Dome vs Flat Skylights:
Aerodynamics That Decide Your DP Rating
Skylight shape fundamentally changes how wind interacts with your roof opening. In Palm Beach County, where design wind speeds reach 170 MPH along the coast, the difference between a dome and a flat profile can mean a 15-30% swing in required design pressure. Understanding the aerodynamic coefficients, deflection limits, and testing standards is the difference between a skylight that survives a hurricane and one that becomes a projectile.
Wind Tunnel Smoke Trail Simulation
Watch how airflow behaves differently over dome and flat skylights. Smooth laminar flow over the dome contrasts sharply with turbulent vortex separation over the flat profile.
Why Shape Determines Your Pressure Coefficient
ASCE 7-22 Figure 30.3-2A assigns external pressure coefficients (GCp) based on skylight geometry, roof zone, and effective wind area. Dome shapes earn a measurable aerodynamic advantage.
Dome Skylight Aerodynamics
Curved surfaces allow incoming wind to accelerate smoothly over the profile without abrupt separation. The attached boundary layer generates lower peak suction at the apex. For skylights with a rise-to-span ratio of 0.2 or greater, ASCE 7-22 permits a reduction factor on GCp values. Wind tunnel studies consistently show dome peak negative pressures 15-30% lower than equivalent flat units in the same roof zone. The membrane action of the curved shell also distributes loads more uniformly to the curb, reducing point stress concentrations.
Flat Skylight Aerodynamics
A flat profile creates a sharp leading edge where wind separates abruptly, forming turbulent vortex structures on the leeward side. These recirculation zones generate intense localized suction that exceeds the average pressure across the surface. Corner and edge zones amplify this effect dramatically. ASCE 7-22 assigns GCp values up to -2.2 for flat elements in roof corner zones (Zone 3). The flat glass must resist this suction purely through bending stiffness, creating deflection concerns in larger spans and demanding thicker laminate assemblies for the same opening size.
GCp Values by Roof Zone — Dome vs Flat
Pressure coefficients from ASCE 7-22 for component and cladding loads on skylights in Palm Beach County at 150-170 MPH design wind speeds.
| Roof Zone | Dome GCp (neg) | Flat GCp (neg) | Dome DP (psf) @ 170 MPH | Flat DP (psf) @ 170 MPH | Dome Advantage |
|---|---|---|---|---|---|
| Zone 1 (Field) | -1.4 | -1.8 | -62.8 psf | -80.8 psf | 22% less |
| Zone 2 (Edge) | -1.6 | -2.0 | -71.8 psf | -89.7 psf | 20% less |
| Zone 3 (Corner) | -1.7 | -2.2 | -76.3 psf | -98.7 psf | 23% less |
Calculation Basis
Values based on qh = 44.85 psf (170 MPH, Exposure C, mean roof height 25 ft, Risk Category II). Net pressures include internal pressure coefficient GCpi = +/-0.18 for enclosed buildings. Actual pressures vary by building height, exposure, and enclosure classification. Use WindLoadCalc for project-specific numbers.
Flat Glass Deflection: The Hidden Failure Mode
Flat skylight glazing resists wind pressure entirely through bending. Under negative (suction) loading, the glass bows outward. ASTM E1300 limits this deflection to L/60 of the short span for insulating glass units (IGUs). For a 48-inch wide skylight, that means a maximum outward bow of just 0.8 inches at full design pressure.
In Palm Beach County's 170 MPH coastal zone, flat skylights in roof Zone 3 can see design pressures exceeding -98 psf. At these loads, standard 1/4" laminated glass deflects well beyond acceptable limits. The result is insulating glass seal failure, moisture infiltration between panes, and progressive degradation that turns a "passing" installation into a warranty claim within 3-5 years.
- L/60 deflection limit per ASTM E1300 for IGUs
- PVB interlayer reduces effective stiffness by 15-20%
- Edge seal strain increases exponentially with deflection
- Repeated cyclic loading (UL 1897) accelerates seal fatigue
- Dome curvature converts bending into membrane compression
Deflection at Design Pressure (48" span)
At -80 psf. Values per ASTM E1300 with load duration factor = 1.0.
Curb Height Requirements & Condensation Management
The curb is the structural bridge between your roof membrane and the skylight assembly. Its height and detailing directly affect water resistance, thermal performance, and uplift capacity.
Minimum Curb Heights
Florida Building Code Section 2405.5 mandates a minimum 4-inch curb above the finished roof surface for residential skylight installations. Commercial projects follow NRCA guidelines requiring 8-inch minimum curbs. For Palm Beach County WBDR locations, experienced contractors spec 12-inch curbs to keep wind-driven rain below the gasket line during sustained 100+ MPH winds. Every additional inch of curb height reduces water intrusion risk by approximately 15% during oblique wind-rain events.
Dome Condensation Path
Dome skylights exploit gravity and surface geometry for passive condensation management. Moisture forming on the inner acrylic surface migrates downward along the curve, collecting at the curb perimeter. Factory-integrated weep slots in the curb flashing channel this collected moisture to the exterior roof surface. In Palm Beach County's 76% average relative humidity, this passive drainage handles condensation without any interior gutter hardware. Double-wall and triple-wall acrylic domes further reduce interior surface condensation by maintaining warmer inside temperatures through the insulating air gap.
Flat Condensation Trap
Flat skylights create a horizontal collection surface where condensation pools directly above occupied space. Without active management, this pooled moisture drips onto floors, furniture, and equipment. Frame manufacturers machine perimeter condensation gutters into aluminum extrusions to capture runoff, routing it through weep holes. However, these channels have finite capacity. During Palm Beach County's summer months with sustained dew points above 75 degrees F, flat skylights without thermal break frames produce condensation volumes that overwhelm standard 3/8-inch gutters within 6-8 hours of continuous AC operation.
ASTM E330 — Structural
UL 1897 — Wind Uplift
UL 1897 vs ASTM E330: Both Are Required
Palm Beach County building officials require both ASTM E330 and UL 1897 test reports as part of the skylight product approval documentation. These two standards measure fundamentally different failure modes, and passing one does not guarantee passing the other.
ASTM E330 applies a single sustained static pressure at 1.5 times the rated design pressure (DP) for 10 seconds. This simulates the worst peak gust a skylight might experience. The assembly must show no structural failure and no permanent deformation after load release. A skylight rated DP-60 must survive 90 psf sustained pressure without breaking.
UL 1897 simulates the cyclic nature of actual hurricane winds. Pressure oscillates between positive and negative values at increasing intensities, stressing gaskets, seals, fasteners, and frame joints repeatedly. A product that passes the single static ASTM E330 test can fail UL 1897 if fasteners loosen under cyclic fatigue or if gaskets creep under repeated compression-release cycles. This is particularly relevant for flat skylights, where the glass flexes with each pressure cycle and progressively strains the edge seal.
Palm Beach County Submission Requirement
Include both ASTM E330 and UL 1897 test reports in your permit package along with FL product approval number, Miami-Dade NOA (if in HVHZ adjacent area), and manufacturer installation instructions with your specific curb detail.
Hail Resistance Ratings: Shape Matters More Than You Think
While Palm Beach County is not a primary hail zone, severe thunderstorms routinely produce 1-2 inch hailstones. Insurance carriers increasingly require hail resistance documentation for skylight approvals.
| Skylight Assembly | Profile | Max Hail Diameter | FM 4431 Class | Insurance Acceptance |
|---|---|---|---|---|
| Polycarbonate Multi-Wall Dome | Dome | 2.0" | SH (Severe Hail) | Preferred |
| Acrylic Double-Wall Dome | Dome | 1.75" | SH | Accepted |
| Laminated HS + PVB + Tempered | Flat | 1.75" | MH (Moderate) | Case-by-Case |
| Tempered Monolithic Glass | Flat | 1.25" | No Rating | Often Denied |
| Laminated (Annealed + PVB) | Flat | 1.0" | No Rating | Denied |
Why Domes Resist Hail Better
The curved geometry of dome skylights deflects hailstone strikes at oblique angles rather than absorbing the full perpendicular impact force. A 2-inch hailstone hitting a dome at a typical 15-25 degree angle from perpendicular transfers approximately 35% less kinetic energy compared to a dead-center strike on a flat surface. Additionally, polycarbonate and acrylic dome materials are inherently more impact-absorbing than glass. Polycarbonate multi-wall domes rated to FM 4431 Severe Hail (SH) classification withstand 2-inch hailstones at terminal velocity without penetration, cracking, or dislodgement from the curb frame.
Flat Glass Hail Vulnerability
Flat glass skylights absorb hail impact perpendicular to the surface, concentrating all kinetic energy into a single point. Tempered glass, despite its strength, shatters completely when the surface compression layer is penetrated by a hailstone exceeding 1.25 inches diameter. Laminated glass with a heat-strengthened outer lite performs better, containing fractures within the PVB interlayer, but the outer lite still cracks visually. For Palm Beach County installations where insurance coverage requires hail resistance documentation, flat skylights typically need sacrificial outer lites or protective screens that add cost and reduce light transmission by 8-12%.
Skylight Permit Documentation Checklist
The Palm Beach County Building Division requires specific documentation for all skylight installations. Missing any item delays your permit review by a minimum of 2 weeks.
Product Approval
Florida Product Approval (FL number) or Miami-Dade NOA showing the skylight assembly meets FBC 2023 requirements. Must list specific model, size range, DP rating, and impact classification. Verify the approval covers your exact installation configuration including curb type, glazing makeup, and frame material. Expired or revoked approvals are rejected without notification.
Wind Load Calculations
Site-specific wind load analysis per ASCE 7-22 showing the required DP for your skylight location. Must identify roof zone (1, 2, or 3), effective wind area, exposure category, and building enclosure classification. The calculated DP must not exceed the product's rated DP from the approval document. Calculations must bear the seal of a Florida-licensed Professional Engineer for commercial projects.
Installation Details
Manufacturer's installation instructions with curb detail, flashing sequence, and fastener schedule. Must show structural attachment of curb to roof framing, continuous load path from skylight through curb to building structure. Include ASTM E330 and UL 1897 test reports. For roof zones 2 and 3, additional engineering for curb anchorage may be required. Include roofing membrane termination detail at curb perimeter.
Frequently Asked Questions
Technical answers to the most common skylight wind load questions specific to Palm Beach County installations.
Why do dome skylights perform better in high wind than flat skylights?
What curb height does Palm Beach County require for skylights?
How does ASTM E330 testing differ from UL 1897 for skylights?
Do skylights in Palm Beach County need impact-rated glazing?
What is the maximum flat glass skylight deflection allowed under wind load?
How does condensation management differ between dome and flat skylights?
What hail resistance rating should skylights have in Palm Beach County?
Get Your Skylight Wind Load Calculation
Know your exact design pressure requirements before selecting dome or flat skylights for your Palm Beach County project. Avoid permit delays with accurate ASCE 7-22 calculations.