Longer than most people expect, and the useful answer separates the parts. A frame and a sealed unit are long life components measured in decades. Seals, springs, friction pads and handles are wearing parts measured in years. A window that feels tired is usually a wearing part rather than a window at the end of its life.
How long a rooflight lasts generally
Set out once, with the reasoning, on how long do rooflights last.
What follows is the component picture specific to a branded timber cored roof window, which is a different question from the life of a unit.
The frame
The longest lived part by some distance.
A laminated timber core, either lacquered or factory coated, is a stable section that does not move much with humidity, and in a normally heated room it is good for a very long time.
What ends a frame is water, not age.
The sealed unit
Decades, and it fails in one particular way.
The edge seal that holds the cavity gas eventually admits moisture, and when it does the unit mists internally rather than degrading gradually.
That is a pane replacement rather than a window replacement, and it is on why is my rooflight misted up.
The sash seal
The first thing to go, and the cheapest to put right.
A rubber gasket compresses over years of closing, and once it has taken a set it stops sealing evenly, which shows as a draught before it shows as anything else.
It is a stocked part on current ranges.
Springs and friction
What makes a sash feel old.
The springs that hold a sash where you leave it weaken, and the friction pads wear, so a window that used to stay put starts to creep or becomes heavy to move.
Both are replaceable without touching the roof.
The handle and the latch
Ordinary hardware wear.
A ventilation latch used daily for twenty years will eventually be loose or stiff, and handles are available against the size code and the range.
The external cladding
Where the coastal position tells.
The aluminium capping over the frame takes weather directly, and on a salt laden site the coating dulls and can eventually pit, particularly on an exposed south west elevation.
What that exposure does is on Goring by Sea.
The flashing
Frequently outlives the window it was fitted to.
A lead or aluminium kit dressed properly and not disturbed will still be sound when the sash is worn out, which is part of why a like for like replacement is quick.
What actually shortens a window
Moisture reaching the timber.
A failed reveal detail, an unventilated bathroom, or a leak left running will rot a frame from behind long before anything else on the window is worn out.
Which is why the reveal is treated as part of the window rather than as decoration.
The lacquer on a pine unit
The one item that asks something of the owner.
A clear lacquered timber sash is a maintained finish, and it wants inspecting and recoating periodically, more often on a sunny elevation and in a humid room.
Left too long, the timber greys and the job becomes larger than a recoat.
A coated frame instead
Where maintenance is the concern. A white polyurethane frame over a timber core is factory finished and wiped rather than painted, which suits a bathroom, a high window or an owner who would rather not think about it.
The comparison is on what is the difference between GGL and GGU.
Blinds and motors
Shorter lived than the window, and expected to be.
Fabric fades, and a motorised opener or blind is an electro mechanical part with its own service life, replaceable independently of the window.
What maintenance actually consists of
Very little, done occasionally. Wipe the sash seal, clear the drainage channels at the sill, check the lacquer on a timber sash, and clean the filter in the ventilation flap.
Twenty minutes a year is the whole of it.
When replacement is genuinely right
Where the frame has gone, or where the performance is the point.
A rotted frame is replaced rather than repaired, and a very old single or early double glazed unit is often replaced for the thermal step rather than because it has failed.
What the drainage channels do
Small detail, large consequence, and almost nobody knows they are there.
The frame carries channels that take the water running off the glass and any condensation forming at the sill, and carry it out below the sash rather than letting it sit against the timber.
Blocked with dust, moss or paint, they hold water against the one part of the window that cannot take it, which is how an otherwise sound unit starts to rot.
The ventilation flap and its filter
A wearing item that is easy to overlook entirely.
Most units carry a trickle ventilator in the top of the sash with a filter behind it, and over years it clogs with dust and, near the sea, with salt.
It pulls out, rinses and goes back, and a clear one makes a noticeable difference to a bedroom overnight.
What a coastal position does over decades
Accelerates everything external and leaves the internals alone.
Salt shortens the life of a coating and of any exposed fixing, so a window a mile inland and one on a seafront elevation age at visibly different rates even though the timber core is doing the same job in both.
What we look at on a VELUX
We look at the frame and the reveal before we look at the sash, because a window that is failing early is almost always telling you about water rather than about wear.
The rest is on the VELUX window installation page.
Thinking about this job? We survey across Worthing, West Sussex and the South Downs and give you a fixed price before any work starts, with a 10-year workmanship guarantee.