Aluminium or uPVC roof lanterns

The two materials are built in quite different ways, and most of the practical differences follow from that.

Aluminium

Lantern frames are extruded from aluminium alloy, so each bar is a hollow profile with a precise cross-section. Because aluminium conducts heat readily, the better systems are thermally broken: an insulating polyamide strip separates the inner and outer metal so cold cannot travel straight through the bar. The visible faces are powder coated or anodised after the bars are cut and welded or mechanically jointed.

uPVC

Frames are extruded from rigid plastic in multi-chambered profiles, which trap air and give the material its insulating value. A plastic bar is not strong enough to span a large lantern on its own, so most uPVC lantern bars carry a steel or aluminium reinforcing core inside the plastic. The finished frame is welded at the corners, and the colour is either mixed through the plastic, which in practice means white, or applied as a foil laminate over the surface.

In both cases the frame is only half the lantern. The glass sits in a gasket or a bonded channel along each bar, and the base of the lantern is a separate frame that sits on the upstand. That base is where most of the weathering detail happens, and it is where the choice of material affects the finished roof least.

Stand under a lantern and look up. The glass is the point, but the bars are the first thing the eye reads, because they are the only solid lines in all that brightness. The thinner they are, the more the room feels like it has an opening to the sky. The heavier they are, the more the lantern reads as a piece of joinery.

Aluminium has the advantage here because metal is stiffer than plastic for the same section. Its bars can be slim without bending under the weight of the glass. A uPVC bar needs more plastic around its reinforcing core to reach the same strength, so it is usually deeper and wider. Neither is better in the abstract. A slim aluminium bar suits a clean modern extension. A broader uPVC bar often suits a Victorian or Edwardian house with a conservatory-style roof, and some people prefer the more substantial look.

The table is a general guide, and lantern systems vary. Judge a bar by holding a sample section next to your ceiling finish.

A roof lantern is a large area of glass and frame in the warmest part of the room, and the frame is the weakest point in its thermal performance. Warm, damp air rises, meets the coldest surface at the top of the room, and condenses. If the cold surface is a glazing bar, you get a line of drips along the bars on a cold morning. If the frame is warm enough, you get nothing.

Aluminium without a thermal break would be a poor choice for exactly this reason, and modern lantern systems do not use it. A thermally broken aluminium bar with a warm-edge spacer in the glazed unit performs well, and Approved Document L sets limiting U-values that a finished lantern has to achieve, whichever material the bars are made from. The material is not what decides whether a lantern is efficient. The system is, and that includes the glass specification, the spacer, the seals and the way the base is insulated into the upstand.

Where the two do differ is the margin for error. A uPVC bar insulates because of its construction, so it is forgiving. A thermally broken aluminium bar insulates because of a thin strip of polyamide, which needs to be continuous and correctly seated at every joint. Good systems do this well, and we use the maker's published data for the specific lantern rather than rely on a general rule about material. If condensation has been a problem in your current room, the cause is more often a cold upstand or an unventilated room than the frame, and our guide to roof lantern upstands and kerbs explains how the base is insulated.

This is where the two materials part company most visibly.

Aluminium takes a powder coat in almost any colour, and a standard architectural range includes the greys, blacks and soft whites that suit new extensions and bifold doors. A common detail is a different colour inside and out: a dark grey outside to sit against the roof covering and the sky, and a white inside to disappear into the ceiling plaster. The coating is baked on and is hard wearing, though it can chip if the lantern is knocked during installation, which is why the bars are protected until the roof is complete.

uPVC is most dependable in white. Foiled colours exist, including woodgrain effects and greys, but the foil is a thin film over a white plastic core, and a scratch or a lifted edge can show the white underneath. White uPVC can also yellow over many years in strong sun, particularly on cheaper formulations, and a lantern is facing the sky all day. Quality varies between manufacturers more than between materials, so it is worth asking what the profile is made from and how it has been tested for weathering.

Think too about the colour you see from inside. From the room, a lantern frame is usually seen against a white ceiling. A white frame on a white lining can look soft and calm. A dark frame gives a graphic outline, a line of ink drawn around the glass, which looks wonderful in a plain white room and heavy in a small one.

The longer and wider the lantern, the more the frame is asked to do. Glass is heavy, and a large double or triple glazed unit weighs a good deal more than a window pane. Along the length of the lantern, the ridge and the bars have to carry that weight without deflecting, and they have to hold the glass flat enough that the seals stay tight.

Aluminium handles this better. A long, wide lantern over an open kitchen and dining room, say four metres or more, is nearly always aluminium, because the bars can stay slim while carrying the load. A smaller lantern over a hallway or a bedroom is within the range of either. If you are weighing a very large uPVC lantern, ask to see the manufacturer's span data and the reinforcement specification for that exact size, because a plastic bar that bows is a fault you see from the room every day.

Devon roofs are not all in the same weather. A lantern in Exmouth or Dawlish takes salt-laden air, a lantern on a Topsham extension sits above the Exe estuary, and a lantern on the edge of Dartmoor takes driving rain and hard frost.

Powder coated aluminium is widely used in coastal homes, though the coating matters. Salt can attack bare or damaged aluminium, so near the sea the powder coat should be a marine-grade or high-durability specification, and any fixings and cappings need to be compatible with the frame. Rinsing the frames with fresh water now and then removes salt deposits. uPVC does not corrode, so salt air is less of a concern for the frame itself, though the metal reinforcement is hidden inside and the fittings and gaskets still age in sun and salt.

On the moor edge and in the Culm and Exe valleys, frost and wet are the main tests. Both materials cope well. The thing to look at is the drainage and the seals rather than the frame, because a lantern in a damp, shaded spot, under trees or by a north-facing wall, collects algae on the glass and the bars whichever material they are. How long a lantern lasts depends on the system and its maintenance more than the material, and how long roof lanterns last goes through the parts that wear first.

A lantern is one part of a roof, and the roof is one part of a house, so the frame should talk to what is around it.

If the extension has aluminium bifold or sliding doors with slim frames, an aluminium lantern in the same colour looks planned. If the back of the house has white uPVC windows and a uPVC door, a white uPVC lantern ties in, and a dark aluminium lantern on top can look like it came from a different house. On a Georgian or Regency terrace in Southernhay or St Leonards, a slim aluminium lantern in a dark neutral often suits the proportions better than a heavy white bar. On a 1930s semi in Heavitree with a pitched-roof extension at the back, the answer depends on the windows and the garden: a plain white lantern can look right, and so can a dark one.

Planning can matter too. On a listed building or in a conservation area, the frame colour and bar profile are part of what the local planning authority looks at, and the Article 4 direction in force (where one applies) may change what is permitted. Exeter City Council or the relevant district council can tell you where your house stands, and our Devon guide to rooflight planning and Building Regulations sets out the usual rules.

The material you pick for the frame has consequences inside. The lantern's base frame sits on the upstand, and below that there is a reveal, the sides of the opening, which is plastered or boarded and painted. The frame and the lining meet at the paint line, and the way they meet depends on the frame.

Aluminium base frames have a clean, sharp lower edge, and the plaster or board can be stopped against them with a stop bead to leave a fine shadow line. The thin inside face also lets the lining sit tight to the glass. uPVC base frames are slightly deeper and softer in outline, and often finish with a plastic trim. A well-formed lining meets either neatly, but the detail at the junction differs, so we agree it before the lantern is ordered.

Inside colour is the other consideration. If you want the frame to disappear, a white interior to match the ceiling does that. If you want the frame to be a feature, a dark interior against a pale lining draws the eye. Either can be right, and it is better chosen alongside the ceiling finish than after it. We cover the ceiling options in roof lantern ceiling finishes, and every one of them goes into the finish schedule that comes with the quote, so the frame colour, lining depth and paint line are written down together before anything is ordered.

In general, uPVC lanterns cost less than aluminium ones of the same size, and the gap widens as lanterns get larger and more specified. The reasons are the raw material, the amount of fabrication and the finish. Powder coating to a chosen colour, thermal breaks, longer spans and more complex ridge details all add cost to an aluminium system.

It is not the full picture, though. The frame is only one line in the price of installing a lantern. The glass specification, the upstand, the roof covering, the structural work to carry the opening, the lining and the finishing all weigh as much. A cheaper frame on a poorly built upstand is no saving, and a more expensive frame on a well-built one is value you can see every day. We give a written quote after a survey, with the frame, glass and finish itemised in the schedule, so you can compare like with like. For general guidance on what moves the price of rooflights, see what rooflights cost.

A short test helps. Start with the doors: if they are aluminium, lean aluminium; if they are uPVC and you like them, lean uPVC. Then look at the span. A long lantern points to aluminium. Look at the house: a slim, dark frame suits some homes and a white, substantial one suits others. And look at the exposure: near the sea, insist on a marine-grade coating if you choose aluminium.

Whichever you pick, the frame is one decision in a connected set: the size of the lantern, the height of the upstand, the glass, the lining and the ceiling it meets. The full picture is on our roof lantern installation page, and if you would like a lantern specified for your own roof, ask for a survey through the quote form and we will bring sample sections for both materials.