Roof Lantern Installation

Quick Answer: A roof lantern is a glazed pyramid or hipped-glass structure set on a flat roof, typically with an aluminium or aluminium-clad-timber framework, sealed double or triple glazed units, and a thermal-broken upstand kerb to comply with Approved Document L. Modern installations must achieve a maximum U-value of 1.5 W/m²K (whole rooflight) under Approved Document L1B for replacements, 1.4 W/m²K for new dwellings under L1A. Thermal expansion and timber kerb/glazing interfaces are the key technical risks; a 1m² lantern can move by up to 6mm under summer/winter cycling.

Summary

Roof lanterns sit between two skill sets: glazing fitter and flat roofer. The lantern itself arrives as a factory-built aluminium or hardwood frame with sealed glazing units bonded to it; the installation work is mostly preparing the kerb upstand on the roof, weather-sealing the lantern-to-roof interface, and ensuring the structural opening is correctly designed. Done correctly, the lantern is a 30–50 year fixture. Done badly, water ingress or condensation problems show up within 2–3 years and the lantern needs to be lifted and refitted.

The product has shifted dramatically since 2010. Older roof lanterns were mostly hardwood with thick glazing bars, single-glazed or early double-glazed units; thermal performance was very poor (U-values 4–6 W/m²K), they leaked condensation, and they discoloured paint and plaster below within a few years. Modern thermally broken aluminium lanterns with argon-filled triple-glazed units achieve U-values of 1.0–1.5 W/m²K, eliminate condensation in normal occupancy, and the frames are slimmer (giving more glass area and more daylight).

For tradespeople, three jobs come up. First, kitchen and dining-room extensions: the most common use, replacing a flat-roof skylight or fitting on a new flat roof. Second, garden room and outbuilding upgrades. Third, replacement of older 1990s-era hardwood lanterns that have failed. Each has slightly different details but the same core sequence: prepare a structurally adequate and waterproof kerb, fit the lantern, weather-seal externally, finish internally.

Key Facts

Quick Reference Table

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Lantern size (external dimensions) Typical glass area Approximate weight Min ridge reinforcement
1000 × 1500 mm 1.5 m² 100–130 kg None (alu profile only)
1500 × 2000 mm 3 m² 200–260 kg None or steel insert
2000 × 2500 mm 5 m² 350–450 kg Steel ridge insert
2500 × 3000 mm 7.5 m² 500–650 kg Steel ridge + rafter inserts
3000 × 3500 mm 10.5 m² 700–900 kg Bespoke steel design
3500 × 4000 mm 14 m² 1000–1300 kg Bespoke steel + structural calc

Detailed Guidance

Structural Opening and Kerb

The structural opening sits below the lantern. For a flat roof on timber joists, the opening is created by trimming joists either side and adding a trimmer (header) at each end. The trimmer takes the load that the cut joists previously carried; standard practice is to double the trimmer for openings up to 1.5m and triple for larger.

For a flat roof on a steel deck or concrete, the opening is cast or cut into the structural deck and the kerb is bolted or built up around the opening.

The kerb (upstand around the opening) lifts the lantern above the roof finish so weather seal is maintained. UK convention is 150 mm minimum kerb height above the finished roof; some specifications use 200–250 mm for better weather performance on exposed sites. Kerb construction is typically 100×150 mm or 150×150 mm timber framed inside the opening, faced with treated softwood or GRP, and lined externally with the roof finish dressed up the sides.

The kerb must be insulated to maintain the thermal envelope. 50–75 mm PIR or mineral wool inside the kerb construction is standard. Without this, the kerb becomes a thermal bridge and condensation forms on the inside face during winter.

Roof Finish and Flashing

The roof finish (usually flat-roof EPDM, GRP, single-ply membrane, or felt) must be dressed up the kerb sides and terminated under a counter-flashing or in a chase. The lantern then sits on top of the kerb, with its base flange overlapping the dressed roof finish.

Three flashing approaches:

Site-formed lead flashing — Code 4 or Code 5 lead, dressed up the kerb and terminated under the lantern flange. Traditional, weatherproof for 50+ years, but skilled work and increasingly rare.

Proprietary aluminium flashing — factory-formed aluminium flashing kits supplied by the lantern manufacturer; bolted to the kerb and terminated under the lantern flange. Faster to fit, lower skill requirement, weatherproof for 30–40 years.

GRP integrated upstand — the kerb itself is a single GRP moulding that the lantern bolts directly to, eliminating site-flashing. Used on new-build extensions where the GRP roofer fits the upstand as part of the roof installation.

The lantern-to-kerb interface is the highest-failure-risk point. Apply two beads of EPDM-compatible butyl tape (or equivalent) between the lantern flange and the kerb, plus a continuous bead of low-modulus silicone or polysulfide as a secondary seal. Mechanical fixings through the lantern flange into the kerb at 200–300 mm centres.

Lifting and Setting the Lantern

Lanterns over 1.5 × 1.5m exceed two-person lifting and need either a suitable crane or a glass-handling vacuum lifter. The HSE Manual Handling Operations Regulations 1992 set the upper guideline for two-person carry at 25 kg per person; a 200 kg lantern is twice that, even with three or four lifters.

Sequence:

  1. Position the lantern over the kerb dry, with the bedding tape and flashing protected
  2. Check it sits flat on the kerb at all four corners
  3. Check the lantern is square (diagonals equal) and aligned with the kerb edges
  4. Lift one side off, apply the bedding seal, lower into place
  5. Mechanical-fix through the flange into the kerb at correct centres
  6. Apply the secondary external sealant bead

For large lanterns, a temporary support frame keeps the unit at the correct height during bedding and squaring.

Thermal Performance and Glass Specification

Modern roof lanterns rely on the glass for thermal performance. The frame is a relatively small fraction of the total area and is thermally broken; the glass dominates the U-value calculation.

Standard specification for current UK Building Regulations:

This specification achieves whole-rooflight U-value of approximately 1.1–1.3 W/m²K, comfortably under the 1.4 W/m²K L1A target. For solar control, a solar coating on the outer pane (e.g. SolarGain Lite) reduces solar heat gain coefficient below 0.4 — important for south-facing lanterns in well-insulated extensions where summer overheating is a real risk under TM52/TM59 thermal modelling.

Self-cleaning coatings (Pilkington Activ, Saint-Gobain BIOCLEAN) on the outer pane use UV light and rainwater to break down organic dirt. Practical benefit: the lantern stays cleaner with much less manual cleaning, which matters for lanterns that are not easily accessible from a window.

Condensation Risks

Condensation on a roof lantern has three possible causes, in order of frequency:

Surface condensation on the inside glass — caused by humid indoor air meeting the cold inner pane. Modern soft-coat low-E glass keeps the inner pane warm enough to stay above dew point in normal occupancy. If condensation appears in a modern lantern, the cause is excessive indoor humidity (laundry drying, blocked extract fans, occupancy without ventilation) — fix the moisture source, not the lantern.

Condensation between the panes (sealed unit failure) — the seal of the double or triple-glazed unit has failed, allowing humid air into the cavity. The unit must be replaced. Modern factory units have 25-year sealant warranties; failure within that period is a warranty replacement.

Condensation on the inside of the frame — frame thermal break has failed, or the frame is non-thermally-broken aluminium. Old hardwood and pre-2000 aluminium lanterns often condense at the frame. Fix is replacement; no remedial repair is reliable.

The kerb/wall junction is also a condensation risk if the kerb is uninsulated. Check the inside face of the kerb during winter; condensation here means the thermal continuity is missing and the kerb needs to be insulated internally.

Frequently Asked Questions

Do I need planning permission for a roof lantern?

Generally no, on a flat roof of an extension or on an existing flat roof — these typically fall within permitted development. Permission is needed for: listed buildings (listed building consent), conservation areas (Article 4 directions may apply), where the lantern raises the roof height above the permitted limit, and where the property's permitted development rights have been removed. Always check with the local planning authority for site-specific guidance.

What's the cost of a roof lantern in the UK?

Typical 2026 UK rates supply-only for a quality thermally broken aluminium roof lantern: £1,200–£2,500 for 1.5 × 1.0m, £2,500–£4,500 for 2.5 × 1.5m, £5,000–£9,000 for 3.5 × 2.5m. Installation adds £800–£2,500 depending on size, access, and roof finish. A typical 2.5 × 1.5m lantern fully installed with kerb and flashings is £4,000–£7,000. Premium ranges (Atlas Iconic, Korniche, Apropos) cost 30–60% more than standard ranges (Korniche, Wickes, B&Q). See the pricing methodology for marking up these jobs for cost markup approach.

How long does a roof lantern last?

A modern thermally broken aluminium lantern with argon-filled sealed units lasts 30–50 years with periodic maintenance (resealant of external silicone every 10–15 years, glazing-unit replacement at 25+ years if seal fails). Older hardwood and pre-2000 aluminium lanterns rarely exceeded 15–20 years; most that remain are now poor performers worth replacing.

Can I install a roof lantern on a pitched roof?

Roof lanterns are designed for flat roofs (typically 1:80 to 1:40 fall). On a pitched roof, the equivalent product is a pitched rooflight (Velux, Fakro) or a continuous rooflight strip. Roof lantern manufacturers occasionally offer pitched-base versions, but these are bespoke and rarely stocked. For pitched-roof daylighting, a Velux centre-pivot or top-hung rooflight is the standard product.

What's the difference between a roof lantern and a skylight?

A skylight is a flat or slightly domed glazed unit set in a roof. A roof lantern is a structured glazed pyramid or hipped form with multiple glass panels meeting at a ridge. A lantern provides more daylight (taller volume), a more dramatic interior space, and a stronger architectural statement. A skylight is cheaper, simpler, and sometimes more practical on a small roof or a low ceiling height where the height of a lantern is intrusive.

Regulations & Standards