Aluminium and timber roof lanterns compared

The one paragraph version

If the extension is modern, if you want the slimmest possible bars, and if you never want to think about the lantern again, specify thermally broken aluminium. If the house is a period property, if the lantern is visible from a principal room, and if you want a warm surface overhead rather than a cold one, specify timber, or timber inside with aluminium outside. If cost is the binding constraint and appearance matters less, aluminium is also the cheaper of the two at almost every size.

What an aluminium lantern is made from

The bars are extruded aluminium alloy, typically 6063 T6, pushed through a die as a hollow profile and cut to length. That process allows a complex section with internal chambers, drainage channels and glazing rebates all formed in one piece, which is why aluminium sections do so much with so little material.

The alloy is not solid metal all the way through the bar. A rafter bar is a shell around voids, with the metal placed where the bending stress is highest. This is the reason an aluminium bar 50mm deep carries what a timber bar of 90mm would.

Every exposed face is powder coated. The raw alloy would oxidise to a dull chalky surface within a few years, so the coating is structural to the finish rather than decorative.

Why aluminium needs a thermal break, and what one is

Aluminium conducts heat roughly a thousand times better than timber. Left as a single piece of metal spanning from the cold outside face to the warm inside face, a bar becomes a cold bridge and will run with condensation every winter morning.

A thermal break solves this by making the bar two separate pieces of aluminium joined by an insulating strip, usually polyamide reinforced with glass fibre, mechanically rolled into channels in both halves. The inner face never touches the outer face.

Break depth varies. A 24mm polyamide break performs meaningfully better than a 14mm one, and the difference shows up as the temperature of the bar you can touch from inside. Any aluminium lantern worth fitting has one. A lantern advertised only as aluminium, with no mention of a break, is not a bargain.

What a timber lantern is made from

Good timber lanterns are engineered laminate rather than solid section. Three or more strips of timber are bonded with the grain alternated, which cancels out most of the movement that solid timber shows across the seasons.

Species is usually European redwood for painted frames, or a hardwood such as European oak or sapele where the timber is to be stained or oiled and seen. Redwood is stable, takes paint well and is the sensible default. Oak is heavier, harder to coat and moves more, and it looks like nothing else from underneath.

Factory finishing matters more than species. A frame that arrives with a factory applied base coat and two top coats, applied under controlled humidity, lasts far longer than one coated on site in an Oxford autumn.

Strength per millimetre, and what that does to the view

Structural performance is the honest reason aluminium dominates. For the same span and the same glass weight, an aluminium rafter bar is narrower on the face and shallower in depth than a timber one.

In round numbers, an aluminium rafter bar faces up at 50 to 65mm wide, and a timber one at 80 to 110mm. Across a lantern with four intermediate bars, that difference removes well over 150mm of solid frame from your view of the sky.

Whether that matters is genuinely a matter of taste. Slim bars read as contemporary and disappear against a bright sky. Deeper timber bars read as joinery and give the ceiling a structure, which on the right house is the point rather than the compromise.

Spans, and where each material stops

Both materials will carry more than most domestic lanterns ask of them, but the ceiling is different.

Aluminium ridge and hip sections, sometimes with a steel or aluminium reinforcing insert slid inside the extrusion, will take a lantern comfortably past five metres on the long axis without an intermediate post. That is why very large lanterns are almost always aluminium.

Timber runs out sooner. Past about 3.5 to 4 metres, a timber ridge needs either a much deeper section, which starts to look heavy, or a steel flitch concealed within it, which is possible and adds cost. Above four metres, the practical answer is aluminium or a composite frame.

Composite frames: timber inside, aluminium outside

The composite lantern is the expensive middle path and it is the best object of the three. The internal frame is timber, giving the warm surface and the traditional profile. The external face is a powder coated aluminium capping, clipped or screwed over the timber, taking all the weather.

The advantage is that the failure mode of a timber lantern is removed. Timber lanterns do not fail structurally. They fail because the coating on the weather face degrades, water gets into the end grain at a joint, and the joint opens. Cap the outside in aluminium and that route is closed.

Expect to pay a clear premium over aluminium for the same size. Whether that is worth it depends entirely on whether anybody looks up.

uPVC lanterns and where they honestly sit

uPVC lanterns exist and they are cheap. The frames are wide, because the material is weak and needs bulk plus internal steel reinforcement to carry glass, so you lose a great deal of sky to white plastic.

They discolour. White uPVC on a south facing roof, exposed to full ultraviolet all day with nothing shading it, yellows in a way that a vertical window frame does not, because a lantern faces the sky squarely.

We will fit one if that is what a client wants and the budget requires it. We will also say plainly that the frame is the part of a lantern you cannot change later, and that this is the wrong place to save the money.

How each behaves through an Oxford winter

The winter test is not cold. It is the cycle: damp air, a frost, then sun on the glass by ten o’clock.

Aluminium moves. A three metre bar expands and contracts by about 2mm across a 30 degree swing between a frosty dawn and a sunlit afternoon, which is why glazing gaskets and the ridge joints on a decent lantern are designed to accommodate movement rather than resist it. A well made unit does this silently. A poorly detailed one ticks.

Timber barely moves thermally but responds to humidity, which is why a laminated section is used and why the coating is doing a moisture job as much as a decorative one. Neither behaviour is a defect. Both are reasons to buy a properly engineered frame rather than the cheapest example of either material.

Maintenance across twenty years

Aluminium: wash the frame when you clean the glass, and clear the drainage channels at the eaves once a year with a soft brush. That is the whole list. A powder coat on a lantern will hold its finish for decades on the north facing facets and will chalk slightly on the south ones.

Timber, externally exposed: inspect annually, and recoat every five to eight years depending on aspect and exposure. Access is the real cost. Recoating a lantern four metres up over a flat roof means a tower or a platform, so the labour dwarfs the paint.

Timber, internally with an aluminium capping: recoat the inside when you redecorate the room, which is to say roughly never in practice, because the internal face is not weathered.

Condensation on the frame itself

Timber almost never shows condensation on its inner face, because timber is a poor conductor and the surface stays close to room temperature.

Aluminium will, if the thermal break is thin or absent. The pattern is distinctive: beads along the bars, not on the glass, worst at dawn in a kitchen, clearing by mid morning. It is not a defect of the glass and no amount of ventilation entirely removes it if the break is inadequate.

Specify the break depth on the order and this stops being a risk. Kitchen condensation generally, and its other causes, is covered on roof lantern condensation in a kitchen.

Colour and finish

Aluminium is powder coated to any RAL reference, and dual colour is straightforward: anthracite outside, white or a soft grey inside. Coating a lantern in a dark colour outside and a pale colour inside makes the bars vanish against the sky from below and read as a crisp object from the garden.

Timber can be sprayed to any colour, oiled, or stained. A stained oak lantern is a completely different object from a white painted one and suits a house where other joinery is timber.

Coating systems, RAL choices and how they weather are covered in depth on roof lantern frame finishes and powder coating, and the question of which colour to pick on what colour should a roof lantern frame be.

What actually drives the price difference

Aluminium is cheaper than timber at almost every size, and the gap widens as the lantern grows. The reasons are manufacturing rather than material.

An aluminium lantern is cut from stock extrusions on a saw, and the corner and ridge connections are machined castings or cleats. It is an assembly operation. A timber lantern is joinery: each bar is machined, jointed, sanded and coated, and the labour does not fall with volume in the same way.

A composite frame carries both costs, the joinery and the extruded capping, plus the work of marrying them. That is why it sits at the top.

How each one is actually installed

The installation is more similar than people expect, because in both cases the lantern is bedded onto the kerb and the weather detail happens below it.

Aluminium frames are supplied as a knock down kit and assembled on the roof, bolted at the corners and the ridge, then glazed facet by facet. That means a large lantern arrives in a car sized van and goes up a ladder in pieces.

Timber frames are more often delivered as a made up frame, which is heavier and needs more hands or a lift to get onto a roof. On a tight terrace access in east Oxford, that is occasionally the deciding factor on its own.

In both cases the frame is bedded on a continuous seal to the kerb top and mechanically fixed down through the base. Whatever the frame, the roof is never left open overnight.

Period houses, conservation areas and what the planners look at

Oxford has a great deal of protected fabric, and where a lantern is visible from a public place the material can matter to a decision.

On a rear addition behind a Victorian terrace, out of public view, aluminium is accepted without comment. On a building where the roof is seen, or on anything listed, a timber framed lantern with traditional bar profiles is a far easier conversation, because it reads as a period conservatory element rather than as a modern glazed box.

The rear addition case specifically is covered on roof lanterns on a Victorian rear addition, and the consent position on do roof lanterns need planning permission.

Side by side

How to make the decision

Ask three questions in this order. Can you see the frame from a room you sit in, or only from the kitchen you stand in? Is the roof visible from anywhere that matters, either publicly or to a neighbour? And who is going to get a tower onto the flat roof in 2033 to recoat it?

Most answers land on aluminium, and that is a good outcome rather than a compromise. Where the answers land on timber, they land there firmly, and the extra cost buys something real.

If you tell us the house, the room and the aspect, we will give a range by email for each material so you can see the gap before you decide. The fixed price follows the survey, and the workmanship is guaranteed for ten years whichever frame you choose.