Timber and polyurethane roof window frames

What is actually under each finish

Both frames are timber. That surprises people who assume the white one is plastic all the way through, and it matters, because the structural behaviour of the two is identical.

The pine version is laminated softwood, finished with a water-based clear lacquer that leaves the grain visible. The white version is the same laminated softwood core with a moulded polyurethane layer bonded over it, typically 1 to 3mm thick, wrapping the sash and frame faces in one continuous piece with rounded corners and no visible joint. Some ranges offer a third option, which is the pine frame factory-painted white rather than moulded, and that is a different product with different behaviour again. All three take the same external aluminium cladding on the weather face, so from the roof they are indistinguishable.

Laminated pine, and why the timber is built up rather than solid

The pine is finger-jointed and laminated into three or four layers with the grain alternating, in the same way a glulam beam is made, and it is done for stability rather than for cost. A solid softwood section 70mm deep and a metre and a half long will cup, twist and bow as it takes up and gives off moisture, and a sash that twists 3mm across its width will not seal against its gasket.

Laminating cancels most of that movement out, because each layer restrains the ones beside it. The timber is also pressure treated before lamination on most ranges, which puts preservative right through the section rather than on the surface. That is why a thirty year old pine roof window with a sound lacquer is often perfectly serviceable timber, and why the failure when it comes is usually at the glass seal rather than in the frame.

The polyurethane skin, and the thing people get wrong about it

The moulded layer is not paint and it is not a laminate film. It is a thermoset polyurethane poured or injected around the prepared timber in a mould, so it cures into one shape with the corners and profiles already formed. There is no mitre, no butt joint and no fastener head anywhere on the visible surface.

That is the whole argument for it. Water sitting in the corner of a sash cannot find a joint, because there is not one. Steam condensing on a bathroom sash runs down a continuous surface and out through the frame’s drainage channel. A brush and warm water takes the surface back to new, and there is no lacquer to renew on a cycle. What it will not do is repel damage: a deep gouge through the skin exposes the timber core, and the only honest response is a touch-in that will always be slightly visible.

The lacquer on a pine frame, and the interval it wants

A clear-lacquered pine frame is a maintained finish. Manufacturers publish a recommendation of roughly every four years in a normal room, and more often in a room with high humidity or strong direct sun on the internal face.

The work itself is undemanding: a light key with fine abrasive paper, dust off, and a coat of the manufacturer’s water-based lacquer with a brush, keeping it off the gasket and off the glass. An hour a window. The reason it gets skipped is not difficulty, it is that the window is above head height and out of the normal cleaning routine, and by the time anybody looks properly the bottom rail of the sash has gone grey and slightly furry where condensation has been sitting on it every winter morning for a decade. At that point it is a sanding job rather than a coating job.

White painted timber, and where the paint film goes first

The factory-painted pine option sits between the two. It gives the white appearance without the moulded skin, at a slightly lower price on most ranges, and it is a good product in a dry room.

Its weakness is the corners. A paint film applied to an assembled frame has to bridge the mitre and the joint between sash rail and stile, and those joints move fractionally with temperature and humidity. Over ten or fifteen years a hairline crack opens along the joint line, water finds it, and the film lifts locally. It is entirely recoverable with filler and a brush, but it is maintenance the moulded version does not ask for. In a bathroom or a kitchen the painted option is the weakest of the three.

Moisture, and the rooms that decide it on their own

A bathroom, shower room, en suite or utility settles this question without further discussion, and so does a kitchen with a hob under the slope. Those rooms put water vapour into the air in bursts, the coldest surface in the room is the glass overhead, and the water that condenses on it runs down onto the bottom rail of the sash and sits there.

A moulded frame handles that indefinitely. A lacquered frame handles it if somebody wipes the rail down and keeps the coating up, and gradually loses if nobody does. The frame’s own drainage channel and weep holes take most of the water away in either case, which is why keeping them clear matters more in a wet room than anywhere else. Ventilation is the other half of the same problem and it sits on the ventilation flaps and filters page.

Thermal behaviour of the frame section

Softwood conducts heat at roughly 0.13 W/mK, which is around four times better than the polyamide-broken aluminium used in flat roof rooflight frames and hundreds of times better than solid aluminium. So a timber-cored roof window frame is a genuinely good thermal edge for the glass, and both finishes share that advantage because both have the same core.

The polyurethane layer is itself a closed-cell material with a conductivity around 0.03 W/mK, so in principle the moulded frame is fractionally warmer. In published whole-unit U-values the difference between the two finishes on the same range is small, usually within 0.1 W/m2K and sometimes identical, because the glass dominates the figure. Do not choose between them on thermal grounds. The numbers that do move a U-value are on the energy efficient rooflights page.

Section width, sightline and the glass inside the same code

The moulded frame has slightly fuller sections than the lacquered one on most ranges, because the skin adds thickness to every face and the mould needs radiused corners rather than sharp arrises.

The consequence is small but real: for the same size code, the moulded version can give a millimetre or two less visible glass on each edge, and the internal profile reads slightly softer and chunkier. Nobody notices it on a single window. On two windows side by side, one of each, it is visible, which is one of several reasons a run of units is specified in one finish. Where the glass area matters, the manufacturer publishes it per finish and it is worth reading rather than assuming.

Weight, hinges and how the sash behaves in the hand

A moulded sash is marginally heavier than the equivalent lacquered one, on the order of a kilogram on a mid-size unit. The hinges, friction settings and springs are specified for the heavier version anyway, so the operation feels the same and the manufacturer’s pitch range does not change between finishes.

Where weight does register is on a large unit in a steep roof. A 1,140 by 1,400mm sash on a 50 degree Forest of Dean slope is a substantial object to rotate, and it wants a hinge in good adjustment whichever finish it carries. That is a hardware question rather than a material one, and it is covered on the handles, catches and child safety page.

How each one reads from a bed or a chair

You look at a roof window frame more than you look at any other frame in the house, because it is overhead and in your eyeline whenever you lie down or sit back.

Pine reads as an object. It has grain, it has warmth, it draws the eye to the aperture, and in a room with exposed purlins, a timber floor or existing joinery it belongs. It also ambers with ultraviolet exposure, and a south-facing pine sash after ten years is a noticeably deeper honey colour than the day it went in, which some people like and some do not expect.

White recedes. Against a white sloping ceiling the frame disappears and the room reads as a hole in the roof rather than a window in a box, which is generally the better outcome in a small bedroom or a bathroom where you want the ceiling to feel continuous. On a north slope, where light is diffuse, white also bounces more of what arrives back into the room.

Colour, staining and matching a finished room

A lacquered pine frame can be stained or painted on site once it is in, and people do, usually to match skirtings or a fitted wardrobe. It works, with two conditions: the existing lacquer has to be keyed properly first, and nothing goes on the gasket, the hinge mechanism or the drainage channel. A paint bead across a weep hole is how a frame starts holding water.

A moulded frame is a poor candidate for painting. Adhesion onto a cured polyurethane surface is unreliable without a specific primer, and a failed coat on a moulded frame looks far worse than an unpainted one. If a colour other than white is wanted, the practical route is a lacquered pine unit finished on site, decided before the reveal is plastered so the decorator does it all in one visit.

Cleaning, marks and fifteen years of hands

The handle bar on a centre pivot gets touched every time the window is opened, and on a lacquered pine frame that shows as a polished band that goes darker over the years. On a moulded frame it wipes off.

The other real-world difference is the bottom rail of the sash. It collects dust, insects, condensation and, in a kitchen, a fine film of cooking grease. A moulded surface takes a damp cloth and a mild detergent and comes back white. Lacquered pine tolerates the same treatment as long as the coating is intact, and starts absorbing marks once it is not. Neither surface likes solvent cleaners, abrasive creams or anything with bleach in it, and both lose their finish quickly if they get them.

The aluminium on the outside, which both of them share

Nothing described above is exposed to the weather. The external face of a modern roof window sash is clad in a powder-coated aluminium profile in a dark grey, usually around RAL 7016 anthracite, and the flashing kit is the same metal family. So from the garden, a pine window and a white one look identical.

That cladding is why frame maintenance is an internal task only, and it is why the timber core survives as long as it does. It also means the finish decision has no bearing whatsoever on what a conservation officer sees, which occasionally comes as a relief on a Cheltenham or Cotswold elevation. The heritage products that do change the external appearance are on the conservation rooflight page.

Price, stock and lead time

Across the ranges the two finishes sit close on price, with the moulded version usually a little above the lacquered one on like-for-like glazing. The gap is small enough that it is rarely the deciding factor, and on some sizes it reverses depending on what a merchant holds.

Availability is the more useful difference. Common sizes in both finishes are stocked, but merchants tend to carry deeper stock in whichever finish sells fastest locally, and on an unusual size code one finish can be several weeks behind the other. Where a job has a fixed date, the finish is confirmed against actual stock before the covering comes off, because a roof window is ordered and on site before anybody opens the roof.

What the survey settles before a finish is fixed

The finish is confirmed at the same visit that fixes the size code, because both go on the same order line and neither can be changed once the unit is made.

What gets recorded: the use of the room and whether steam is generated in it, whether there is a radiator or heat source below the window, the orientation of the slope and how much direct sun the internal face will take, what other joinery and finishes the room already has, whether more than one unit is going in and therefore has to match, and whether anybody in the household is realistically going to re-coat a frame every few years. That last question is asked plainly, because the honest answer decides between two products that are otherwise very close. The costs page explains how the specification turns into a figure.