above a roof deck specifically recommends removing the skylight and replacing or extending the existing curb to accommodate the insulation thickness, ventilation space if applicable, additional sheathing, and the minimum curb height required by the roofing manufacturer.
What Happens During a Roof Replacement?
During a commercial roof replacement, the contractor first determines the final elevation of the new roof assembly. This includes the roof deck, insulation, tapered insulation where applicable, cover board, membrane, and other roofing components.
The curb extension is then designed so the skylight remains sufficiently elevated after all roofing layers are installed. The roofing membrane is properly flashed up the extended curb, and the skylight frame or counterflashing is reinstalled to create a weather-resistant transition.
This is particularly important when tapered insulation is used. Insulation thickness can vary across a low-slope roof to create positive drainage, meaning the required curb height may need to account for the highest finished roof elevation near the skylight. NRCA guidance notes that curb dimensions need to accommodate varying insulation thicknesses commonly used for tapered insulation systems.
Can Insulation Simply Be Added Without Raising the Curb?
Sometimes, but it should not be assumed. Adding insulation without modifying the curb reduces the amount of exposed curb above the new roof surface. Energy-code guidance also recognizes that adding insulation can reduce base flashing height and recommends evaluating whether curb heights should be increased to maintain appropriate flashing dimensions.
In some projects, the roofing manufacturer may approve an alternative detail or variance, but this should be confirmed before installation because inadequate flashing can affect waterproofing performance and potentially roofing warranty requirements.
The Bottom Line
When roof insulation becomes thicker, skylight curbs are generally raised by extending the existing curb or replacing it with a taller curb. The new height should be calculated from the final finished roof elevation—not simply from the roof deck—and should provide sufficient space for insulation, roofing materials, flashing, and proper skylight attachment.
For commercial buildings undergoing reroofing, insulation upgrades, or roof restoration, evaluating skylight curb height before work begins can prevent flashing problems, water intrusion, and costly modifications later. A qualified commercial roofing contractor should coordinate the curb extension with the skylight manufacturer, roofing manufacturer, project specifications, and applicable building requirements.
Related Questions
- How are ductwork and equipment supports raised when insulation is added?
- How does existing equipment height limit insulation thickness decisions?
- What is the difference between a curb-mounted and deck-mounted skylight?
- What flashing plies are required at walls and curbs?
- What is required to properly abandon and infill an unused curb?
- Can skylights be permanently removed and infilled during a reroof?
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