How long does a roof window last?

Why glass in a roof works harder than glass in a wall

A sealed unit is two or three panes separated by a spacer bar, with a desiccant inside the spacer absorbing the small amount of moisture trapped at manufacture, and two seals around the edge: a primary butyl seal that stops vapour crossing, and a secondary structural seal holding the sandwich together. That assembly is durable and it is not eternal.

In a roof it works harder than the identical unit in a vertical window, for three reasons that are all about position. The glass takes direct sun for hours, so the edge seal cycles through a much larger temperature range every day, and on a south or west facing Gloucestershire slope in July that range is considerable. The pane sits close to horizontal, so rain lies on it rather than running straight off, and the edge seal spends far more of its life wet. And ultraviolet exposure on a roof is simply higher than on a wall. Fifteen to twenty-five years is therefore normal, and units glazed on the same day tend to reach that point within a couple of years of each other, which is why a roof with four windows often needs attention all at once. That is covered at replacing a failed or misted rooflight.

Timber, factory finish, and the coat that extends everything

The traditional roof window frame is laminated pine, vacuum-impregnated and given a factory lacquer or paint finish under controlled conditions. That finish is what stands between the timber and thirty years of Gloucestershire weather, and keeping it intact is the single largest thing an owner controls.

Kept sound, a timber sash outlives the glass in it. Allowed to dull, chalk and eventually break, the timber starts taking on moisture at the exposed grain, the finish lifts further, and the sash swells and stops closing evenly against its gaskets. The fix is unglamorous: a light rub down and a fresh coat of the manufacturer’s water-based lacquer every four to five years on an exposed slope, a little less often on a sheltered one. It takes an afternoon and it adds a decade. What to use, and where the sash finish is different from the frame finish, is on can a roof window be painted.

The other timber question is the one behind the plaster. A reveal with insulation running continuous and tight to the frame, and a vapour control layer taped to it, keeps the frame perimeter warm and dry. A 20mm void behind the plasterboard at the head is a cold bridge, and a cold bridge puts moisture into the timber it touches every February.

Polyurethane sashes, and the rooms they last longest in

Every major range now offers a sash built around a timber core moulded in one piece of white polyurethane, with no joints in the surface for water to enter and no coating to maintain. In a bathroom, a kitchen, a utility or a wet room, where the humidity swings hard twice a day, that construction reaches the far end of the range where a lacquered timber sash in the same position would be at the near end.

It is not maintenance-free, it is finish-free. What it removes is the recoating cycle and the risk that the cycle gets forgotten. Bathroom specification is at fitting a roof window in a bathroom.

Gaskets, hinges and the parts that stay renewable

The moving parts are the good news in this, because they are the components designed to be looked after rather than lived with.

Gaskets go hard, flatten and shrink at the corners over ten to twenty years, faster on a slope that takes full afternoon sun. Kept clean with soapy water and given a light silicone treatment once a year they stay supple much longer, and when they do reach their end they are a stocked spare part on current ranges rather than a reason to change the window. Hinges, friction pads and the sash catch are adjustable throughout the unit’s life, and a sash that has started to drop or catch on one corner is usually asking for adjustment rather than anything more.

The ventilation flap filter in the head is the part most often forgotten entirely. It pulls out, rinses under a tap and goes back, and in a kitchen or bathroom it wants doing more than once a year. A blocked filter means a closed vent, and a closed vent means more moisture reaching the coldest surface in the room.

Lead flashings, and the fifty year component

The perimeter is the part with the longest span and the least attention paid to it. Code 4 lead for most flashings around a roof window, code 5 where it is exposed or forms a gutter, and correctly detailed it outlives everything else on the slope.

Correctly detailed means one thing above all: every piece fixed at the head only and left free along its length, in runs no longer than about 1.5 metres, lapped 150mm. Lead expands and contracts several millimetres in a metre run between a frosty Cotswold dawn and a July afternoon, and lead nailed down both sides has nowhere to go, so it tears itself along the fixing line within a few winters. Cracked lead along a row of nail holes is the most common finding when a thirty-year-old unit comes off a Gloucestershire roof, and it is a fixing decision rather than a material failure.

Underneath the metal, the underlay matters just as much and is the same story. Dressed properly around the frame it does its job silently for the life of the covering. Slit straight and pushed over the frame it starts causing trouble much sooner. The reasoning is at do rooflights leak.

Recognising the end of a service life

A window generally tells you well in advance, and the signs are specific enough to be read from the room below.

Cloudiness or streaking between the panes that never wipes off means the primary edge seal has let go and the cavity has taken on moisture. There is no way to dry that cavity out again, and any product claiming otherwise is selling a hole drilled in a piece of glass. Draught, whistling, or daylight visible at the edge of a closed sash means the gaskets have perished. A sash that drops out of square, will not latch, or has become stiff through its whole travel is usually a hardware or a trimmer question rather than a glass one. Bare or blackened timber at the bottom rail of the sash means the finish gave way some time ago.

Where one of those turns up on a unit past twenty years old, the arithmetic tends to favour a new complete window. The glass, the gaskets, the sash finish and the flashing kit are all the same age and all at the same stage, so changing the glass alone leaves the rest to follow within a few seasons.

Replacement, and the standard a new unit is held to

Putting a new window into an existing opening is quick work compared with forming a new one. The old unit comes out, the timber is checked with a moisture meter and anything soft is cut out and replaced in treated stock, the underlay is renewed locally and dressed properly around the new frame, insulation is reinstated tight and continuous, and a new flashing kit ties the covering back in. On a like-for-like swap in a finished loft room the whole thing is frequently done from outside, leaving the internal plaster and decoration untouched.

Thermal performance has moved a long way in the meantime: a 1990s double-glazed roof window sits around 2.8 to 3.0 W/m2K and a current unit runs around 1.0 to 1.4, with solar control coatings and laminated inner panes that did not exist when the old one was made. The numbers are on energy efficient rooflights.

Severn Skylights Gloucester fits new roof windows and replaces units that have reached the end of their service life, across Gloucester and thirty miles by road. The workmanship carries a ten-year guarantee, written against the perimeter details that decide longevity, and the glass and frame carry the manufacturer’s own warranty as a separate document. Brand-specific pane codes are on the VELUX window installation page. For a view on where your window sits in its life, email info@severnskylightsgloucester.co.uk with a photograph of the sash data plate and one from inside the room.