---

📋 Part of our Skylight Guide

Related: Glass Skylight Systems | Skylight Leaks

A roof skylight is part of the building envelope, not simply a piece of glass inserted into a roof.

Before cutting the roof, the project needs to resolve the structural support, glazing, weatherproofing, thermal performance and operating requirements. The exact requirements depend on the roof construction, skylight type, climate, occupancy and locally adopted building and energy codes.

For U.S. projects, current North American fenestration standards also matter. AAMA/WDMA/CSA 101/I.S.2/A440-26 (NAFS-26) is the latest edition and covers windows, doors, roof windows, tubular daylighting devices and unit skylights.

The practical sequence is:

Define the opening → verify structure → select the skylight → specify glazing → detail the roof interface → verify performance → install according to the approved details.

Quick Summary

The key point: a skylight should be specified as a roof-envelope system, not as a piece of glazing.

---

What Should Be Decided Before Cutting the Roof?

The roof opening should not be sized from the glass alone.

Before construction, define:

• skylight type
• rough opening
• curb or curb-less configuration
• structural support
• roof slope
• roof covering
• flashing system
• glazing configuration
• operating method
• electrical requirements for motorized units
• interior shaft dimensions
• insulation and air-sealing interfaces

A skylight changes the roof structure and weather barrier simultaneously.

That means the opening detail should be coordinated between the roofing contractor, framing/structural team and skylight manufacturer.

---

1. Skylight Framing: The Opening Is a Structural Detail

The skylight frame transfers its loads to the supporting roof structure.

Those loads can include:

• dead load from the skylight
• glass weight
• roof live load where applicable
• snow load
• wind pressure and suction
• maintenance loads where applicable

The required section size therefore depends on the product dimensions and project design conditions.

Do not specify a generic aluminum frame thickness or curb dimension as though it were universal.

For unit skylights, the applicable product standard should be checked against the project requirements. Under the current North American fenestration framework, NAFS-26 includes unit skylights and provides requirements for testing and rating.

For commercial projects, structural design also needs to reflect the applicable adopted building code and site-specific wind, snow and other environmental loads.

---

2. Skylight Glazing: Safety Comes First

Because skylight glazing is installed overhead or at a roof slope, the glazing specification needs to address both impact/breakage safety and structural loading.

The exact permitted glazing depends on the adopted code and skylight configuration.

For example, the current IBC provisions for sloped glazing and skylights permit specified types of glazing including laminated glass, fully tempered glass and heat-strengthened glass, subject to the detailed conditions in the code. The same section also contains requirements related to framing and structural loading.

So the article should not state:

“Every skylight must use laminated glass.”

A more accurate specification approach is:

Use glazing that satisfies the applicable adopted code, tested product configuration and project safety requirements.

Common configurations include:

Laminated glass

Two glass layers are bonded with an interlayer. When fractured, the interlayer helps retain the broken glass.

This can be particularly useful where glass-retention after breakage is important.

Fully tempered glass

Thermally treated glass with increased strength compared with annealed glass. When it fails, it generally breaks into relatively small fragments.

Combination IGUs

A skylight may use different glass constructions on the exterior and interior leaves, depending on structural, safety, solar and thermal requirements.

The correct configuration should come from the actual skylight system and code path rather than a generic material preference.

---

3. Skylight Flashing Is a Roof-Envelope Detail

The skylight itself is only one part of the weatherproofing system.

The critical interface is where:

roof covering + underlayment/membrane + flashing + curb + skylight frame

come together.

The purpose of flashing is to control water around the roof penetration and direct it away from the opening.

A good flashing detail should account for:

• upslope head flashing
• side flashings
• downslope sill/ apron condition
• roof underlayment
• membrane continuity
• drainage
• fastener penetrations
• roof movement
• compatible materials

Current U.S. code provisions for roof assemblies establish flashing requirements and require coordination with the relevant roof construction and manufacturer’s instructions.

This is why it is not enough to specify:

“Provide aluminum flashing.”

The specification should define the complete roof-to-skylight interface.

---

4. Curb Height and Roof Slope

Curb requirements depend on the skylight type, roof slope, occupancy and adopted code.

For example, model residential-code provisions address conditions in which a unit skylight installed on a relatively low-slope roof is mounted on a curb projecting above the roof plane unless otherwise permitted by the manufacturer’s instructions. California’s residential provisions provide one example of this type of rule.

This should not be presented as a universal “all U.S. skylights need a 4-inch curb” rule.

Instead:

Confirm the required curb configuration from the adopted local code, roof system and skylight manufacturer’s installation instructions.

That distinction is important because U.S. building regulation is adopted and amended at the state and local level.

---

5. Thermal Performance: Specify the Rated Product

A skylight is part of the building’s fenestration.

Thermal specification should consider:

• U-factor
• SHGC
• visible transmittance
• glazing build-up
• frame construction
• spacer
• curb/interface
• operating configuration

For the 2024 IECC, fenestration products including skylights have their U-factors determined using NFRC 100. SHGC and visible transmittance are determined using NFRC 200. The IECC also provides default values where labeled values are unavailable.

That means a useful U-factor specification is:

Provide an NFRC-rated skylight meeting the project U-factor requirement.

Not:

“Use Low-E glass.”

Low-E can be part of the solution, but the relevant procurement value is the rated product performance.

NFRC currently certifies residential skylights for U-factor, SHGC, visible transmittance, air leakage and condensation index, and maintains a certified product directory.

---

6. SHGC Matters Too

Skylights receive solar radiation from above, so U-factor alone does not describe their energy performance.

Depending on climate and building design, solar heat gain can materially affect:

• cooling load
• glare
• interior comfort
• daylighting strategy

The 2024 IECC distinguishes U-factor from SHGC and visible transmittance, and NFRC certification provides these ratings for certified products.

A skylight specification should therefore consider:

U-factor + SHGC + VT

rather than treating Low-E as a complete thermal specification.

---

7. Fixed vs. Venting Skylights

A fixed skylight provides daylight without intentional ventilation.

A venting skylight can open to provide outdoor air when the building design permits it.

An operable system may be:

• manually operated
• electrically operated
• remotely controlled
• integrated with building controls

Some products can also include rain sensors or other automatic controls, but these should be treated as product options, not universal skylight requirements.

Ventilation also needs coordination with:

• insect protection
• rain exposure
• interior humidity
• control wiring
• maintenance access
• smoke-control requirements where applicable

A skylight that opens for ventilation should therefore be specified as an operable building component, not simply as a window with a motor.

---

8. Condensation at a Skylight

Condensation is often treated as a glass problem, but the actual issue is surface temperature relative to indoor dew point.

Potential contributors include:

• high interior humidity
• low exterior temperature
• glass U-factor
• frame thermal performance
• spacer
• curb condition
• interior shaft geometry
• air leakage around the opening

Therefore:

Low-E glass does not guarantee zero condensation.

The correct approach is to evaluate the complete installation.

This is particularly important where the skylight is connected to a long insulated roof assembly and the curb creates a local thermal bridge.

---

9. Air and Water Performance

The skylight must perform as a complete product, not just as glazing.

Relevant performance areas include:

• air leakage
• water penetration
• structural load
• operating force where applicable
• thermal performance

The current NAFS-26 framework includes testing and rating provisions for unit skylights and roof windows.

For a project specification, request the actual product rating rather than relying on phrases such as:

“weather resistant”

or

“tested for severe weather.”

A professional submittal should identify the tested/rated product and its applicable performance designation.

---

10. Wind, Snow and Roof Loads

A skylight is exposed to the same environmental forces as the roof.

The design may need to account for:

• positive wind pressure
• wind suction
• snow
• ice
• roof live loads
• thermal movement
• seismic effects where applicable

The required performance therefore depends on project location and roof geometry.

Do not publish a universal:

“Skylight supports X psf wind load.”

That would be meaningless without knowing the product, test size, design pressure, installation configuration and local design criteria.

The correct procurement question is:

What design pressure and environmental loads apply to this opening, and what tested/rated product meets them?

---

Skylight Specification Matrix

---

What a Skylight Manufacturer Should Provide

Before ordering, request more than a product brochure.

Product documentation

• model number
• dimensions
• frame material
• glazing configuration
• operating type
• finish
• performance rating

Structural information

• design pressure
• tested size
• permissible dimensions
• installation configuration
• roof/curb requirements

Thermal information

• U-factor
• SHGC
• VT
• condensation information where available

Installation information

• rough-opening requirements
• curb detail
• flashing sequence
• underlayment/membrane interface
• sealants
• fasteners
• drainage

Compliance documentation

• applicable NAFS rating
• NFRC certification where applicable
• required labels
• test reports
• installation instructions

NFRC maintains a searchable certified-products directory for skylights and other fenestration products, allowing project teams to verify certified product ratings rather than relying solely on manufacturer marketing claims.

---

What Not to Specify

Avoid specifications like:

“Aluminum skylight with tempered glass.”

Too little information.

Or:

“Low-E skylight with flashing.”

Still incomplete.

A better specification identifies:

product + dimensions + loads + glazing + U-factor + SHGC + flashing + curb + operation + applicable ratings + installation details.

That gives the manufacturer and contractor something that can actually be reviewed.

---

Pre-Cut Roof Skylight Checklist

Before cutting the opening, confirm:

---

Frequently Asked Questions

What should I consider before installing a roof skylight?

Define the opening, structure, skylight type, glazing, flashing, thermal performance, roof interface and operating method before cutting the roof. The skylight should be coordinated with the roof assembly rather than treated as an isolated product.

What glass is used in a skylight?

Skylight glazing must comply with the applicable code and product requirements. Depending on the configuration, common solutions include laminated, fully tempered or heat-strengthened glass. The exact permitted construction varies with the adopted code, glazing arrangement and application.

Is laminated glass required for every skylight?

Not universally. U.S. model-code provisions allow several types of glazing for sloped glazing and skylights subject to detailed conditions, so the actual requirement must be checked against the adopted code and project configuration.

Do skylights leak?

They can, particularly when the roof-to-skylight interface, flashing, drainage or installation is improperly designed or executed. The skylight and roof should be treated as one weather-management detail rather than relying on the glazing unit alone.

What is the most important part of skylight flashing?

The complete interface is important: head, sill, side flashings, underlayment or membrane, curb, roof covering and drainage. A single strip of flashing is not a substitute for a coordinated roof-penetration detail.

What U-factor should a skylight have?

There is no single U-factor suitable for every U.S. project. The target depends on the adopted energy code, climate zone, building type and project design. Under the 2024 IECC, skylight U-factors are determined using NFRC 100 where rated values are used.

Does Low-E glass make a skylight energy efficient?

Low-E glazing can improve thermal and solar performance, but it is only one component. The frame, spacer, glass build-up, curb and installation details also affect overall performance.

What is NAFS-26?

NAFS-26 is the 2026 edition of AAMA/WDMA/CSA 101/I.S.2/A440, the North American Fenestration Standard/Specification. It covers windows, doors, roof windows, tubular daylighting devices and unit skylights, and provides performance, testing and rating requirements.

Should a skylight be NFRC certified?

Where the project or applicable energy-code pathway requires rated fenestration performance, NFRC certification provides third-party verified ratings. NFRC’s residential program covers skylights and provides ratings including U-factor, SHGC, VT and condensation index.

Can a skylight be added to an existing roof?

Yes, but the opening must be coordinated with the existing structure, roof covering, insulation, air barrier, flashing and interior finish. A retrofit skylight is a roof-modification project, not simply a product replacement.

Should I choose a fixed or venting skylight?

Choose fixed when daylight is the primary objective. Choose operable when ventilation is part of the building strategy. For motorized units, also coordinate controls, power, maintenance and weather-related operating requirements.

---

Final Takeaway

A good roof skylight specification starts before the roof is cut.

The sequence is:

Structure → opening → skylight system → glazing → thermal performance → flashing → drainage → operation → installation

For U.S. projects, the exact requirements depend on the locally adopted building and energy codes. Current North American product standards also matter: NAFS-26 is now the latest edition of AAMA/WDMA/CSA 101/I.S.2/A440, while the 2024 IECC uses NFRC 100 for U-factor and NFRC 200 for SHGC and VT for fenestration products such as skylights.

The key procurement question is not:

“What skylight should I buy?”

It is:

“What skylight system, glazing, structural rating, flashing detail and energy performance does this roof opening require?”

That is the information that should be resolved before the first cut is made.