Gaskets are the cheapest component on a rooflight and the one whose failure produces the widest range of symptoms. On a thirty year old unit they are almost always the first thing worth changing, and they are almost never the first thing anybody suspects.
What they do
A rooflight has gaskets in two distinct roles, and they fail differently.
Glazing gaskets sit between the glass and the frame, bedding the sealed unit in its rebate and weathering that junction. They are compressed once at installation and stay compressed.
Weather seals sit between an opening sash and its frame, and they are compressed and released every time the unit is operated. On a fixed unit there are none.
Both are elastomer, both rely on staying elastic, and both stop doing their job when they no longer spring back.
How they perish
Three mechanisms, working together over decades.
- Ultraviolet. Breaks down the polymer at the surface. An overhead unit takes more than a vertical one and the seal on an opening sash is exposed along its whole length whenever the unit is open.
- Compression set. An elastomer held compressed for years gradually stops returning to its original thickness. It takes a permanent set.
- Thermal cycling. Expansion and contraction, daily, for thirty years, working the material and the joint it sits in.
The result is a gasket that is rigid rather than resilient, frequently cracked at the corners, and no longer filling the gap it was made for.
The symptoms, and why they mislead
| Symptom | Usually blamed on | Actually |
|---|---|---|
| Draught near the rooflight | The unit, or the installation | The seal no longer compressing |
| Louder in rain than it was | The glass | The same gap, passing sound |
| Water at one point in heavy rain | A leak in the flashing | A crack in the seal at a corner |
| Powered unit stops short or reverses | A failing motor | Higher closing force against a rigid seal |
| Whistling in strong wind | Nothing anybody can name | Air through a gap under pressure |
| Sash no longer feels tight when shut | The catch | Nothing left to compress |
Six symptoms pointing at four different components, and one cause. It is the most misdiagnosed thing on a rooflight.
The thumb test
The whole diagnosis takes about ten seconds and needs no tools.
Open the unit, find the seal, and press it firmly with a thumb. A serviceable gasket compresses noticeably and springs straight back when released. A perished one is rigid, gives very little, and stays where you pushed it.
Then run a finger along it looking for cracks, particularly at the corners, where the material is stretched round a radius and fails first.
What else to look for
Four visual indicators, all from inside with the unit open.
Shrinkage, which shows as a gap at a corner where the two ends of a length no longer meet. Surface crazing, a fine network of cracks along the exposed face. Flattening, where the seal has a permanent groove worn into it. And chalking or powdering on the surface, which is the same ultraviolet degradation that affects a coating.
Any of those on a unit over twenty years old is expected rather than alarming.
Why they get missed
They are cheap, they are not the interesting part, and their failure produces symptoms that look like something else.
Somebody with a draughty, noisy rooflight that has started letting a little water in during a storm reaches for a replacement unit. An installer who has not pressed the gasket agrees. A new unit goes in, and it fixes it, at forty times the cost of the actual repair.
The gasket gets replaced as a side effect of the new window, and nobody learns anything.
Replacing them
On most units it is straightforward and it is a service visit rather than an installation.
Weather seals on an opening sash generally pull out of a groove and push back in, in continuous lengths, with the corners either mitred or formed by a single length turning the corner. Glazing gaskets are more involved, because reaching them means taking the beads or caps off, which is why they are usually done during a reglaze rather than on their own.
Both are ordered against the type and size code on the data plate, and both are stocked as parts for as long as the model is supported.
Doing it properly
- Identify the unit from the data plate and order the correct profile. Generic seal is not the same shape.
- Remove the old seal completely, including anything stuck in the groove.
- Clean the groove out, since thirty years of grit sits in there.
- Fit the new seal without stretching it, because a stretched gasket shrinks back and leaves gaps at the corners.
- Where a length turns a corner, form the corner rather than butting two cut ends.
- Close the unit and check for even compression along the whole edge.
Step four is where most gasket jobs go wrong. Pulling it tight to make it fit is exactly what produces a gap in two years.
Generic seal, and why it disappoints
Universal gasket by the metre is available and it is worth understanding what you get.
The profile of a manufacturer’s seal is designed for the specific groove and the specific gap it fills. A generic profile of roughly the right size sits in the groove and compresses differently, which means it seals unevenly and wears differently.
It works, after a fashion, and it is a reasonable answer where no original part remains available. Where the original part is available it is a false economy, because the difference in price is small and the difference in fit is not.
What it costs against what it saves
We publish no prices, and the shape of this one is worth describing because it is unusually favourable.
A set of weather seals is a small material cost and a short visit. A replacement rooflight is the unit, the access, the covering work, the collars and the making good.
Where the symptoms are draughts, noise and a stiff mechanism, and the glass is clear and the frame is sound, the gasket is the entire repair. It is the single best value job in this trade and it is the one nobody asks for.
The powered unit case
Worth its own note because it produces a specific and expensive misdiagnosis.
An actuator closing a sash has to compress the seal, and a hardened seal takes considerably more force. The controller reads that increased resistance at the end of travel as an obstruction, and does what it is designed to do: stops, or reverses.
The owner sees a window that will not close properly and concludes the motor is failing. The motor is working correctly and reporting a real condition. Replacing the gasket resolves it, and replacing the actuator does not.
The coastal angle
Gaskets perish through ultraviolet and compression rather than through salt, so a coastal unit is not much worse off than an inland one on this specific component.
What the coast adds is grit. Fine sand blowing off the beach settles in the seal channel, and grit trapped against an elastomer both abrades it and holds it away from full contact. It also raises the closing force, compounding the powered unit problem above.
Wiping the seal along its whole length with the unit open, twice a year, is the maintenance that matters, and it takes a minute.
Materials, and which ages best
Gaskets are made from a small number of elastomers and they do not all age alike.
EPDM is the common choice for weather seals and it is a good one: it resists ultraviolet and ozone well, stays flexible over a wide temperature range, and is what most current products use. Silicone performs better again at temperature extremes and costs more. Older units, particularly from the 1970s and 1980s, frequently used materials that have not lasted nearly as well, which is part of why a unit of that age is invariably found with rigid seals.
It is not usually a specification decision, because you get what the manufacturer fits. It does explain why a thirty year old unit has hard seals and a fifteen year old one frequently does not.
Glazing gaskets and the sealed unit
A connection worth understanding, because the two ages together.
The glazing gasket beds the sealed unit in its rebate and keeps water off the edge seal of the glass. Where it has hardened and no longer seals, water sits at the edge of the sealed unit, which is exactly the condition that shortens the life of the edge seal.
So a perished glazing gasket does not only produce a draught. It accelerates the failure of the expensive component next to it, which is one more reason a reglaze includes new gaskets as a matter of course rather than as an option.
When they are no longer available
On a unit old enough that the model is unsupported, the original profile may not be obtainable.
At that point the options are a generic profile with the caveats above, or accepting that the unit has reached the end of what can economically be done to it. Where the glass and the frame are otherwise sound, a generic seal is worth trying; where they are not, it is the last of several arguments for replacement.
The data plate is what answers the availability question in a phone call.
Expect to do it once
Setting the expectation properly, because people are surprised that anything on a rooflight is consumable.
Over the life of a well made rooflight, expect the weather seals to need replacing at least once. That is not a defect and it is not a sign of a poor product. It is a rubber component in ultraviolet doing a mechanical job for decades.
Knowing that in advance turns a worrying symptom at year eighteen into an expected service item.
Do it with something else
Where a visit is happening anyway, the gaskets are worth including.
During a reglaze the beads are off, so the glazing gaskets are accessible and should be renewed as a matter of course. During an actuator retrofit the unit is open and the seals are in front of you. During a service the same.
Done alongside something else it is minutes of labour and the material cost. Done as a separate visit it is a call-out for a small job, which is a poorer proposition for everybody.
What we press for
We press the gasket before we do anything else on a unit reported as draughty, noisy, leaking at one point, or refusing to close. We order the manufacturer’s profile against the data plate rather than fitting generic seal where the original is available, and we fit it without stretching it. And we say when a set of gaskets is the whole job, even though it is a fraction of the alternative.
Six symptoms, one cause, ten seconds to test. It is the first thing to check and the last thing anybody checks.
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.