Roof lantern frame finishes and powder coating

How a powder coat is actually applied

The aluminium extrusion is cleaned, chemically pretreated, then hung in a booth where dry polyester powder is sprayed with an electrostatic charge that makes it cling to the metal. The whole piece then goes through an oven at around 180 to 200 degrees, where the powder melts, flows and cures into a continuous film.

It is not paint and it does not behave like paint. There is no solvent, nothing to evaporate, and the cured film is tougher and thicker than a wet-applied coat. It also cannot be applied on site, because the oven is the whole process.

Pretreatment, the step nobody sees

The single largest difference between a coating that lasts thirty years and one that lifts in eight is the chemistry that happens before the powder arrives. The extrusion is degreased, etched to remove the natural oxide layer, and given a conversion coating that gives the powder something to bond to.

Older systems used chromate. Most coaters now use chrome-free alternatives, which perform comparably when the process is controlled. What matters to you is that the coater works to a recognised standard such as Qualicoat or BS EN 12206, which sets the etch weight, the film build and the testing regime. Ask the lantern supplier which standard their coating house works to. If they know the answer, that is informative in itself.

Film thickness, and the number to ask for

The standard is a minimum of 60 microns of cured film, with 70 to 80 typical on architectural work. Below 60 you get thin cover on sharp arrises, which is exactly where a coating fails first.

Thicker is not automatically better. Past about 120 microns the film becomes brittle and can chip when handled. The useful specification is a consistent 60 to 80 microns including on edges and corners, which is a function of how the piece is hung and sprayed rather than how much powder is thrown at it.

Gloss, satin and matt in real daylight

Sheen is quoted as a gloss percentage measured at 60 degrees. Gloss is 80% and above, satin runs around 30% to 50%, matt sits at 30% and below.

On a lantern this matters more than on a window, because a lantern is seen against the sky from outside and against a bright ceiling from inside. High gloss on a large frame throws hard reflections and shows every ripple in the extrusion. Matt absorbs light, hides small imperfections, and makes the bars read thinner because there is no bright highlight running along them. Matt does hold dirt slightly more readily on a horizontal surface, and it marks a little more visibly if it is scuffed.

Our default recommendation for a lantern is a matt or low satin exterior. It is the finish most current extensions use and it is the one that ages least conspicuously.

The colours everyone picks, and why

RAL 7016 has become the standard almost everywhere, which is a good reason to consider it and also a reason to look at 7039 or 7015 if you want the house to look less like the one three doors down. The specific choice for a given roof and brick is covered on our page about frame colour.

Different colour inside and out

Dual colour finishing means the outer face is coated one colour and the inner face another, usually anthracite outside and white or a pale grey inside. It is a genuinely useful option on a lantern because the two faces do completely different work: outside the frame should disappear against the sky, inside it should reflect daylight down into the room rather than absorbing it.

It is done by masking and coating in two passes, so it adds cost and lead time, typically a week or two and a modest percentage on the frame price. On a thermally broken system the break itself gives a natural division line between the two colours, which makes the result cleaner than it sounds.

Matching a lantern to existing doors and windows

This is where projects come unstuck. A lantern in RAL 7016 matt and bifold doors in RAL 7016 satin, standing next to each other in the same daylight, will not look like the same colour. The RAL number matches and the sheen does not, and sheen is what the eye reads at a distance.

Three things have to line up: the RAL number, the sheen percentage, and the texture. Get all three from the door supplier in writing and pass them to the lantern supplier. Where the doors are already fitted, the surer route is to send a sample of the door frame to the coater and have the lantern matched to the object rather than to a code.

Batch variation, and how to avoid a mismatch

Powder is made in batches and batches vary slightly. Two frames coated eight months apart in the same RAL can differ enough to be visible when placed side by side, particularly in mid greys.

Where a lantern and a set of doors are going into the same elevation, order them together and ask that they are coated in the same batch. Where a second lantern is being added later to an extension that already has one, expect a small difference and place the two so they are not seen in the same view if you can. Nobody notices a difference across a room. Everybody notices one across a joint.

Textured and metallic finishes

A fine texture, sometimes sold as a sablé or matt textured finish, has a slight grain that hides handling marks and fingerprints better than a smooth coat. It is a good choice on the internal face of a lantern within reach, and on frames that will be cleaned often.

Metallic and pearl effect powders contain aluminium flake and look excellent when new. They are less forgiving over time, because any localised touching in shows as a flat area where the flake orientation is different. If a frame is likely to be marked in transit or on site, a plain finish is the safer specification.

How long it lasts on a roof

A properly pretreated architectural powder coat on a lantern in Oxford should hold its colour and gloss for twenty five to thirty years, with slow, even fading that you will not notice year to year. Manufacturers commonly warrant the finish for ten to twenty five years subject to a cleaning schedule.

Failure, when it happens, is nearly always traceable to pretreatment or to damage. A scratch through to bare aluminium admits moisture under the film, and the coating then lifts outward from that point over several years. That is why a mark on a frame is worth attending to promptly rather than living with.

Cleaning that keeps the finish and the warranty

Twice a year, warm water and a mild detergent with a soft cloth or sponge, then rinsed. Nothing abrasive, no solvents, no cream cleaners, no pressure washer at close range. That is the whole regime, and it is almost always a written condition of the finish warranty.

On a lantern the accessible part is the internal face and the external frame near the eaves. The upper external surfaces are usually cleaned at the same time as the glass, which is a job for a pole system or a person on the flat roof, covered on our page about cleaning a roof lantern. Keeping the drainage channels at the eaves clear of moss and leaf debris matters as much as the appearance, because standing dirt holds moisture against the coating.

Anodising as an alternative

Anodising is an electrochemical process that thickens the aluminium’s own oxide layer, then dyes it. The finish is part of the metal rather than a film on top of it, so it does not chip and it wears exceptionally well.

The trade-offs are real. The colour range is limited to natural silver, bronzes, greys and black. Colour consistency between extrusions is harder to control than with powder, because the result depends on the alloy. And it costs more. On most domestic lanterns powder coat is the right answer, and anodising is worth considering where the frame is very large or where a metallic finish is wanted for its own sake.

Timber lanterns and their finishing schedule

A timber lantern is finished either with a factory-applied microporous stain or an opaque paint system, usually three coats applied under controlled conditions before it leaves the works. Factory finishing is considerably more durable than anything brushed on site.

The maintenance interval depends on exposure and colour. On a sheltered rear addition, expect to inspect at five years and recoat at eight to ten. On an exposed elevation, or a dark colour taking full southern sun, closer to five. The recoat is a wash, a light abrade and a fresh coat, not a strip back, provided it is done before the film breaks.

Timber inside with aluminium cladding outside removes the maintenance entirely from the weather face and keeps the timber in the room. It is the expensive option and it is the one we would choose for a period house. The full comparison is on our page about aluminium and timber lanterns.

Lead times: stock colour against special

Most manufacturers hold stock in 7016, 9005 and one or two whites, and can produce a lantern in those within their normal lead time. A non-stock RAL adds one to three weeks, because the coater has to buy or mix powder and schedule a run. Dual colour adds a similar amount again. A matched-to-sample colour can add four weeks.

None of that is a reason to abandon the colour you want. It is a reason to settle the colour early, because a lantern is on the critical path of an extension and a colour decision made late holds up a roof. We ask for the finish specification at order stage for exactly this reason.

Marks, scuffs and what can be done on site

A powder coat cannot be recoated on site, because the process needs an oven. What can be done is a touch-in with an air-drying paint matched to the RAL, applied with a fine brush into the damaged area. On a small mark on a bar four metres up, this is invisible from the floor and it stops moisture reaching the aluminium, which is the point of doing it.

Larger damage, or damage on a visible internal face, means the affected section going back to the coater to be stripped and recoated, which on a lantern usually means removing that member. That is a good reason to insist frames are delivered with their protective film intact and kept wrapped until the last moment, and a good reason not to have a lantern stood in a garden for three weeks waiting for a roof.

We inspect finishes on delivery, before anything goes up, because a marked frame is far easier to deal with on the ground. For a range by email, send the size and the RAL you are considering, or call 01865 704245 and we will talk through the finish options for your roof before anything is ordered.