Guide · Roof Lantern Installation

Aluminium and timber roof lanterns compared

18 sections 11 minute read

Aluminium is the default and it deserves to be. It carries further on a thinner section, it does not move much with temperature, and it needs nothing doing to it for the life of the building. Timber is the right answer in a narrower set of cases: period houses, conservation areas, and rooms where the warmth of a wooden ceiling line is the point rather than an afterthought.

That is the summary. What follows is the detail behind it, because the two materials differ in ways that are not obvious from a brochure photograph, and because the honest comparison is not one against the other in the abstract but each against a particular roof, a particular room and a particular house.

The short answer, and the two questions behind it

Two questions settle most of this. How wide is the lantern, and what does the house want to look like from inside?

Width settles it structurally. Beyond about 2.5 metres across, a timber lantern needs bars deep enough that they start to read as a pergola, while an aluminium system with a steel reinforced ridge will carry three metres and more on a section under 60mm wide. Below that width both materials are perfectly capable and the question becomes appearance, upkeep and budget. That is a genuine choice rather than a compromise, which is why this page bothers with the comparison at all.

A lantern is a triangle in section. The rise at the ridge is half the width multiplied by the tangent of the pitch.

What an aluminium lantern is made of

An aluminium lantern is an assembly of extruded profiles: a base ring that sits on the kerb, rafter bars running from the base up to a ridge extrusion, hip bars where the corners are hipped, and pressure plates and capping that clamp the glass down onto gaskets from outside.

The important detail is the thermal break. Each profile is manufactured as two separate aluminium sections joined by a polyamide strip, so there is no continuous metal path from the cold outer face to the warm inner face. Without it, aluminium conducts heat roughly a thousand times faster than timber and the inner bars run cold enough to stream with water in a kitchen in January. Any lantern worth fitting is polyamide broken throughout, ridge and hips included, not just on the rafters.

What a timber lantern is made of

A modern timber lantern is not solid sections cut from a plank. The bars are engineered laminated timber, built up from three or more strips glued with the grain alternated, which is dimensionally far more stable than solid stock and lets the section carry more for its depth.

Nearly every timber lantern made now is capped externally with an aluminium or lead profile. The wood provides the structure and the internal face, and the metal takes the weather and the ultraviolet. A wholly exposed timber lantern with the wood on the outside is a heritage detail rather than a current product, and where it is specified for a listed building it is understood to be a maintained element.

Side by side on what actually differs

	Aluminium	Timber with external capping
Typical rafter bar width seen from below	40mm to 60mm	60mm to 90mm
Practical unsupported width	Up to 3m and beyond with a reinforced ridge	Comfortable to about 2.5m
Thermal conductivity of the frame material	High, managed by a polyamide break	Low, inherently
Movement across a 40 degree swing	About 1mm per metre	About 0.2mm per metre lengthwise
Internal finish	Powder coated, any RAL	Painted, stained or oiled, recoatable
Upkeep of the internal face	Wipe over	Recoat every eight to fifteen years
Weight for a 2000 by 1500 unit	Lighter	Heavier, by a useful margin

Sightlines, and the width of a bar

The visible width of a rafter bar is the number people react to once the lantern is in. Aluminium systems reach 40mm to 50mm on a standard bar because the extrusion can be shaped where the load is and hollowed where it is not. Timber has to be solid through its whole section, so equivalent strength arrives at 60mm to 90mm.

Across a lantern with six bars that difference is 150mm to 250mm of extra solid material in the ceiling. Whether that reads as heavy or as substantial depends on the house. In a Georgian or Victorian room it often reads as correct. In a plaster-white modern extension it reads as clutter. Bar thickness in its own right is worked through on the sightlines page.

Span, and how far each material carries

Span is where the materials genuinely part company. An aluminium ridge is commonly reinforced with a steel or a heavier gauge insert running its full length, which turns the ridge into a beam and lets the rafters sit shorter and thinner. Timber ridges are timber, and depth is the only lever available.

In practice a timber lantern above about 2.5 metres in the short direction needs bars deep enough to cast their own shadows, and the maker will often introduce an intermediate purlin bar running horizontally partway up the slope. That bar collects dust, interrupts the view and is the moment most people change their minds. If the lantern needs to be wide, aluminium is the material.

Thermal behaviour, and the break in the middle

Whole-unit U-values for the two materials land closer together than the raw conductivities suggest. A good thermally broken aluminium lantern with a decent argon filled double glazed unit will report a whole-product U-value around 1.2 to 1.4 W/m²K. A timber lantern with the same glass reports something similar, occasionally a shade better, because the frame is not fighting its own material.

Where timber pulls ahead is at the edges rather than in the average. The linear thermal bridge along the frame perimeter is smaller, so the inner surface temperature of a timber bar sits a degree or two warmer than an aluminium one under the same conditions. That is a small number with a visible consequence, which the next sections cover.

Thermal movement, and what it asks of the seals

Aluminium expands about 23 microns per metre per degree. Over a three metre ridge going from a frosty minus two overnight to a black-capped 60 degrees in August sun, that is close to 4mm of movement along the length. The system is designed for it: gaskets are compressible, glass sits on setting blocks with clearance, and end details slip rather than lock.

Timber moves about a fifth of that along the grain, but it moves across the grain with moisture rather than temperature, and that movement is seasonal rather than daily. Both are entirely manageable. Both go wrong in the same way, which is a fixing overtightened on a cold morning by someone in a hurry. It is one of the reasons a lantern is set with the manufacturer’s torque figures rather than by feel.

Cold surfaces, and how each behaves over a hob

In a kitchen, the frame is the coldest thing in the ceiling and it is the first surface water finds. This is where the polyamide break earns its cost, and where timber has a quiet natural advantage.

Every serious lantern of either material carries a drainage channel along the inside of the base, which collects any water that does form on the glass edge and takes it out through weep holes to the outside face. The channel matters more than the material. A timber lantern without one will still wet the plaster below it, and an aluminium lantern with a well designed one stays dry through a winter of boiling pans. Why kitchens are the hard case is covered on the condensation page.

What each reads like from the room below

Aluminium reads as line. The bars are thin, matt and even, the corners are crisp, and the eye goes past them to the sky. In a room with flat plaster and square reveals that is exactly the intent, and a dark grey or black internal finish makes the bars disappear against the brightness behind them.

Timber reads as structure. The bars have depth, the light catches the sides of them, and the ceiling gains something closer to the character of a roof than of an opening. In a room with exposed brick, a beam, or joinery of any weight, timber joins the conversation rather than sitting apart from it. Neither is better. They are different intentions, and the room usually already knows which one it is.

What each reads like from the garden

From outside, capped timber and aluminium are hard to tell apart at anything over ten metres, because the external face of both is a metal capping and a glass slope. The differences that show are the width of the capping, which follows the bar beneath, and the profile of the ridge.

Where it shows most is on a house being viewed from an upper storey or from a neighbouring garden. Timber systems tend to carry a chunkier external capping and a slightly heavier ridge. On a period house that is a plus. Colour is a bigger factor than material at that distance, and the finish options for both are covered on the finishes page.

Upkeep over ten and twenty years

An aluminium lantern wants the glass washed and the external gaskets looked over. That is the whole list. A polyester powder coating on a properly pretreated extrusion holds its colour and its film for decades, and the marine grade coatings used on coastal work hold longer still.

A timber lantern wants the internal face recoated at some point. On an oiled or waxed finish that might be every eight years, on a modern microporous paint system fifteen or more, and in a bathroom sooner than in a dining room. It is a morning’s work with a brush from a stepladder, not a trade job, but it is real and it should be part of the decision rather than a surprise. The external capping needs nothing.

Daylight from a single large rooflight over a kitchen extension

Timber species, and how the wood is finished

The usual species is engineered European redwood, sometimes described as Scandinavian pine, laminated and finger jointed. It takes paint well, is dimensionally reliable, and is what most stock products are made from. Oak is available from a smaller number of makers, costs considerably more, and is specified where the wood is meant to be seen as oak rather than as a painted bar.

Finishes divide into three. Factory applied paint, usually a white or an off white, which is the most common and the most durable. Stain, which keeps the grain visible and darkens the timber. And oil, which is the most natural looking and the most frequently recoated. Factory finishing beats site finishing every time, because the bars are coated on all faces before they are assembled and the end grain is sealed.

The hybrid, timber inside and aluminium outside

The comparison is less binary than it looks, because the hybrid product exists and it is a genuinely good answer. The structural bar is aluminium, thermally broken and doing the span, and a timber cover profile clips to the inside face so that the ceiling reads as wood.

This buys the aluminium sightline and span with the timber appearance, and the internal timber is a cosmetic layer rather than a structural one so a knock does not matter. It costs more than either pure system, the lead time is longer, and fewer manufacturers offer it. Where a listed building or a conservation officer wants timber visible internally but the opening is wide, this is often the only route that satisfies both.

Cost, and where the difference actually sits

For a like for like size and glass specification, timber generally sits above aluminium, and the gap widens as the lantern gets bigger because the timber sections have to grow to keep up. At small sizes the two are close enough that the decision should be made on appearance rather than on the figure.

The other cost worth naming is installation. A timber lantern of the same size is meaningfully heavier, and weight decides whether a unit goes up by hand over a scaffold or needs mechanical lift and an extra pair of hands. On a first floor flat roof over a Chelmsford side return with garden access, that is a genuine line in the price. Indicative figures are set out under costs.

Where timber is the right call across Essex

Timber earns its place on the older stock, and Essex has a lot of it. The timber framed and pargeted houses around Thaxted, Coggeshall and Saffron Walden, the weatherboarded cottages in the Blackwater villages and around Stock, and the Victorian brick terraces in the older centres of Chelmsford, Maldon and Colchester all sit better under a wooden ceiling line than a metal one.

It is also the pragmatic choice where a conservation officer is involved. A painted timber lantern with a visible glazing bar module reads as historically plausible in a way a slim black aluminium grid does not, and that reading is often what unlocks the consent. On a listed building, consent is a separate process from planning and it wants establishing before anything is ordered.

Where aluminium is the right call

Aluminium suits everything else, and that covers the great majority of lantern work. Post war and modern extensions, the trussed roof estates at Great Notley, Beaulieu Park and South Woodham Ferrers, the new town housing across Harlow and Basildon, and any job where the lantern is wide, the ceiling is flat plaster, or the owner wants a ceiling that stays as it was fitted.

It is also the answer on exposed sites. The bungalows on reclaimed land at Canvey and the properties along the Blackwater and Crouch estuaries take salt-laden wind, and a marine grade powder coating on aluminium handles that with no attention at all. Sunspire fits both materials and specifies from the roof outward, because the building narrows the choice long before preference does.

Handling, lead times and installation day

Stock aluminium lanterns are frequently available inside two weeks and are delivered as a kit of bars, glass and gaskets that is assembled on the kerb. Timber lanterns are more often made to order at four to six weeks, and are commonly delivered part assembled, which makes them awkward as well as heavy.

On the day the sequence is the same for both: kerb checked for level and square, base ring bedded and fixed, bars set, glass laid onto the setting blocks, gaskets and capping fitted, then the flashing dressed and the internal reveal made good. The roof is closed and watertight before anyone leaves, whichever material is going in. The rest of the process, from survey through to the Building Control notification, is set out on the roof lantern installation page.

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