Natural Lighting in Large-Span Buildings: Skylight and Polycarbonate Strategies

Lighting is one of the largest controllable energy loads in a warehouse, and daylight is free at the point of use. A well-designed daylighting scheme can cut daytime artificial lighting energy by 60–80%, and in many markets it also earns credits under green building schemes. Done badly, it creates glare, heat gain and roof leaks. The difference is entirely in the specification.

How Much Daylight? The Skylight Ratio

The roof light area is usually expressed as a percentage of the roof plan area. Practical ranges:

• 5–8%: minimum sensible for general warehousing — visible improvement with modest heat gain risk.
• 10–12%: the standard target for distribution centres and factories, giving an average daylight factor around 2–4% under an overcast sky, sufficient for most handling tasks.
• 15%+: only where the roof structure and climate can absorb the solar gain, or where the climate is predominantly overcast.

Daylight factor is measured on the working plane (typically 0.85 m) and varies dramatically with distance from the roof light. Uniformity matters as much as average: patchy daylight is worse than slightly dimmer, evenly distributed light.

Roof Light Types

1. Polycarbonate profiled sheets matched to the roof profile. The cheapest option and easy to install, but typically only 1.0–1.5 mm thick, with poor insulation, no fire rating, and a tendency to discolour and become brittle after 10–15 years. Choose UV co-extruded sheet with a 10-year warranty and never use it where a fire rating is required.
2. Double-skin polycarbonate panels (10–25 mm multiwall). Much better thermal performance (U-value around 2.5–3.5 W/m²K) and less condensation, at higher cost. Good for conditioned buildings.
3. Glass-reinforced polyester (GRP) rooflights with a fire-retardant resin. Better fire performance and UV stability than plain polycarbonate; commonly used in Europe.
4. Insulated rooflights / light panels matching sandwich panel thickness, for cold-climate or conditioned buildings. These maintain the envelope’s insulation line and fire rating.
5. Roof monitors and clerestories (vertical glazing in a raised roof section) admit light at a low angle, reduce overhead solar gain and can be openable for smoke ventilation. They cost more structurally but give the best combination of daylight and heat control.
6. Light tubes / tubular skylights for deep, small areas; excellent for isolated zones but limited in total area.

Placement Rules That Control Glare and Heat

• Place rooflights in rows parallel to the ridge, aligned with bay spacing, so they read as part of the architecture and are easy to flash.
• Concentrate roof lights over aisles rather than over racking, so daylight reaches the working plane.
• In hot climates, reduce or omit rooflights facing the equator and rely on north-facing (or south-facing in the southern hemisphere) monitors.
• Consider diffusing or frosted sheets to spread light and reduce direct glare and shadow patterns on the floor.
• Keep rooflights clear of the sprinkler and smoke vent layout; coordinate at design stage.

Waterproofing: Where Rooflights Fail

1. Order rooflight sheets from the same manufacturer and the same profile as the roof sheet, with matching side laps and fixings.
2. Upgrade the fixing: rooflights need more fasteners than steel sheet, generally at every other corrugation at supports and every third at intermediate purlins.
3. Use gasketed sealing washers and avoid over-tightening, which crushes the sheet.
4. Never cut or drill polycarbonate on a hot day without supporting it; thermal expansion is roughly 3–5 times that of steel.
5. Detail a drip edge and maintain the minimum roof slope; water ponding behind a lap is the usual cause of a “leaking skylight”.

Combine Daylight With Controls

Daylight only saves energy where lighting is controlled. Specify daylight-linked dimming or stepped switching with photocell sensors, and split the lighting circuits into zones parallel to the roof light rows so the innermost zones switch off first. Without controls, a daylit warehouse often runs exactly the same lighting load as a windowless one.

What to Tell Your Supplier

Provide the building dimensions, roof profile, climate, whether the building is conditioned, and the target skylight ratio. Pingchuang will lay out the rooflight positions in the roof sheet and purlin drawings, specify sheet type and thickness, and coordinate with the smoke vent and sprinkler design.

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