Do flat rooflights hold water after rain?
A flush flat rooflight will hold a thin film of water on the glass after rain, and on a unit set at a very shallow angle a shallow pool can sit in the middle of the pane for an hour or two before it clears. That is normal behaviour for a near horizontal sheet of glass. It is not a fault, it is not a sign of a failed installation, and it does not put any load on the unit that the glass was not designed for.
What matters is telling that ordinary behaviour apart from two things that are not ordinary: water that never clears at all, and water sitting on the roof surface around the unit rather than on the glass. This page separates the three. The wider detailing is covered on the flat roof rooflight installation hub.
What is actually sitting on the pane
Three separate things get described as water on the glass and they behave differently.
The first is a residual film, a couple of tenths of a millimetre thick, left behind everywhere on the pane once the bulk of the rain has run off. The second is a genuine pool, held in the shallow dish that any large pane forms under its own weight. The third is beading, where the water has broken into discrete droplets that sit proud of the surface instead of spreading. All three are surface effects. None of them is inside the unit.
Surface tension, and why a film stays behind
Water sticks to glass. The adhesive force between water molecules and the silica surface is stronger than the cohesive force holding the water to itself, which is why a droplet spreads on clean glass rather than balling up.
Gravity has to overcome that adhesion to move the film, and on a surface at five degrees the component of gravity acting down the slope is less than a tenth of its full value. Below about three degrees the film effectively stops moving and waits to evaporate. That is why a rooflight at the shallow end of the range clears by drying rather than by draining, and why it takes noticeably longer on a still, humid, overcast day than on a breezy one.
The dish in the middle of a large pane
A sealed unit is not a rigid plate. Under its own weight, a large pane supported on four edges deflects downwards in the middle, and the deflection scales steeply with the span. A unit around a metre square deflects a fraction of a millimetre. A unit two and a half metres long can sit several millimetres low at its centre.
That shallow dish is where a pool collects. Manufacturers allow for it, because the same deflection limit governs wind and snow load, and the volume of water involved is trivial next to a snow load. Practically, it means a very large flush rooflight will hold a visible sheet of water in the middle after heavy rain and it will look like a mirror from inside. It clears.
Drying marks, and the dust that makes them
The mark left behind is not the water, it is what the water was carrying. Rain arrives with particulates in it, and the film picks up more from the glass surface as it moves. When the film evaporates the solids stay, concentrated wherever the film was last to go.
In Essex that means two seasonal peaks. Late spring gives a fine yellow deposit from oilseed rape and tree pollen across the arable belt around Chelmsford and Braintree, and it is at its worst on the first rain after a dry spell. Late summer gives a general road and field dust. Both show up as tide lines around where a pool sat, which is why a large flush unit develops a faint oval mark in the middle over a season.
Self cleaning coatings, and how they change the sheet
A self cleaning outer surface is a pyrolytic titanium dioxide layer fired onto the glass at manufacture. It does two things and it is worth understanding both, because it changes how water sits.
Ultraviolet light triggers a photocatalytic reaction that breaks down organic dirt on the surface. Separately, the coating is hydrophilic, meaning water spreads across it as a continuous sheet instead of forming droplets. That sheeting action carries the loosened dirt off the pane and, importantly, dries without the droplet edges that leave spotting. The coating does not stop water sitting on a shallow pane. It changes the mark it leaves when it goes.
| Surface | How water behaves | What it leaves behind |
|---|---|---|
| Plain float glass | Spreads, then breaks into droplets as it thins | Ring marks at each droplet edge |
| Aged, weathered glass | Patchy wetting, strong droplet formation | Heavy spotting, needs washing |
| Self cleaning pyrolytic coating | Sheets continuously, dries evenly | Little visible marking |
| Silicone or fluoropolymer treatment | Beads and rolls off quickly | Clean while it lasts, wears in a few years |
Where the water goes at the frame edge
Water reaching the perimeter of the glass meets the frame, and this is the part of the design that does the real work. A flush rooflight frame carries a drainage channel around the inside of its outer edge, sized to take the run-off from the pane and route it to weep openings on the low side.
Those weeps discharge onto the top of the kerb, above the level the roof covering has been dressed to. That sequence matters: glass to channel, channel to weep, weep onto the membrane, membrane to the roof drainage. Nothing is relying on a bead of sealant to keep water out, which is why the detail survives twenty winters.
Keeping the weeps working
The one maintenance item on a flat rooflight is the perimeter channel. Leaves, moss, blown grit and, on Essex roofs near arable land, a surprising amount of chaff, all collect in the low corner where the water leaves.
A blocked weep does not cause a leak straight away. It causes the channel to fill, which raises the water level against the glass edge seal and gives it a working head of water instead of a trickle. Five minutes with a gloved hand each autumn clears it. If a unit that used to drain in an hour now holds water all day, look at the low corner before assuming anything about the glass.
Water on the roof, which is a different question entirely
Everything above concerns water on the pane. Water lying on the roof covering around the kerb is a separate matter and a more serious one, because it is the condition that tests every part of the perimeter detail.
Standing water that persists more than forty eight hours after rain is ponding, and it means the roof is not draining as designed. The fall that a flat roof needs is set out on the minimum fall for a flat roof with a rooflight, and where ponding has already produced marks on a ceiling, why a flat rooflight leaks works through the causes in order.
What is worth acting on, and what is not
Water that clears within a few hours of the rain stopping, marks that wash off, and a film that dries unevenly on a still day are all normal and want nothing done about them.
Three things are worth a look. Water still sitting on the pane a full day later with no rain since. A repeating mark that does not shift with a wash, which usually means the outer surface has weathered and the coating, if there was one, has done its time. And water standing on the covering against the kerb, which is about the roof rather than the rooflight. Sunspire fits new units and replaces those that have reached the end of their life, and a survey settles which of the three you have.
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