Aluminium and timber roof lanterns compared

What the two materials actually are before they reach a roof

Lantern aluminium is almost always 6063 alloy in a T6 temper, extruded through a die into hollow multi chamber sections and cut to length. That alloy was developed for architectural extrusion because it takes a complicated die profile cleanly and it takes a powder coating well. It arrives at a fabricator as lengths of profile with internal chambers, screw ports and gasket grooves already formed, and the fabricator cuts, notches and assembles rather than shapes.

Lantern timber is rarely a solid stick of anything. It is engineered: three or more laminations of Scandinavian redwood or a durable hardwood, finger jointed for length and glued up so that the grain of each lamination opposes its neighbour. That construction exists for one reason, which is that a solid section of the same size would twist. Laminated stock stays straight, takes a factory applied paint or stain system, and can be machined to a profile that a solid section would split along.

Thermal conductivity, and the polyamide strip that makes aluminium usable

Aluminium conducts heat at around 160 watts per metre kelvin. Softwood conducts at about 0.13. That is a difference of more than a thousand times, and it is the single most important fact about the two materials in a roof.

A solid aluminium rafter running from outside to inside is a heat drain and a condensation surface. The fix is a thermal break: one or two strips of glass reinforced polyamide, typically 14 to 24mm wide, rolled into grooves in the extrusion so that the outer face and the inner face are two separate pieces of metal held together by plastic. Polyamide conducts at about 0.3, so the break does the work of a piece of timber a fraction of its width. The lanterns that streamed with water down every rafter in the 1980s were not badly made, they simply had no break in them.

Break width is worth asking about, because it varies between systems and it is the difference between an internal frame surface that stays above dew point in January and one that does not.

Timber’s thermal advantage, and where its weakness sits instead

A timber rafter needs no break. The whole section is insulation, its internal surface runs close to room temperature, and condensation on the frame itself is rarely the complaint on a timber lantern.

The weak point moves to the joints and the finish. Timber takes up and gives off moisture with the air around it, moving across the grain by up to about 2 per cent between a dry summer and a wet winter. A 100mm deep rafter can therefore change dimension by a millimetre or two through the year, every year, and every glued joint, every screw fixing and every bead of sealant has to accommodate that movement without opening. Where a paint film cracks at a joint, water gets into end grain, and end grain wicks water tens of times faster than a face. That is the failure sequence on old timber roof glazing, and it starts at a hairline crack in a corner.

Section depth for the same span, and the arithmetic behind it

Stiffness in a beam depends on the material’s modulus of elasticity and on the shape of the section. Aluminium’s modulus is roughly 70 gigapascals against about 10 for softwood, so aluminium is around seven times stiffer for the same shape. Timber cannot be extruded into a deep hollow box either, so it loses on both counts.

In practice, for the same ridge length and the same glass weight, a timber ridge and rafter set has to be noticeably deeper and wider than an aluminium one. A ridge that an aluminium system handles with a 60mm deep extrusion and a steel core will want 150mm or more of timber, and that depth is visible from the room as a heavier grid. It is also why the largest lanterns are aluminium: past about three metres of ridge, timber sections get deep enough that the frame starts to dominate the glass.

Corner joints: cleats and gaskets against mortise and tenon

An aluminium lantern is assembled mechanically. The eaves beam corners are made with an internal cleat or a machined corner casting, drawn together with screws and bedded on a gasket or a low modulus sealant. Rafters land on the eaves beam and the ridge in pressed or die cast connectors, and the whole assembly relies on the accuracy of the cuts rather than on the skill of the fitter.

A timber lantern is joinery. The corners are mortise and tenon or a machined comb joint, glued with a structural adhesive, and the rafters are birdsmouthed or housed into the ridge and the eaves beam. It is a stronger joint in the right hands and a slower one, and it cannot be adjusted on site the way a screwed aluminium corner can. Timber lanterns therefore arrive built and finished far more often than aluminium ones, which brings its own lifting problem.

Weight per metre and what that means on a scaffold

Aluminium weighs about 2,700 kilograms per cubic metre against roughly 500 for redwood, so metal looks heavier by density and is lighter in practice, because the extrusion is a thin walled hollow and the timber section is solid and much larger.

A typical aluminium lantern rafter runs somewhere around 2 to 3 kilograms per metre. A timber rafter of equivalent capacity runs 5 to 8. Across a 1200 by 2000mm lantern that is a frame weighing perhaps 35 kilograms in aluminium against 70 or more in timber, before a single pane goes on. On a Quedgeley driveway that difference is a shrug. On a Chalford or Rodborough cottage where the frame has to be hand carried up a flight of steps from a shared lane, it decides how many people are on site that morning, and it goes in the price at survey rather than appearing later.

Maintenance across twenty five winters

Polyester powder coating on aluminium is cured onto a chromate or a chrome free pretreatment at around 200 degrees. On a domestic roof away from the coast, a good architectural coating holds its colour and its gloss for decades, and the maintenance schedule is washing it. Nothing has to be done to keep the guarantee alive beyond keeping salt and builder’s debris off it.

A factory finished timber lantern arrives with a microporous paint system applied under controlled conditions, and it is genuinely good. It still has a cycle. Expect to inspect the external paint film every couple of years and to redecorate the exposed faces somewhere between years six and ten, sooner on a south west elevation taking driving rain off the Severn, later on a sheltered north facing rear. The inspection matters more than the interval: a cracked film at a rafter foot dealt with in the spring is a light sanding and two coats, and the same crack left for four winters is a new rafter.

Condensation on the internal face of each material

Water forms on any surface below the dew point of the air touching it. In a kitchen at 20 degrees and 60 per cent relative humidity, the dew point is about 12 degrees, so any internal surface colder than that will run.

On a well broken aluminium lantern the internal frame face typically sits a couple of degrees below the room air, which keeps it clear in normal use and can bring it close on a still frosty night with a pan boiling. On timber the internal face sits within about a degree of room temperature and effectively never runs. Where aluminium wins back some ground is that the frame is dimensionally stable and its drainage channels stay the shape they were extruded, so any water that does form is collected and taken out through the weep holes rather than sitting in a joint. Condensation in a kitchen specifically is covered on the roof lantern condensation page.

Aluminium clad timber, and what it does and does not solve

The hybrid puts a thin extruded aluminium capping over a timber structural core, clipped or screwed on with a drained cavity behind it. Outside it is powder coated metal and needs no painting. Inside it is timber, warm to the touch and paintable to match joinery.

It genuinely solves the external maintenance cycle and the internal cold surface at once. Two things it does not solve. The section is deeper than aluminium alone, because the timber is still doing the structural work, so the sightline is wider. And it is the heaviest of the three options per metre, which matters on a valley site and on a hipped lantern with a long ridge. It is also the most involved to make, and it carries the longest lead time of anything discussed here.

Where uPVC lanterns end up

uPVC lanterns exist, usually as a reinforced extrusion with a steel or aluminium insert in the main members. They are the cheapest route to glass overhead and they perform reasonably on paper, because the plastic itself conducts at about 0.17.

The reason they are uncommon on a well detailed extension is dimensional. uPVC has a coefficient of thermal expansion around twice that of aluminium and it softens at a temperature a dark roof surface can reach in July, so the sections have to be fatter and the reinforcement heavier to stay straight. The result is a wide white grid across the ceiling and a limit on how large the lantern can be before deflection at the ridge governs. On a rear extension in a Gloucestershire conservation area it is also the material least likely to be consented.

Delivery: a knock down kit against a pre assembled frame

How the frame arrives changes the day on site. Aluminium lanterns are normally delivered as a knock down kit of cut and notched sections with a setting out drawing, and are assembled on the kerb. That is a real advantage on a tight site: the largest single item is a rafter length, everything goes through a normal doorway, and nothing has to be lifted as a whole.

Timber lanterns and clad timber lanterns are frequently delivered assembled and finished, because the joints are glued and the paint film is applied in a factory. A 1200 by 2000mm assembled frame is an awkward, heavy, expensive object that has to be craned or carried to roof level in one piece. On a Stroud valley terrace with no vehicle access to the elevation, that single fact has been known to decide the material.

Cheltenham and the wolds, where consent chooses the material

In central Cheltenham, through Lansdown, Montpellier, Pittville and the Bath Road terraces, a great deal of the stock is listed and nearly all of it sits in a conservation area with an Article 4 direction over parts of it. Lanterns there go on rear additions rather than into historic roofs, and the frame material is frequently a condition of the consent rather than a choice. What gets approved on those elevations is typically a slim painted timber or timber cored frame in a dark colour, because it matches the sash joinery already on the building and because a wide silver aluminium grid does not.

The same logic applies to garden rooms and orangeries behind Cotswold stone in Painswick, Bibury, Northleach and Chipping Campden. A painted timber lantern behind a coped stone parapet reads as part of the building. It is worth taking pre application advice with photographs before ordering a frame, because timber lead times run to eight or twelve weeks and a refusal at that stage is expensive in time as well as money. The wider consent picture is on the conservation areas and listed buildings page.

Quedgeley, Bishops Cleeve and the case for aluminium

On the modern estates at Quedgeley, Abbeydale, Bishops Cleeve, Ashchurch and Yate, and on interwar semis at Longlevens and Hucclecote, there is no consent constraint on the rear elevation and the arguments run purely technical.

Aluminium wins most of them there. The frame is slimmer for the same glass area, the anthracite powder coat matches the doors and windows that were fitted with the extension, the kit comes through the side gate in pieces, and nothing needs decorating in year eight. Where the extension is wide and the customer wants a long ridge, aluminium is the only material that will do it without a heavy grid overhead. The exception is a room where somebody wants timber visible from the sofa, and that is what the clad option exists for.

Lead times, and the order the decisions come in

The material decision has to be early, because it changes the whole programme. Aluminium lanterns are commonly four to six weeks from a confirmed kerb dimension. Timber and clad timber are frequently eight to twelve, longer for a non standard colour or a heritage profile.

The sequence that avoids trouble: agree the material and the pitch at survey, build and weather the kerb, measure the finished kerb, then order. The kerb goes up first on every lantern for exactly this reason, and who builds it is set out on the roof lantern kerbs page. Ordering a timber lantern off a drawing twelve weeks before the kerb exists is how a frame arrives that does not fit a real building.

What actually separates the two on price

No figures are published here, and the honest reason is that the gap between an aluminium and a timber lantern is not a fixed percentage. It moves with size, with colour, with how the frame reaches the roof and with what the planners have asked for.

What moves it: the fabrication method, since machined joinery takes longer than cut and screwed extrusion; the factory finish, since a paint system on timber is several controlled coats and a powder coat is one pass through an oven; section sizes, because a timber lantern uses considerably more material for the same span; whether the frame arrives assembled and needs mechanical lifting; the lead time, which affects the programme rather than the frame; and the long term upkeep, where timber carries a redecoration cycle that aluminium does not. The costs page explains how the indicative range and the fixed price after survey work. Other written answers, including what colour a lantern frame should be, sit under guides.