Rooflights for Steel Buildings | Polycarbonate Specification

Q: What is a barrel vault rooflight?

A barrel vault rooflight is a curved GRP panel shaped to match the adjacent metal roof sheet profile. It sits in the roof sheet run between standard purlins, providing natural light without a separate framing system.

Q: What percentage of a portal frame roof should be rooflights?

Common UK practice is 5-10% of roof area. CIBSE LG10 recommends approximately 5% for most industrial buildings. Higher rooflight area requires compensating insulation improvement elsewhere for Part L compliance.

Q: What is the difference between TP(a) rigid and TP(b) fire classifications for rooflights?

TP(a) rigid materials show no sustained flaming or burning droplets and have no area restriction in most building classes. TP(b) materials have area restrictions under Approved Document B. Polycarbonate multiwall 16mm+ typically achieves TP(a) rigid.

Q: Can polycarbonate multiwall be used as a rooflight in a composite panel roof?

Yes, but rooflight bays need specific framing as composite roofs have no continuous purlin below the panel run. Some composite panel manufacturers provide purpose-designed rooflight kerb assemblies.

Rooflights for metal cladding roofs

Rooflights are a standard element of UK portal frame roofs, sitting within the metal sheet runs to provide natural daylighting. The two main types are GRP barrel vault rooflights (matching the profile of the adjacent metal sheet) and polycarbonate multiwall flat sheets (used in special glazing bars or carrier systems). All light transmission and U-value figures shown are indicative — verify from current manufacturer data sheets before specifying.

GRP barrel vault Polycarbonate multiwall Fire classifications Design guidance

GRP barrel vault rooflights

GRP (glass-reinforced polyester) barrel vault rooflights are curved to match standard metal sheet profiles. They are laid directly in the roof sheet run with no special framing required, making them the most common rooflight type in agricultural and industrial portal frame buildings.

Manufacturer / Product	Widths (mm)	Light transmission — opal (%)	Light transmission — clear (%)	U-value single skin (W/m²K)	U-value double skin (W/m²K)	Fire (single)	Compatible profiles
Brett Martin BarrelVault GRP rooflight	600, 900, 1000	35	75	5.7	2.3	TP(b) single skin; TP(a) rigid double skin options available	1000mm cover-width box profiles, 1000mm trapezoidal; 900mm for some corrugated runs
Hambleside Danelaw SkyGlaze GRP barrel vault	600, 1000	38	72	5.7	2.4	TP(b) single; TP(a) rigid double skin	1000mm cover-width box and trapezoidal profiles
Xtralite GRP barrel vault rooflight	600, 900, 1000	37	74	5.7	2.3	TP(b) single; TP(a) rigid double	Designed to coordinate with standard 1000mm metal cladding profiles

Polycarbonate multiwall sheets

Polycarbonate multiwall sheets offer significantly better thermal performance than single-skin GRP, making them the preferred rooflight type in heated buildings. They are used flat in purpose-made carrier systems or on site-formed curved mounting rails.

Manufacturer / Product	Thickness (mm)	Construction	U-value (W/m²K)	Light transmission (%)	Weight (kg/m²)	Fire rating
Brett Martin Sunpal multiwall polycarbonate sheet	10	Twin wall	2.9	75	1.7	TP(a) rigid (16mm+); TP(b) (10mm)
	16	Twin wall	1.9	72	2.3
	25	Three wall	1.4	67	3.1
	35	Five wall	1.1	62	4
Palram Palsun polycarbonate sheet	10	Twin wall	2.9	74	1.7	TP(a) rigid (16mm+)
	16	Twin wall	1.9	70	2.3
	25	Three wall	1.4	66	3.1

Corrugated polycarbonate

Corrugated polycarbonate sheets with the same pitch geometry as steel corrugated sheeting — used as a direct drop-in rooflight in agricultural and light-industrial corrugated roof runs.

Product	Pitch (mm)	Cover width (mm)	Thickness (mm)	U-value	Light trans. clear	Light trans. opal	Fire
Brett Martin — Corrugated PC rooflight	76.2	840	0.8	5.5	80%	55%	TP(b)

Fire classifications for rooflights

Classification	What it means
TP a rigid	TP(a) rigid — thermoplastic materials that when tested show no sustained flaming and no flaming droplets. Suitable for use in roofs without area restriction in most classifications.
TP b	TP(b) — thermoplastic material with limited flame spread. Area restrictions apply in buildings under the Building Regulations.
grpClass3	GRP (glass-reinforced polyester) single-skin rooflights are typically Class 3 to BS 476 Part 7. Double-skin with gel coat may achieve Class 1.

Design guidance

Condensation risk — single-skin GRP: Single-skin GRP rooflights have U-values of ~5.7 W/m²K — significantly lower thermal performance than the surrounding insulated roof. Condensation on the underside is common in heated buildings. Double-skin or polycarbonate multiwall is preferred in buildings with high humidity. 16mm twin-wall polycarbonate at U=1.9 W/m²K is a much better thermal match to an insulated composite panel roof.

Rooflight area: No single fixed UK regulation, but common design practice is 5–15% of roof area as rooflights. CIBSE LG10 recommends 5% for most industrial buildings. Rooflights must be accounted for in Part L compliance calculations — U-value and area both contribute to the building's thermal envelope. Higher rooflight area typically requires higher insulation elsewhere.

Common questions

What is a barrel vault rooflight?

A barrel vault rooflight is a curved GRP panel shaped to follow the cross-section profile of the adjacent metal roof sheet. It sits in the roof sheet run, secured between standard purlins or on purpose-made fixing bars, and provides natural light through the roof without a separate framing system. The barrel profile sheds rainwater and provides some additional stiffness over a flat panel.

What percentage of a portal frame roof should be rooflights?

There is no single fixed requirement, but common UK practice is 5–10% of the roof area as rooflights. CIBSE LG10 recommends approximately 5% for most industrial buildings to achieve adequate daylighting. The Building Regulations Part L require rooflights to be included in the thermal envelope calculation — higher rooflight area generally increases the demand for insulation elsewhere in the building envelope.

What is the difference between TP(a) rigid and TP(b) fire classifications?

TP(a) rigid thermoplastic materials show no sustained flaming and no flaming droplets when tested, and can be used in roofs without area restriction in most building classifications under the Building Regulations. TP(b) materials have limited flame spread but can produce burning droplets — area restrictions apply. Most polycarbonate multiwall sheets of 16mm or greater achieve TP(a) rigid. Single-skin GRP is typically TP(b) or Class 3. Always confirm from the manufacturer's current certification and check Approved Document B for your building use class.

Can polycarbonate multiwall be used as a rooflight in a composite panel roof?

Yes, but the installation is different from a profiled metal roof. Composite panel roofs do not have a continuous purlin below the panel run, so rooflight bays need to be specifically designed with framing to support the polycarbonate sheet edges. Some composite panel manufacturers provide purpose-designed rooflight kerb assemblies. Alternatively, GRP barrel vault rooflights can be used in composite panel systems with specialist kerb details — check with your cladding manufacturer for their approved rooflight detail.