Aluminium and timber roof lanterns compared

A timber roof lantern is built from engineered laminated softwood or hardwood, usually finished with a factory-applied microporous paint or stain. The timber you are buying today is not the solid single-section joinery of a Victorian conservatory. It is laminated stock, glued up from thin layers with the grain arranged to cancel out movement, then machined and coated in a controlled factory environment before it ever reaches your roof. Most quality timber lanterns are engineered redwood or a durable hardwood such as sapele, and many are clad on the outside with a thin aluminium capping so the weather never touches the wood at all.

An aluminium roof lantern is an extruded system. The bars are hollow aluminium profiles, powder-coated to a chosen colour, with the structural strength coming from the metal itself rather than from any bulk. Good aluminium lanterns are thermally broken, which means the inner and outer faces of each bar are separated by a rigid polyamide barrier so the cold outer metal is not in direct contact with the warm inner metal. Without that break the frame would run cold and gather condensation along every bar on a winter morning. The thermal break is the single feature that separates a proper aluminium lantern from a cheap one, and it is worth naming when you compare quotes.

So the honest starting point is that both are modern engineered products. The choice is not old-fashioned timber against high-tech metal. It is two different ways of making a strong, weathertight frame, each with a genuine set of trade-offs.

The most visible difference between the two materials is the width of the bars. Aluminium is strong for its cross-section, so an aluminium lantern can carry the same pane of glass on a slimmer bar than timber needs. In practice aluminium sightlines sit around 50 to 65mm, while a timber lantern of the same size wants more like 70 to 95mm of frame to hold the load safely. That difference reads clearly from underneath. More glass and less frame in the aluminium version, more visible structure and a chunkier grid in the timber one.

Whether that matters depends on the house. On a plain modern extension with clean plastered reveals, the thin aluminium grid looks right, and it lets more daylight down into the room because there is less bar shading the glass. On a period property, a weatherboarded cottage in one of the Leigh conservation areas, or a 1930s semi where the rest of the joinery is painted timber, a slim metal grid can look out of place, and the deeper timber bars sit more comfortably with the age of the building.

Span is the other side of this. Because aluminium carries more load per bar, an aluminium lantern reaches a larger clear size before it needs an intermediate structural bar breaking up the glass. If you want a single large lantern over a knocked-through kitchen and dining space, aluminium usually gets you there with a cleaner grid. A very large timber lantern can be built, but it will carry more bars to do it, and above a certain size the timber option starts to look and feel heavy.

People assume timber is the warmer frame because wood feels warm to the touch and metal feels cold. As a raw material that instinct is correct: timber conducts heat far more slowly than aluminium. But a lantern is not a raw material, it is an assembly, and the numbers close up once you account for the thermal break in the aluminium and the glass area in both.

The frame is only the edge of a lantern. Most of the visible area is glass, and the glass is usually the same units regardless of which frame you buy. What the frame decides is the U-value at the perimeter, and how much that perimeter drags down the performance of the whole unit. A thermally broken aluminium lantern and a timber lantern, glazed with the same double-glazed units, will land within a narrow band of each other on the whole-unit U-value. Timber often edges it, but by a margin most rooms will never feel.

Read that table and the gap that matters is not aluminium against timber. It is the presence or absence of a thermal break in the metal. A thermally broken aluminium lantern and a timber one are close enough that the frame is not where you should decide your winter comfort. An aluminium lantern with no thermal break, which still turns up on the cheapest quotes, is a different and worse product, and it will stripe your ceiling with condensation on a cold morning. If you are comparing an aluminium price against a timber price, first make sure the aluminium one is thermally broken, or you are not comparing like with like.

Here is the part that gets lost in every aluminium-against-timber argument. Neither frame does anything about the heat and glare pouring through the glass on a sunny afternoon. That is entirely a function of the glazing you specify, and it is the number that actually decides whether the room under the lantern is usable in summer.

A roof lantern faces close to straight up, so it collects far more solar energy per square metre than any wall window. On a clear July afternoon a roof surface can be receiving in the region of 700 watts per square metre. A 1.5 by 1.0 metre lantern is 1.5 square metres of glass, so roughly a kilowatt of solar energy is arriving at it. With ordinary clear double glazing, whose G-value is around 0.6, about 600 watts of that lands in the room as heat. Swap to a solar-control unit with a G-value nearer 0.3 and you halve it. That swap has nothing to do with whether the frame around it is metal or wood.

This is where Southend changes the maths. Most English seaside places face north, east or west. This stretch of the Essex coast faces south across the Thames Estuary, so the rear extensions and loft conversions that carry roof lanterns here are pointed straight at the summer sun with an open horizon and reflected light coming off the water. A lantern over a south-facing 1930s semi, of which south Essex has an enormous stock, is a solar collector with a dining table under it. Whichever frame you choose, if the glass is clear the room will overheat, and if the glass is solar-control it will not.

Glare works the same way. A low G-value earns its keep in a kitchen or a home office where sun on a worktop or a screen ruins the afternoon, and the frame material makes no difference to any of it. You can read the full case for specifying the glass before the unit on our page on solar-control glazing. The short version: on any south-facing or west-facing plane along this coast, spend the argument on the G-value, not on the frame.

This is where the materials genuinely separate, and it is the honest reason many people lean one way or the other.

Aluminium is close to maintenance-free. A powder-coated finish holds its colour for decades, it does not rot, it does not swell, and it does not need repainting. Wiping the glass and clearing leaves out of the gutter detail is about the extent of it. Aluminium also does not move with the weather the way timber does, so seals and joints stay put. In a coastal setting this matters. Salt-laden air off the estuary is hard on finishes, and a quality powder coat with a marine-grade specification shrugs it off better than a painted surface will.

Timber is a living material even after it is engineered and coated. It moves a little with humidity and temperature, and its factory coating, good as modern microporous paints are, is a coating, and coatings weather. A timber lantern will want its finish inspected and, in time, refreshed to keep the wood protected, particularly on the exposed outer faces. This is exactly why the better timber lanterns are clad externally with aluminium: it gives you the timber look and feel on the inside, where you see it, and a maintenance-free metal skin on the outside, where the weather is. If you want real timber but not the upkeep, the externally capped version is the sensible middle path, and it is the one that survives a salt-air location best.

One point that is often confused. A frame that has reached the end of its life, or a sealed glass unit that has misted because its edge seal has gone, is not something to nurse along: it is replaced, and swapping a tired lantern for a properly specified new one is a clean job. If a lantern is letting water in around the upstand, that is a roofing question for a roofer to look at first, and no new glazing will settle it until the upstand and flashing are sound.

Both materials span a wide price range, and the ranges overlap, so a blanket statement that one is cheaper than the other is misleading. What follows is industry-typical guidance for a supplied-and-installed roof lantern on a suitable existing upstand, not a quote, and the figures move with size, glass specification and access.

As a rule, aluminium tends to sit lower for the frame itself and asks nothing further over its life. Timber usually costs more at the outset and carries a small ongoing upkeep cost, though the externally capped versions push that upkeep close to zero and cost the most to buy. Notice the row that is identical across both columns: the glass upgrade. Moving to solar-control glazing costs the same whichever frame you choose, and on a south-facing lantern it is the line that changes your summer, so it is the last place to economise.

Two costs sit outside the table and apply to either material. A new or enlarged roof lantern is notifiable under Building Regulations: Part L covers the thermal performance and Part K covers the safety glazing, since anything overhead needs laminated glass on the inner pane. We make the Building Control notification to Southend-on-Sea City Council, or the relevant local authority for your address, as part of the installation. And if the existing structural opening or the upstand needs work to carry the new lantern, that is a separate line on any honest quote, whatever the frame is made of.

The right answer follows the building, the aspect and how much upkeep you want to sign up for. A few patterns hold across the local stock.

• Modern rear extensions on 1930s and post-war semis. Aluminium usually wins here. The clean plastered reveals suit a slim grid, the larger clear spans mean a single lantern rather than a divided one, and the maintenance-free finish is welcome. If the extension faces south, put the argument into solar-control glass.
• Period villas and Edwardian seafront properties. Timber, or externally capped timber, often sits better with the age and detailing of the building, particularly where the existing joinery is painted wood. The deeper bars look intentional rather than mean.
• Conservation areas and the Leigh Article 4 zones. The Leigh Cliff and Leigh conservation areas carry an Article 4 Direction that removes permitted-development rights for roofing and window changes, so a planning application is needed where it usually would not be, and the planners may have a view on appearance. Timber, or capped timber in a heritage colour, tends to be the safer conversation, though this is general guidance and the local authority is the place to confirm what any specific address needs.
• Coastal and cliff-top exposure. Anywhere taking the full salt-laden air off the estuary, aluminium with a marine-grade powder coat, or externally capped timber, are the finishes that last. An uncapped painted timber lantern in that exposure will ask for attention sooner.
• Big single-span kitchens. When the brief is one large lantern with as little frame as possible over a knocked-through space, aluminium reaches the size with a cleaner grid and more daylight.

Across all of these, the frame decision comes second to the aspect. Work out which way the roof plane faces before you settle on a material, because a south-facing lantern in the wrong glass is a hot room whether it is framed in metal or wood, and a north-facing one can take clear glass and all the daylight either frame will give it.

You can settle this without a showroom visit. Ask the questions in the right order and the material almost picks itself.

1. Which way does the roof plane face? Orientation first. It decides the glass, which matters more than the frame.
2. If the quote is aluminium, is it thermally broken? This is the difference between a warm frame and a cold one. A price that undercuts everything else is often a lantern with no thermal break, and it is not the same product.
3. What is the G-value of the glass? On a south-facing or west-facing lantern you want a number nearer 0.3 than 0.6, and it costs the same to specify whichever frame you pick.
4. Is the whole-unit U-value quoted, not just centre-pane? Whole-unit figures include the frame and are the honest comparison, and they are what Building Control looks at.
5. If it is timber, is it externally capped? Capped timber gives you the look inside and no upkeep outside, which is the version that lasts in coastal air.
6. Who notifies Building Control, and does the upstand need work? Both apply whatever the frame. Get them named on the quote so there are no surprises.

Frame material is a real choice, and there are good reasons to prefer one over the other on a given house. Aluminium for slim sightlines, large spans and zero upkeep. Timber for period character and a warmer look, ideally capped for a coastal setting. But it is the second decision, not the first. The first is the glass, because that is the number that decides whether you can sit under the lantern on a July afternoon. You can see how a lantern goes in, upstand and all, on our page on roof lantern installation, and if you want the frame options and the glass specification set out side by side for your own roof, with the G-value written down for each plane, ask us for a specification and we will start with the compass.