Flat Roof Skylights and Rooflights: Upstand Heights, Kerb Details, Planning Considerations and Thermal Performance

Summary

Rooflights are the most effective way to bring daylight into the rooms below a flat roof, and demand for them has increased significantly as loft conversions, kitchen extensions, and garden room additions have become standard domestic projects. However, rooflight installation involves three separate technical disciplines — structural kerb construction, waterproof membrane upstand, and glazed unit thermal performance — and failures in any one of them can create costly problems.

The most common site errors are: undersized upstands that allow ponding water to breach the membrane termination at the kerb; kerbs that are not thermally broken, creating significant cold bridging; and glazed units specified below the minimum U-value required by Building Regulations, which can trigger enforcement action on a replacement installation or affect EPC scores on sale.

This article covers domestic and light commercial rooflights on flat roofs. It is relevant to flat roofers (who install the kerb and waterproof upstand), general builders (who often construct the structural opening and timber kerb), and building control officers assessing Part L compliance. Planning restrictions for conservation areas and listed buildings are covered separately in the planning section below.

Key Facts

• Minimum upstand height — 150mm above finished roof level (FRL) measured to the top of the membrane, per BS 6229:2018 Clause 9.3
• Kerb construction — timber (minimum 47 x 150mm C24 structural softwood) or prefabricated GRP/aluminium proprietary kerb; must be structurally independent of the opening framing
• Kerb height from deck — kerb must achieve the 150mm upstand after allowing for the membrane thickness (typically 4–12mm depending on system); structural deck to top of kerb is commonly 165–175mm total
• Membrane upstand at kerb — same requirements as parapet upstands; minimum 150mm above FRL, terminated under or behind the rooflight frame; EPDM, GRP, or torch-on all acceptable
• Rooflight frame overlap — the frame/kerb cap must overlap the waterproof upstand by minimum 50mm to form a weather rebate; manufacturers specify exact overlap dimensions per product
• U-value for replacement rooflights — ≤1.6 W/m²K (centre-pane) under Approved Document L 2021; this applies to like-for-like replacements as well as new installations
• New-build flat roof rooflight U-value — Part L 2021 (new dwellings) targets 1.6 W/m²K for rooflights; better-performing units contribute to overall fabric energy efficiency target
• Triple-glazed rooflights — achieve 0.8–1.2 W/m²K; significantly better than standard double-glazed (1.5–1.8 W/m²K); recommended for Passivhaus or low-energy new build
• Condensation risk — frames with no thermal break are prone to condensation on the inner frame in cold weather; look for warm-edge spacer bars and thermally broken frame profiles
• Solar gain — south-facing rooflights with no shading increase summer overheating risk; solar control glass (g-value 0.3–0.4) or external blinds recommended for >1m² units facing south
• Structural opening size — trimmer joists required each side of the opening; minimum trimmer depth to match joist depth; engineer's input required for openings >600mm in any existing structural deck
• Permitted development limit — rooflight must not project more than 150mm above the roof plane; applies to Class A permitted development (Part 1 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015)
• Conservation areas — no rooflights permitted on principal elevation roof planes under Class A permitted development in conservation areas; planning application required
• Listed buildings — rooflight installation requires listed building consent regardless of planning status; Conservation Officer approval typically needed
• Access hatch vs rooflight — an access hatch with no glazed element is a structural opening only; Building Regulations Part L thermal requirements still apply to the hatch panel itself (≤1.6 W/m²K recommended)

Quick Reference Table

Detailed Guidance

Kerb Construction: Timber vs Prefabricated

Timber kerbs are built on site from structural softwood. The kerb box sits on the structural deck framing and is constructed before the membrane is applied. Key dimensions: the kerb height above deck surface must be sufficient to achieve 150mm above the finished membrane top — budget 165mm as a working minimum to allow for membrane thickness. The kerb must be constructed from preservative-treated timber (UC4 treatment minimum); untreated softwood will rot within a few years from membrane moisture.

Corner joints on a timber kerb should be half-lapped or mortised, not butted and nailed. Butted corners open as the timber dries and leave gaps in the membrane upstand at the corners — this is one of the most common leak points on site-built rooflights.

A vapour control layer (VCL) should be carried over the top of the kerb box before the membrane upstand is applied. Without this, warm moist air from the building interior can enter the kerb box, condense on the cold rooflight frame, and drip into the room below.

Prefabricated GRP or aluminium kerbs are manufactured to standard or bespoke sizes. They arrive pre-formed with upstand flanges designed to receive specific membrane systems. GRP kerbs are thermally inferior to timber (higher conductivity) but dimensionally stable and quick to install. Aluminium kerbs require a thermal break between the inner and outer sections — without this, the aluminium conducts cold directly through to the interior frame.

Prefabricated kerbs typically include a pre-formed corner, eliminating the corner joint failure mode of site-built timber. They are the preferred option on commercial and larger domestic projects.

Membrane Upstand at the Kerb

The membrane upstand at a rooflight kerb follows the same principles as a parapet upstand but requires careful detailing at the four corners. Inside corners (deck-to-vertical) are the most vulnerable points.

For EPDM systems: pre-moulded inside corner pieces are applied first, bonded with contact adhesive; the field sheet is then lapped over them with minimum 100mm overlap. The upstand sheet is bonded to the kerb face and terminated under the frame sill using a termination bar. Pre-formed outside corners are used where the kerb top changes direction.

For GRP systems: the laminate is applied continuously from the flat field up the kerb face, using a cove fillet (minimum 50 x 50mm, 45° angle) at the base junction to eliminate the right-angle fold. The laminate is turned over the top of the kerb and trimmed flush with the inner edge. The rooflight frame is then set on top and sealed with low-modulus silicone.

For torch-on felt systems: the base sheet is turned up the kerb face and nailed through a timber batten; the cap sheet is then turned up in the same way. All four kerb corners must be cut and lapped — this is technically demanding and is a frequent source of callbacks on poorly executed felt work.

Thermal Performance and U-Values

The thermal performance of a rooflight is measured as the U-value of the complete unit (frame and glazing combined), sometimes called the Uw value. Centre-pane U-values (Ug) are always lower than the whole-unit value and can be misleading in marketing materials.

For Building Regulations compliance under Approved Document L (2021), the relevant value is the whole-unit U-value. A replacement rooflight that achieves ≤1.6 W/m²K satisfies the regulations as a like-for-like replacement. A new installation in a new dwelling must be included in the SAP calculation — a worse-performing unit will require compensating improvement elsewhere in the fabric.

Condensation on the inner face of the glazing (surface condensation) is prevented by:

1. Warm-edge spacer bars (low-conductivity polymer spacers between the glazing panes) — reduce edge condensation
2. Thermally broken frame profiles — prevent the outer frame temperature transmitting to the inner face
3. Adequate room ventilation — high humidity rooms (bathrooms, kitchens) with poor ventilation will cause condensation on even well-specified glazing

Interstitial condensation within the kerb assembly is prevented by a correctly positioned VCL on the warm side of the insulation, as with the main roof build-up.

Planning and Permitted Development

Standard residential properties in England — permitted development under Class A of Part 1, Schedule 2 of the GPDO 2015 allows rooflight installation on flat roofs subject to conditions:

• The rooflight must not project more than 150mm above the existing roof surface
• The development must not result in the highest part of the rooflight being higher than the highest part of the existing roof
• Does not apply to the principal elevation (front of house) facing a highway

Conservation areas — Class A permitted development rights are removed for roof alterations visible from a highway. A householder planning application is required. Conservation Officers vary significantly in their approach; some will accept flush or near-flush rooflights, others will refuse any flat roof rooflight on principal elevations. Pre-application advice is strongly recommended before specifying.

Article 4 Directions — local planning authorities can remove permitted development rights through Article 4 Directions in specific areas. This is common in sensitive urban areas beyond conservation areas. Check with the local planning authority before assuming permitted development applies.

Listed buildings — any alteration to a listed building, including rooflight installation on outbuildings or extensions, requires listed building consent (separate from and in addition to planning permission). The bar for approval is high; Conservation Officers generally prefer reversible installations with minimal visible impact.

Wales, Scotland, Northern Ireland — separate planning and building regulation frameworks apply. The permitted development limits in these nations differ from England in detail. Always verify against the specific national framework.

Common Sizes and Rough Opening Dimensions

Rooflights are sold by external kerb size or by the glazed area. Rough opening sizes (the hole in the structural deck) are typically 25–50mm smaller than the external kerb dimension on each side to allow for structural trimming.

Common domestic sizes (external kerb dimensions):

• 600 x 600mm (single, small bathroom/hallway)
• 600 x 900mm (standard single)
• 1000 x 1000mm (popular square format)
• 1000 x 2000mm (common walk-on or feature unit)
• 2000 x 2000mm (large feature unit — structural engineer input always required)

Walk-on rooflights (trafficable glazing) must meet BS EN 1991-1-1 imposed load requirements and are typically specified with toughened laminated structural glass. These require a minimum 6.8mm laminated inner pane. Their U-values are typically worse than standard units due to thicker glass and deeper cavities.

Frequently Asked Questions

Does my rooflight installation need building regulations approval?

Yes, in almost all cases. A new rooflight opening in a structural roof requires Building Regulations approval under Part A (Structure — for the trimming of the structural deck) and Part L (Thermal — the unit must meet U-value requirements). Replacing a like-for-like rooflight with a compliant unit can be self-certified by a competent person registered with a scheme such as FENSA (for windows and rooflights). If you are not registered, you must notify Building Control.

What is the difference between an upstand rooflight and a walk-on rooflight?

An upstand rooflight sits on a raised kerb and projects above the roof surface — these are the most common type for residential flat roofs. A walk-on (or trafficable) rooflight is set flush or near-flush with the roof surface and is designed to bear foot traffic loads. Walk-on units use structural laminated glass, are significantly more expensive, and have higher U-values. The framing and kerb details differ substantially; they should not be substituted for one another.

My rooflight is leaking. Is it the membrane or the frame?

The most common leak source is the membrane upstand at the kerb, not the sealed unit itself. Run a garden hose around the base of the kerb (not the frame) for 15 minutes — if water appears inside, the upstand is the cause. If the base of the kerb is dry and water appears when you direct the hose to the frame/glazing interface, the frame seal or the frame-to-kerb joint is the source. Sealed unit failure (internal misting) is a warranty issue with the glazing manufacturer and is not a structural leak.

Can I install a rooflight in a cold flat roof?

Yes, but the condensation risk is higher than in a warm flat roof build-up. In a cold roof, the insulation is between the joists, below the deck, meaning the structural deck and kerb are cold. The kerb box should be insulated internally (PIR board inside the kerb box walls) and a VCL applied on the warm side before the membrane. Cold flat roofs with rooflights should always have adequate cross-ventilation in the cold void — minimum 25mm clear air path maintained. See flat roof insulation for cold vs warm roof build-up details.

How do I size the structural opening?

For openings up to 600mm wide in a domestic timber deck, trimmers each side of the opening (same section as the existing joists) are typically adequate. For openings 600mm–1200mm, doubled or increased-depth trimmers are required. For openings over 1200mm, or where the opening runs parallel to span direction, a structural engineer should be consulted. The structural trimming frames the rough opening; the kerb box is then built within the rough opening and is not itself structural.

Regulations & Standards

• BS 6229:2018 — Flat roofs with continuously supported flexible waterproof coverings; upstand requirements at rooflight kerbs follow the same 150mm rule as parapets

• Building Regulations Approved Document L (2021 edition) — Part L1A (new dwellings) and Part L1B (existing dwellings); sets U-value requirements for rooflight installations and replacements

• GPDO 2015 Schedule 2 Part 1 Class A — Permitted development rights for rooflight installation on residential flat roofs in England

• BS EN 1991-1-1 — Eurocode 1: Actions on structures; Part 1-1 General actions — Densities, self-weight, imposed loads; relevant for walk-on rooflight structural glass sizing

• Planning Policy Guidance (England) — MHCLG guidance on permitted development and conservation area restrictions

• NHBC Standards Chapter 7 — Covers rooflight kerb construction requirements in NHBC-registered new build dwellings

• Approved Document L 2021 — MHCLG; thermal performance requirements for windows and rooflights

• Planning Portal Permitted Development Guidance — Summary of Class A permitted development for householders

• NFRC Rooflight Guidance — National Federation of Roofing Contractors technical guidance

• BS 6229:2018 — BSI; flat roof code of practice including upstand requirements

• flat roof membrane types — membrane choice affects kerb upstand detailing method

• flat roof insulation — cold vs warm roof affects condensation risk at rooflight kerb

• flat roof parapet detailing — upstand principles are shared between parapets and rooflight kerbs

• building regs part l — thermal compliance context for rooflight U-value requirements

• flat roof falls and drainage — falls away from rooflight kerbs are essential to prevent ponding at the upstand