You're right, NFPA 72 doesn't specifically address skylights in the way you're describing. However, your analogy to beam pockets is a good starting point. Here's a breakdown of how to approach this, combining code understanding with practical considerations

Understanding the Issue:
Purpose of Smoke Detectors

To detect smoke early and provide a timely alarm.

Smoke Stratification

Smoke rises but can be trapped in certain areas, such as beam pockets, skylight wells, or other recessed spaces. This can delay detection if detectors are only located on the main ceiling.

Obstructions

The skylight well itself could obstruct the free flow of smoke to the corridor's main ceiling-mounted detectors.

NFPA 72 Guidance and Considerations



NFPA 72, Section 17.7.
3.2.5 (and similar sections for other detector types) regarding Beam Construction

This section (and similar sections for other detector types) are frequently interpreted to include similar obstructions. It generally states that detectors need to be placed in each bay formed by beams running the longer direction, if the beam depth exceeds a specific percentage of the ceiling height (typically 4").

Engineering Judgement (Performance-Based Design)

If you believe your corridor detectors adequately cover the area and detect smoke quickly enough, you might be able to argue against detectors in the skylight wells. This would require a strong justification and may need approval from the AHJ. Consider factors like air currents, anticipated fire scenarios, and building materials.

AHJ (Authority Having Jurisdiction)

Ultimately, the AHJ has the final say. It's wise to contact them early in the process to discuss your design and get their perspective on the skylight issue. They may have local interpretations or specific requirements.

Applying it to Your Situation


Treat the Skylight as an Obstruction

Given the dimensions of the skylights (48"x48"x48") and the ceiling height (108"), the skylight depth is a significant portion of the ceiling height (48" / 108" = ~44%). This strongly suggests treating it like a beam pocket for detector placement.2.

Detector Placement within the Skylight



Recommended

The safest and most code-compliant approach is to install a smoke detector within each skylight well. Mount it as high as possible within the well, near the top, without obstructing the skylight itself.

Consideration

The detector location should be accessible for testing and maintenance.

Type of Detector

Use a compatible detector type, likely the same as the ones you're using in the corridor.3.

Alternative: Engineering Judgement and AHJ Approval (More Difficult)



Demonstrate Adequate Coverage

You'd need to prove that the corridor detectors will detect smoke within a reasonable timeframe, even with the skylights obstructing airflow.

Consider CFD (Computational Fluid Dynamics) Modeling

For complex situations, CFD modeling can simulate smoke spread and demonstrate detector effectiveness. This is a more expensive approach.

Early Consultation

Crucially, discuss this approach with the AHJ

before finalizing your design. Get their agreement in principle.

Recommendations



Go with the safe approach

Install smoke detectors in the skylight wells. This is the most defensible approach from a code perspective and provides the best protection.

Document everything

Keep detailed records of your design decisions, including calculations, assumptions, and any consultations with the AHJ.

Consult with the AHJ

This is the most important* step. Get their feedback and approval on your design.

In summary, the most prudent approach is to treat the skylight wells as obstructions similar to beam pockets and install smoke detectors within each well. This ensures better smoke detection and minimizes potential code compliance issues. Flag for review