Guide · VELUX Window Installation
Timber and polyurethane roof window frames
Almost every roof window sold in the UK comes in two versions of the same product: a lacquered pine one and a white one. The white one is not plastic in the sense that a uPVC casement is plastic. It is normally the same timber core with a moulded polyurethane skin over it, which is why the two versions share sizes, hinges, flashings and glazing and differ mainly in how they behave in a damp room.
That is the short answer, and it is the one that decides most jobs. What follows is how each frame is actually built, what each asks of you over twenty years, and which rooms in an Essex house tend to end up with which.
What a polyurethane roof window is built from
Start with the core, because it is usually wood. A length of laminated softwood is machined to profile, then a rigid polyurethane is moulded around it under pressure so that the finished section has a continuous white skin with no visible joint at the corners.
The polyurethane is not a paint film and it is not a foil wrap. It is a structural coating several millimetres thick, cured hard, with the frame section formed in one operation. Some ranges take this further and build the sash from polyurethane over a smaller timber section, or from a multi chamber profile with no wood in it at all. The finish looks identical from the room and the difference only shows on a section drawing.
What a timber roof window is built from
Redwood pine, laminated, machined and then finished at the factory with a clear or lightly tinted lacquer. That is the specification across the major ranges and it has barely changed in decades.
The lacquer is water based on current products, applied in multiple coats, and it is a surface treatment rather than a barrier. Under it the wood is visible, with the grain and the glue lines of the lamination showing. The frame is jointed at the corners in the traditional way, which is a perfectly sound construction and also a set of joints that a white moulded frame does not have.
Laminated pine, and why the wood is built up in layers
The timber in a roof window is never a single solid piece. It is built from several lamellae glued together with the grain alternating, and often finger jointed along its length as well.
The reason is movement. A solid section of pine 70mm deep will cup and twist as it takes up and gives off moisture, and a frame that twists in a roof plane opens a gap the weather will find. Laminating puts the tendencies of each layer against each other so they cancel out. It also lets the manufacturer use shorter, more stable lengths of wood. A laminated frame is straighter at ten years than a solid one, which is the entire point.
The moulded shell, and the corners that are not joined
The advantage of a polyurethane frame is not that it looks cleaner. It is that the outer surface is continuous. There is no corner joint at the cill, and the cill corner is where water collects on any window and where a timber finish gives up first.
On a roof window this matters more than on a wall window, because the frame is lying back and the lower corners sit in a puddle every time the glass sheds rain. Condensation running off the inner pane goes to the same two corners. A moulded shell has nothing there for water to get into, which is most of the reason the white version exists.
Moisture in the room, and the frame that suits it
The choice is usually made by the room rather than by taste. A bathroom generates a large volume of warm vapour in short bursts, a kitchen does the same for longer, and a utility room with a tumble dryer can be worse than either.
In those rooms the polyurethane frame is the straightforward specification. In a bedroom, a living room, a study or a landing, the moisture load is modest and a lacquered timber frame will sit there indefinitely looking exactly as it did. Both frames carry the same glass and the same weathering, so this decision is about the internal surface, not about whether the window keeps rain out.
If the room has a shower, a hob or a dryer in it, specify the white polyurethane frame and stop thinking about it.
Where water actually settles on a roof window frame
Two places, consistently. The bottom corners of the sash, where run off from the inner pane collects, and the horizontal top face of the cill member, where anything that condenses on the glass eventually arrives.
On a lacquered frame those areas need the lacquer intact. Where a family shower runs twice a day into a small room, the lacquer at those corners takes a repeated wet and dry cycle that the rest of the frame never sees, and it is the first place the finish goes dull. On a moulded frame the same water sits on a surface that does not care. Neither situation is dramatic on a sensible ventilation regime, but it explains why the same window ages differently in two different rooms of the same house.
The thermal difference between the two frames
Less than people assume, and in a direction that surprises them. Pine conducts heat at roughly 0.13 W/mK. Rigid polyurethane sits lower, in the region of 0.03 W/mK for the foamed grades, though the dense moulded skin on a window frame is closer to the timber than that.
Because the polyurethane frame is normally built over a timber core, the whole section performs a little better than bare pine and the published whole window U-values for the two versions of the same model usually differ by a small margin, if at all. The frame is not where the thermal battle is won on a roof window. That is decided by the glazing, the insulation collar around the frame and the detail where the plasterboard meets the unit, which is set out on our page about energy efficient rooflights.
Factory finish, and what arrives on site
Both frames turn up finished. The timber one has its lacquer on, the white one has its shell on, and neither wants painting before it goes in. That is worth saying because a decorator with a tin of undercoat will happily coat a polyurethane frame that did not need it and will not hold the paint well either.
What does need attention on site is protection. A finished frame stood on its cill on a concrete floor picks up marks, and a white shell shows every scuff a lacquered pine frame would hide. Units stay in their packaging until the opening is ready and the sash is lifted out before the frame goes up the ladder.
Recoating a timber frame, and the interval it asks for
A lacquered roof window in a dry room is left alone. In a room with a moisture load, the working interval quoted by manufacturers is a light recoat every three to five years, and in practice it means a clean, a very light key with fine abrasive paper, and a coat of the manufacturer’s water based lacquer on the sash and cill.
It takes an hour with the sash rotated into the cleaning position. The point at which it becomes worth doing is when the finish looks matt where it used to look satin, not when marks appear. Left long enough, the wood beneath darkens and no amount of lacquer takes that back out.
Living with a white frame over twenty years
Polyurethane frames are washed and that is the whole of it. The finish is through the material rather than on top of it, so a scratch shows white underneath rather than a different colour.
The two things that do happen over a long life are worth knowing. Direct sun on a south facing slope will very slowly shift the tone of any white surface, so a unit replaced on the same elevation twenty years later may not match its neighbour exactly. And the surface picks up marks from rubber and adhesive, so tape left on a frame through a plastering phase is worth removing promptly.
How each frame reads in a finished room
This is a design decision and it is worth taking seriously, because a roof window is a large object in a small ceiling. Pine adds warmth and visible grain, and in a room with exposed timber, a wooden floor or a period feel it belongs. It also reads as a window rather than as a hole.
White disappears against a white ceiling and a white reveal, which is exactly what is wanted in a modern bathroom or a plain plastered loft bedroom. Where a reveal is painted a strong colour, a white frame draws a clean line around the glass and pine muddies it. There is no universally right answer, only a right answer for the room the unit is going into.
Hinges, fixings and what the frame has to hold
The ironmongery is the same across both versions in a given range, which is a useful fact. Hinges, catches, ventilation flaps and blind brackets are designed to fix into the timber core, so a polyurethane frame is drilled and screwed into wood exactly as a pine one is.
That is why the two versions share every accessory in the catalogue and why a blind, an awning or a motor kit bought for one fits the other. It also means the fixing brackets that anchor the frame to the trimmed rafters are identical, and the installation is the same job either way.
Weight, handling and getting the unit up a stair
A polyurethane framed unit is usually marginally heavier than the pine equivalent of the same size, because the shell adds material to a core that is already there. On a 780 x 1400 unit the difference is a couple of kilograms and nobody notices.
What does matter is the glass. A double glazed roof window at that size runs around 35kg to 45kg complete, and a triple glazed one considerably more. The sash comes out of the frame for the lift in both cases, which halves the awkward part. Access up a narrow Victorian stair with a turn in it is the real constraint, not the frame material.
What separates the two on price
The polyurethane version of a given model typically carries a modest premium over the lacquered pine version, in the order of ten to fifteen per cent on the unit. It is not a different class of product and it does not change the flashing, the fitting or the structural work.
Set against that is what each asks of you afterwards. If a lacquered frame in a bathroom is going to want an hour of attention every four years, the premium on the white unit is bought back inside a decade. In a dry bedroom there is nothing to buy back and pine is the better value.
Essex rooms, and which frame usually wins
The pattern across the county is consistent enough to be useful. Loft bathrooms and en suites in converted Victorian terraces in Chelmsford and Colchester take polyurethane, every time, because the rooms are small, the ventilation is often a single extract and the moisture load is concentrated.
Kitchen extensions with a hob under or near the opening take polyurethane for the same reason. Loft bedrooms, studies and landings on the interwar semis across the commuter towns take pine as often as not, and in the timber framed houses around Thaxted, Coggeshall and Saffron Walden a pine frame is frequently the better fit for the building. Where a conservation frame is required the question changes entirely, and that is covered on our conservation rooflight page.
What the order should specify
The frame material is a single character in most manufacturers’ model codes, which makes it easy to get wrong and easy to check. A quote should name the model, state the finish in words as well as code, and confirm the glazing separately.
Ask for the finish written out: “white polyurethane” or “clear lacquered pine”, not just a letter. Ask which components are timber and which are moulded, since ranges vary. And settle it before the order goes in, because the finish is not something that gets changed once a unit is on a lorry. When we specify a VELUX installation, the frame finish is agreed at survey alongside the glazing and the flashing kit, so all three arrive matched.
Keep reading
More on velux window installation
Roof window escape openings and the rules
Read the guide →Roof window blinds and when to specify them
Read the guide →Roof window flashing kits by roof covering
Read the guide →Roof window sizes and how they are coded
Read the guide →Roof window ventilation flaps and filters
Read the guide →
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