Install a Velux or Rooflight Dome: Choose and Fit

Bringing daylight in through the roof is one of the most effective moves you can make to turn a dark room into a bright space: overhead daylight delivers 30 % more luminance than an equivalent vertical window. But depending on whether your roof is pitched or flat, the right tool isn’t the same: Velux for sloped roofs (pitch > 15°), rooflight dome for flat roofs (pitch < 5°). Mix the two and you get water ingress on the first heavy rain. This guide breaks down both solutions, helps you pick the right one for your project, and details the self-build installation of a rooflight dome on a flat roof (the Velux roof window has its own dedicated guide).

Velux or dome: roof pitch decides 90 % of the time

The number one technical criterion is roof pitch. A Velux-type roof window is designed to sit flush in a sloped covering: its flashing seal works by overlap and run-off, exactly like a tile. On a flat roof, water doesn’t run off: it pools during heavy rain, and a Velux fitted on shallow pitch becomes a water ingress reservoir.

Conversely, a rooflight dome (also called skylight, dome rooflight or flat roof rooflight) is designed to sit proud above the roof covering on a kerb upstand of 15 cm minimum, which keeps it clear of any rainwater pool. It stays watertight on flat roofs or very shallow pitches.

Special case: 5 to 15° pitch

On intermediate pitches (low-pitch roofs typical of converted industrial buildings, garages, outbuildings, contemporary extensions), you have two choices:

• Dome on pitched kerb (slope-compensated kerb): the kerb cancels the pitch to bring the dome back level. Standard solution for this kind of roof.
• Low-pitch Velux (GGL model with EDL flashing for metal sheet): possible but with a mandatory raised kit below 15° (Velux ZCE reference, 80-150 €).

Anatomy of a rooflight dome

A rooflight dome isn’t just a transparent dome. It’s a five-layer assembly built up in a precise order, and each layer has a distinct role. Understand the anatomy first, fitting follows naturally.

The dome (the translucent part)

The dome is the visible piece, made of polycarbonate (the standard) or, less commonly, PMMA / acrylic (premium). Three features to check:

• Skin count: single (Uw 5, cheap but condenses), double (Uw 2.5, standard), triple (Uw 1.5, RE2020-grade).
• Shape: pyramidal, hemispherical, flat, faceted. Pyramidal is the most common in housing.
• Tint: opal translucent (diffuses light, removes glare), clear (open view but glares), bronze or grey (filters heat in southern exposure).

The frame (peripheral border)

The frame is a PVC or aluminium border that acts as the watertight junction between the dome and the kerb upstand. It receives the dome fixing screws and the brush or compression seals that ensure airtightness. On opening models, it houses the lifting mechanism (pneumatic ram, electric motor, manual hand-winder).

The kerb upstand (the raise)

This is the critical point of the installation. The kerb upstand is a rectangular box (or round) made of treated timber or metal, fixed to the roof, raising the dome by 15 cm minimum above the finished covering. Its function:

1. Keeps the dome out of water: no puddle can reach the frame
2. Provides surface for waterproofing upstand: the EPDM or bituminous membrane runs up the kerb, never directly onto the dome
3. Receives perimeter insulation (PIR, mineral wool), which avoids thermal bridging
4. Compensates for pitch on low-slope roofs

The waterproofing (the membrane)

The flat roof is sealed by an EPDM membrane (synthetic rubber, 50-year lifespan), a bituminous membrane (twin-layer torched, 25-30 years) or a PVC membrane (welded synthetic, 20-30 years). The membrane runs up all four faces of the kerb by at least 10 cm above the covering level (15 cm in highly exposed zones), then is clamped under the dome frame by an aluminium clamping profile and EPDM seal.

The perimeter insulation

The continuous roof insulation (mineral wool, PIR or XPS, 16 to 22 cm in RE2020 builds) must wrap around the kerb from the outside to avoid thermal bridging: without perimeter insulation, cold penetrates through the kerb, condenses on the inside around the dome, and rots the timber kerb in 5 years.

The four types of rooflight domes

Fixed dome

The simplest, the cheapest. It doesn’t open: pure daylight only. Ideal for circulation areas (stairwell, corridor, central hallway) where overhead light changes everything, with no need for ventilation.

Manual opening dome

Lifted by a telescopic pole or a wall-mounted hand-winder. Allows natural extraction of warm air by stack effect (hot air rises and exits through the top, drawing in fresh air through low openings). Very effective in summer in a kitchen or a bathroom without mechanical ventilation.

Motorised dome

24 V electric ram on mains power or solar autonomous (integrated panel, backup battery). Operated by wall switch, remote control or smart home (Home Assistant, SmartThings). Often paired with a rain sensor that automatically closes the dome during a shower.

Smoke ventilation dome (NSHEV)

NSHEV stands for Natural Smoke and Heat Exhaust Ventilator (in French: DENFC, Dispositif d’Évacuation Naturelle de Fumée et de Chaleur). It’s a motorised dome with rapid opening in case of fire (pneumatic ram fed by a CO2 cartridge, or electric ram with safety power supply). Mandatory in several cases:

• Stairwells of more than 50 m² in collective housing
• Public buildings (ERP) by category
• Storage premises > 300 m²
• Workshops subject to ICPE fire regulations

In a detached self-built home, NSHEV is generally not required. But if you build a large open-plan space (loft, converted workshop), it can be imposed by the safety review when filing your planning permission.

Regulations and planning

New build

If the dome appears on the plans submitted for planning permission, no extra paperwork. Just check:

• Right-of-view rules in the Civil Code (1.90 m straight view, 0.60 m oblique view: rarely a constraint since a dome looks at the sky)
• PLU prescriptions on outside appearance (in protected zones or near a Listed Monument, the architect of historic buildings can require an opal non-protruding dome or fully ban domes)

Renovation or addition

To add a dome to an existing roof, file a planning notice (French Cerfa form n°13703). Processing: 1 month (2 months in conservation areas). Documents required:

• Site plan, block plan
• Roof plan before / after
• Elevation of relevant facades if the dome is visible from the street
• Distant and close-up photos

RE2020 thermal performance

The total glazed area of a RE2020 home must represent at least 1/6 of the habitable floor area (article R162-1 of the French construction code). Domes count toward this ratio. The maximum allowed Uw for a dome in new build is 2.5 W/m².K (vs 1.7 for a Velux roof window and 1.4 for a vertical window). In practice, target Uw ≤ 1.8 to avoid penalising the Bbio coefficient. See our guide understanding RE2020 for the other requirements.

Fitting a rooflight dome on a flat roof: step-by-step method

Fit the dome ideally before final waterproofing of the roof, on a dry, clean substrate. Plan one day for two people for a standard 100 × 100 cm dome.

Step 1: Marking out the trimmer location

Measure the desired position from inside, then transfer it to the roof by four marker holes drilled from inside (drill + 8 mm wood bit) at the four corners of the future opening. On the roof, these four holes mark the inner face of the kerb upstand, not the dome opening: the kerb is 5-7 cm thick, so the hole in the deck will be smaller than the dome.

Step 2: Cutting the substrate

On a timber-framed flat roof, cut the boarding and joists flush with the inner face. On a concrete flat roof, diamond circular saw + breaker (heavy duty, plan 2 days of equipment hire: 80-150 €).

Golden rule: you may cut up to two joists without a structural study. Beyond that, get it validated by a structural engineer (50-200 €) or fit sized lateral trimmers.

Step 3: Building the trimmer

If one or two joists are cut, build a timber trimmer with the same section as the existing joists (typically 75 × 200 mm or 100 × 220 mm), assembled with metal angle brackets (Simpson Strong-Tie ETB, 4-6 € each). The cut joists hang off the trimmer’s high cross-pieces.

Check squareness with a tape (measure both diagonals: must be equal within 2 mm).

Step 4: Setting and fixing the kerb upstand

The kerb upstand (bought prefabricated or built on site from 22 mm OSB boards + PIR insulation) is set directly on the substrate (timber roof or concrete slab) with a 5 cm overhang all around the trimmer opening. It’s fixed by:

• On timber frame: 8-12 coach screws 6 × 80 mm into the joists around the perimeter
• On concrete slab: 8-12 expansion bolts or chemical anchors HILTI HSL or equivalent

Check horizontality with a 60 cm spirit level: if your roof has a 1-3 % drainage slope, the kerb itself will be slightly tilted. For a level dome, buy a slope-compensated kerb (sometimes labelled “compensating kerb”).

Step 5: Perimeter insulation of the kerb

Slide a rigid PIR or high-density mineral wool board (60-80 mm thick) onto the four outer faces of the kerb, flush with its upper face. This detail prevents thermal bridging. Screw the insulation onto the kerb with rosette screws designed for insulation (10 per face).

Step 6: Waterproofing membrane with upstand

This is the most technically delicate step. Depending on your waterproofing type:

• EPDM: roll out the membrane across the whole roof, make an X-cut above the opening, fold the triangles upward and bond them against the vertical kerb faces with EPDM-specific adhesive (Firestone QuickPrime + bonding adhesive). Minimum upstand height 15 cm above the covering level.
• Twin-layer bitumen: the first layer folds up the same way, the second layer is torched (proper training essential, hire of gas torch + bottle 80 €). Better to bring in a professional waterproofer for this part if you’re not trained.
• Reinforced PVC: hot-air welding, specific equipment (700-1,200 € for the unit), outsource it.

Step 7: Fitting and clamping the frame

The pre-assembled frame (supplied with the dome) sits on the upper face of the kerb, over the waterproofing upstand. It’s clamped by:

• 8 to 12 screws through into the kerb (screws supplied)
• A compressible foam tape (in the kit) that ensures airtightness between frame and kerb
• An aluminium clamping profile around the perimeter that pinches the EPDM membrane against the kerb

Step 8: Fitting the dome

The dome clips or screws onto the frame depending on the model. On opening domes, connect the ram (manual or electric) before closing the dome. On motorised domes:

• Run the 24 V electric cable from the consumer unit (usually via a 230/24 V transformer in the loft, 30-50 €)
• Fit a wall switch and optionally a rain sensor on the dome

Step 9: Insulation reinstatement and interior finish

Inside, reinstate the insulation of the rafters or ceiling up to the inner face of the kerb, bonding the vapour barrier to the frame edge with high-Sd adhesive (Sd > 18 m: Pro Clima, Tescon, Siga). Fit a lining: plasterboard, MDF, or factory-supplied lining. The geometry should run straight down to encourage condensation runoff.

Waterproofing weak point: the membrane / kerb junction

In 9 out of 10 dome leaks, the defect is in the same place: the junction between the roof waterproofing membrane and the vertical face of the kerb upstand. Three principles to follow without exception.

1. Minimum upstand height of 15 cm

The waterproofing membrane rises on the kerb by at least 15 cm above the finished covering level (gravel, paving on pads, green roof). In highly wind-exposed zones (>120 km/h, coastal facade), increase the upstand to 20 cm.

2. Never bond directly onto the dome

The waterproofing never touches the polycarbonate dome. It stops on the upper face of the kerb, where it’s clamped by the frame’s aluminium profile. Bonding the membrane onto the polycarbonate expands at different coefficients and ends up debonding within 18-24 months.

3. Mandatory perimeter clamping profile

The aluminium clamping profile (supplied with the frame or bought separately 30-60 €) mechanically pins the membrane to the kerb with screws every 30 cm + foam tape. Without this profile, the membrane moves with thermal cycling, the adhesive seal fatigues and a leak starts at the corner.

Thermal performance: don’t get caught out

A standard double-skin dome has a Uw of 2.2-2.5 W/m².K. Compared to a vertical window (Uw 1.3) or a Velux (Uw 1.2), that’s half as insulating. Three levers to improve this score without breaking the budget.

Triple-skin polycarbonate

Instead of standard double skin, go for a triple-skin dome: roughly Uw 1.5, extra cost 80-150 €. No change to the install, just the model choice.

Insulating glazing replacement

On premium domes, the polycarbonate can be replaced by insulating double or triple glazing (laminated glass + argon spacer). Uw 0.9-1.2, but extra cost of 800-1,500 € over polycarbonate, and trickier install (heavier, requires 3-4 people for handling).

Over-insulated kerb

A 25 cm tall kerb instead of 15, wrapped on all sides with 80 mm PIR, gains 15-20 % perimeter insulation vs a standard kerb. The cheapest option to boost performance. Take advantage if you’re building your own kerb.

Cost of a fitted dome (2026)

Self-build install

Install by waterproofer / roofer

Install only: 400-700 € ex-VAT per dome (1 day, 2 people). VAT 10 % in renovation, 20 % in new build. All-in price:

Self-build saving: 300-1,200 € per dome depending on model. On an extension project with 2-3 domes, the saving reaches 1,000-3,000 €.

Common mistakes to avoid

Decision tree: Velux or dome for your project?

Standards and resources

• DTU 43.1: Waterproofing of flat and pitched roofs with masonry decks (main reference for fitting on concrete slab)
• DTU 43.4: Roofs with timber decks and derived panels (timber flat roof)
• NF EN 1873: Product standard for plastic point rooflights
• NF EN 12101-2: Product standard for NSHEV (smoke ventilation)
• RE2020 (decree 2021-1004): French thermal regulation 2020, energy performance of new buildings
• CSTB: technical approvals on domes and rooflights
• Velux France: flat-roof dome range CFP, CVP, CSP
• Skydome: French manufacturer of domes and rooflights
• Adexsi: NSHEV and smoke ventilation specialist
• Firestone EPDM: EPDM RubberCover technical guide

Pre-installation checklist

Going further

• If you’re hesitating between dome and roof window, also read our guide install a Velux roof window which details fitting on a sloped roof
• Before drilling, validate your roof type: fit a metal sheet roof, build a watertight flat roof or understand waterproofing classes
• For overall thermal performance: understand RE2020 in self-build and insulate sloped ceilings in converted lofts
• For the other openings on the same facade: fit a double-glazed window, install a sliding patio door or fit a conservatory or bioclimatic pergola