How to specify rooflights

How do you specify rooflights?

To specify skylights or rooflights on a flat roof, say whether the unit is glass or plastic and cite the standard that covers it, BS EN 1873 for individual plastic rooflights or BS EN 14963 for continuous ones. Then set the safety and performance: a laminated inner pane, a non-fragile classification to ACR[M]001 or the CWCT notes for glass, a whole-unit U-value no worse than the Approved Document L limit of 2.2 W/m²K measured horizontally, a kerb that gives a 150 mm waterproofing upstand, and the external fire performance the roof needs.

A rooflight on a flat roof has two groups of people to protect. Those below need glass that will not fall on them if it breaks, and those above, usually maintenance workers, need a unit they will not fall through if they step or trip onto it. The two are different properties and a laminated inner pane alone does not guarantee both. Add weathertightness at the lowest point of the roof, a thermal weak spot in the insulation and a fire rating for the roof covering, and a rooflight clause carries more risk per square metre than most of the roof.

This guide covers Uniclass Pr_30_59_72_31 Framed rooflights, the code NBS Source uses for fixed flat glass rooflights and kerb-mounted domes. The same group includes Pr_30_59_72_42 Individual rooflights, Pr_30_59_72_17 Continuous rooflights and Pr_30_59_72_77 Skylights. Roof windows in pitched roofs are a different product, covered in the roof windows guide.

What does a rooflight clause need to cover?

Some properties are specific to rooflights. Others belong in every product clause. A clause that leaves either out gives the contractor room to substitute.

Fields marked Never guess depend on the project and should never be filled from a default or a previous job. If the information is not available yet, the clause should say so with a TBC and a reason.

Which decisions does the clause depend on?

Most properties follow from the product you choose. These depend on the project, so they need a decision from the designer before the clause is written.

Which standards should the clause cite?

Cite the product standard the declaration of performance refers to, the code of practice for installation, and the test method behind each value you set. A value without its test method cannot be checked against a submittal.

BS EN 1873

Prefabricated accessories for roofing. Individual rooflights of plastics. Product specification and test methods

Plastic rooflights with upstands, up to 2.5 m span and 3.0 m length, on roofs up to 25°. It does not cover rooflights that contribute to the roof's strength.

In the clause "Plastics rooflight to BS EN 1873. Submit declaration of performance."

BS EN 14963

Roof coverings. Continuous rooflights of plastics with or without upstands. Classification, requirements and test methods

Continuous plastic rooflights such as barrel vaults and ridge lights.

In the clause "Continuous rooflight to BS EN 14963."

ACR[M]001

Test for non-fragility of large element roofing assemblies, accessories and ancillary products

The industry drop test that classifies roof assemblies, including rooflights, as non-fragile Class A, B or C, or fragile. For glass it refers to the CWCT technical notes.

In the clause "Non-fragile to ACR[M]001 for the assembly as installed."

BS EN 12600

Glass in building. Pendulum test. Impact test method and classification for flat glass

The impact classification for each pane, used to check a laminated inner pane offered as a substitute.

In the clause "Inner pane laminated, Class 1(B)1 to BS EN 12600."

BS EN 14179-1

Glass in building. Heat soaked thermally toughened soda lime silicate safety glass. Definition and description

Heat soak testing to reduce spontaneous breakage of toughened panes from nickel sulphide inclusions.

In the clause "Outer pane heat soaked toughened to BS EN 14179-1."

BS EN 13501-5

Fire classification of construction products and building elements. Classification using data from external fire exposure to roofs tests

The external fire classification for roof coverings and rooflights, such as BROOF(t4).

In the clause "External fire performance: BROOF(t4) to BS EN 13501-5."

BS EN 12101-2

Smoke and heat control systems. Natural smoke and heat exhaust ventilators

The product standard for rooflights that also act as smoke vents.

In the clause Only for smoke vents: "Natural smoke and heat exhaust ventilator to BS EN 12101-2."

BS 6229

Flat roofs with continuously supported flexible waterproof coverings. Code of practice

Flat roof design, including the 150 mm waterproofing upstand at abutments and penetrations such as rooflight kerbs.

In the clause "Kerb upstand to BS 6229, not less than 150 mm above finished roof level."

Confirm the current edition of each standard at every issue. Avoice's State of UK Specifications 2026 found a withdrawn standard cited in 40% of specifications at contract issue.

For the regulations behind these choices, see Part L and the Future Homes Standard, Approved Document B explained, or search the Avoice Regulations library.

How do you write it in CAWS?

In CAWS, rooflights are written in L10 alongside windows. NBS Source lists flat glass rooflights and dome rooflights against L10/460 Rooflights, so the type clause covers the unit, its glazing, its kerb, its performance and how it opens, for one rooflight type.

The roof waterproofing upstand and its flashing to the kerb sit in the roofing section, for example J42 for single-ply. The L10 clause and the roofing clause must agree on who supplies the kerb and who turns the membrane up it. The general L10 clauses cover evidence, site dimensions, protection and building in, and the type clause should refer to them.

Clauses the type clause relies on

L10/110

Evidence of performance Non-fragility, U-value and fire evidence for the size and kerb supplied.

L10/120

Site dimensions Kerb or upstand opening checked before units are ordered.

L10/710

Protection of components Units protected and roof openings guarded until the rooflight is fixed.

L10/750

Building in Coordination of kerb, insulation and membrane upstand at the opening.

Clause numbers vary between NBS versions and practice masters. 460 follows the L10 reference NBS Source gives for rooflights, and the general clauses follow typical NBS L10 specifications; check each exists in your own L10.

How do you write it in Uniclass?

Uniclass has a system code for this: Ss_30_30_72_72 Rooflight systems, which also covers dome lights, lantern lights and skylights. The system clause describes the installed rooflight: the opening and trimming, the kerb or upstand, the waterproofing and flashing, and the performance the installed unit must give.

The rooflight unit is the product clause. Putting the kerb, upstand and membrane in the system clause keeps the interface between the rooflight supplier and the roofer in one place, which is where most leaks and thermal bridges on flat roof rooflights start.

In NBS Chorus, Uniclass product clauses also carry an NBS reference (for framed rooflights, 45-25-99/410). Use whichever system your practice's masters use, but keep CAWS and Uniclass references out of the same specification.

What goes in the spec, and what goes in the schedule?

The specification defines each type once. The schedule and drawings say where each type goes.

For coordination between these documents, see Why your window and door schedules don’t match your specification.

What are the common mistakes?

Most problems are a missing property, a property filled with the wrong kind of value, or a decision left silently open.

• !
  Laminated inner pane taken as non-fragile. It protects people below, not above. Ask for non-fragility evidence separately.
• !
  Centre-pane U-value quoted. Approved Document L needs the whole-unit Ud, horizontal, on the developed area. A centre-pane figure flatters the unit.
• !
  Kerb too low once the roof is finished. Paving, ballast or a green roof raises the finished roof level. Measure the 150 mm upstand from the top of the finish.
• !
  Toughened inner pane without heat soaking. A spontaneous break drops glass into the room. Use laminated glass, or heat soak any toughened inner pane after a risk assessment.
• !
  Plastic rooflights near the boundary. Approved Document B limits their size and position. Check before the roof plan is fixed.
• !
  No access for cleaning. Flat glass gathers dirt where water ponds at the edge. Plan how the glass will be cleaned and say so in the O&M information.

How Avoice helps

Avoice drafts rooflight clauses from your roof plans and rooflight schedule, and flags what is still open.

1. 1
   Drafts from your drawings

   It reads rooflight types, sizes and opening types from the roof plan and schedule, and writes the L10 or Uniclass clause with the glazing, kerb, U-value and non-fragility lines your information supports.

2. 2
   Leaves a TBC with a reason

   If the frame colour or the g-value from the overheating assessment is not yet known, the line says TBC with the reason, and the item joins the project TBC register to close before tender.

3. 3
   Writes in your format

   CAWS L10 or Uniclass, built from your practice's own master clauses.

4. 4
   Checks against everything else

   QA/QC flags a rooflight marked as an AOV on the fire strategy with no BS EN 12101-2 line, a kerb height that conflicts with the roof build-up, or a withdrawn standard in a glazing clause.

5. 5
   Keeps product data in one place

   Non-fragility certificates, U-value calculations and fire classifications sit in the Product Room linked to the clause, so a substitute rooflight can be checked against the size and kerb drawn.

Related guides: How to specify roof windows, How to specify single ply roofing membranes, How to specify polycarbonate roofing.

Software can draft and check. The designer still chooses the product and is responsible for the specification.