Do rooflights leak?
A correctly installed rooflight does not. The reputation comes from a generation of units fitted in the nineteen seventies and eighties, mostly domed polycarbonate on a shallow timber kerb with a felt covering dressed loosely up the side, and a great many of those did let water in eventually. Modern units are not the same product and modern detailing is not the same work. Where water does appear around a rooflight today, the cause is almost never the glass and almost never the frame. It is the two or three hundred millimetres of roof immediately around it.
This page sets out where water actually gets in, in rough order of frequency, and how to tell water ingress from the thing it is most often confused with. How the perimeter is built to keep water out is covered on how a rooflight is made watertight.
The unit is rarely the problem
A sealed glazing unit is a manufactured item made to a standard by a handful of firms, and it either performs or it fails visibly by misting between the panes. It does not quietly pass water through the glass.
The frame is similar. It arrives factory assembled, gasketed and drained, with weep channels designed to take any water that gets past the outer seal back out to the surface. Both are the reliable half of the installation. What varies from one job to the next is the perimeter, and that is where a survey looks first.
Upstand height, the most common cause on a flat roof
The figure quoted across the industry is one hundred and fifty millimetres from the finished roof surface to the top of the kerb, and it is derived rather than invented: it combines the depth of water that can stand on a flat roof with the height wind driven rain can be pushed up a vertical face.
Cut that height to make a unit sit lower and you have removed the margin. In ordinary rain nothing happens. In a February storm with the roof holding water and the wind driving across it, the water reaches the top of the kerb and goes over. That is the single most frequent cause of water appearing around a flat rooflight, and it is designed in rather than developing over time. The kerb detail is set out on upstands and kerbs.
A flashing kit that does not match the covering
On a pitched roof the equivalent fault is a flashing kit chosen for convenience rather than for the covering. Each kit is made for a profile depth, and the apron has to be dressed down into whatever is on the roof.
A low profile kit fitted to a deep pantile leaves the apron bridging each trough with a void underneath. Wind lifts it, rain drives under it, and water enters at the head of the frame and runs down inside the roof. Nothing about that shows on the day. It shows in the third or fourth winter, when somebody notices a mark on the ceiling below and blames the window. The kit is ordered by profile, not by what is nearest in the van.
A missing head drainage channel
Water running down a slope has to get past the frame, and it does that by being carried around the head and rejoining the drainage plane either side. That route is formed deliberately in the underlay, supported on a batten or on a proprietary channel supplied with the window.
Where it is not formed, water arriving at the top of the frame has nowhere to go except sideways along the underlay, and the underlay at that point is cut. On a breathable membrane the natural drape between rafters does part of the job by accident, which is why the fault sometimes takes years to show. It is invisible once the tiles go back, which is precisely why it is worth asking about before they do.
The list, in order of how often it turns up
- Upstand too low on a flat roof, or a kerb built off the deck rather than through the insulation.
- Mismatched flashing on a profiled tile, with the apron unsupported over the troughs.
- Underlay laps running the wrong way, so water travels under the membrane instead of on top of it.
- Ponding against the kerb, where the falls direct water into the unit rather than past it.
- A drainage path that has closed, usually leaves and moss at the head of the unit.
- Age, where a unit fitted in the eighties has reached the end of its working life.
Condensation, which is not water getting in
Before anything else is considered, the first job is establishing which of two quite different things is happening, because they look similar on a ceiling and have nothing in common.
Surface condensation forms on the room side of the glass when warm moist air meets a cold surface, runs down the inner pane, collects at the bottom of the frame and drips onto the reveal. It is physics rather than a fault, it is worst in kitchens, bathrooms and in a converted loft, and it is managed with ventilation and with a warmer inner pane. The clue is that it appears on cold mornings and after showers rather than during rain. A vertical sill return, which sets up a convection current up the face of the glass, does more about it than most glazing upgrades.
Telling one from the other
Two questions settle it in most cases, and they can be answered from inside without anyone going onto the roof.
| Observation | Points to water ingress | Points to condensation |
|---|---|---|
| Timing | During or shortly after rain | Cold mornings, after cooking or showering |
| Position | Head of the frame, or a mark out on the ceiling | Bottom of the glass and the sill |
| Wind | Worse in driving rain from one direction | Unaffected by wind |
| Between the panes | Not affected | Misting inside the unit means a failed seal |
Misting between the two panes is a third category again. That is a sealed unit that has lost its seal and taken on moisture, and it means the unit has reached the end of its working life and wants replacing.
Drainage that quietly closed itself
A great many water problems around rooflights start as a drainage route that stopped working rather than a detail that failed. Leaves collect at the head of a unit on a pitched slope. Moss builds along the upper edge of a kerb. A flat roof outlet silts up and the water that used to leave now stands.
Water that remains on a roof more than forty eight hours after rain is ponding, and ponding against a kerb is the condition that finds any weakness in a detail that would otherwise have lasted. Clearing debris from around the perimeter each autumn is the single most useful habit available, and it takes five minutes with a gloved hand in October.
Why the guarantee sits on the workmanship
Everything on this page points the same way. The glass is reliable, the frame is reliable, and the outcome is decided by four things that are invisible once the job is finished: the height of the upstand, the fall of the roof around it, the laps of the flashing, and where the covering terminates.
All four are inexpensive to get right during installation and expensive to put right afterwards. That is why the ten year guarantee Sunspire offers is on the workmanship rather than on the brand of glass, and why a survey looks at the roof for considerably longer than it looks at the product brochure. Sunspire fits new rooflights and replaces units that have reached the end of their working life, across Chelmsford and thirty miles of Essex around it.
Tell us about your roof
We start with the building, the covering and the slope. Then we tell you what will suit it, and what it will cost, as a fixed written number.
- Surveyed before it is priced
- 10-year workmanship guarantee
- Building Control notification handled
- New installations and replacements