Do flat rooflights hold water after rain?

The difference between a film and a pool

Look at the pane from a window above, or from a ladder, and it is easy to tell which you have. A film covers most of the glass, moves when you watch it, and is gone within half an hour of the rain stopping. It is surface tension holding a thin layer on a shallow slope, and every glazed roof does it.

A pool has an edge. It sits in one place, usually in the middle of the pane or in one corner, it has a visible depth, and it survives a dry morning. That means the glass is deflecting, or the frame is not sitting level on the kerb, or the unit was set with too little slope in the first place. It is worth photographing and worth a conversation.

Why flat glass is never actually flat

Manufacturers give their units a built-in slope, typically between three and five degrees, formed either in the frame itself or by fitting the frame onto a kerb that has been tapered.

Three degrees sounds like nothing. It is about 52mm of drop over a metre, so across a 1.5m pane the low edge is roughly 78mm below the high edge. That is enough to move water once it is moving, and not enough to move it quickly. Five degrees clears noticeably faster and shows fewer marks, at the cost of a unit that reads as more obviously tilted when seen from a garden or an upstairs window. That trade-off is worth deciding at the survey rather than accepting whatever the default is.

Where the water goes once it leaves the pane

It runs off the low edge onto the roof covering below, and that is the point most people have not thought about. A 2m by 1m rooflight sheds the rain that falls on two square metres onto one narrow strip of covering immediately downslope of the kerb.

That concentrated discharge is why the covering downslope of a unit weathers faster than the rest of the roof, why a gravel finish there gets scoured into a channel, and why the fall on that side of the roof needs to actually run somewhere. On a roof with a shallow or badly set fall, that discharge is what creates a persistent wet area against the downslope face of the kerb.

Tide marks and the ring of dirt on the glass

The complaint we hear far more often than standing water is that the glass looks dirty in a pattern. A pale ring, or a series of parallel arcs, appears on the pane and cleaning it removes them for a fortnight.

That is evaporation. Rainwater carries dust and airborne salts; where the water sits longest before drying, the residue is left behind as a line. Under a tree, or anywhere near a busy Oxford road, that residue includes sap and traffic film and it bonds harder. The marks are cosmetic and tell you nothing about the seal. They are simply the record of where water last stood.

What self-cleaning coatings do and do not do

A self-cleaning outer pane carries a thin coating that does two things: it breaks down organic dirt in daylight, and it makes the surface hydrophilic so water sheets rather than beads.

Sheeting is the useful part on a shallow-pitched rooflight. Instead of thousands of individual droplets each drying in place and leaving a spot, the water forms a continuous film and takes the loosened dirt off the low edge with it. The result is a pane that needs cleaning perhaps twice a year rather than four times, and looks better in between. It does not do anything about mineral deposits from hard water, and Oxfordshire water is hard, so the drips that come off a roof edge onto the glass still leave marks. It is a worthwhile upgrade and not a substitute for a ladder.

Water sitting in the joint between glass and frame

On a bonded or externally drained unit there is a shallow rebate where the pane meets the frame, and along the low edge of the unit a shallow band of water can stand there after rain. Sometimes leaves and moss collect in it.

A well-made unit is designed for that. The glazing rebate is drained through small weep slots in the frame, so trapped water finds its way out rather than sitting against the edge seal of the sealed unit. Those slots are the size of a matchstick and they block with debris. Clearing them when the glass is cleaned is the one bit of maintenance a flat rooflight genuinely needs, and it is the thing that prolongs the life of the sealed unit more than anything else.

Water between the panes is a different matter

Misting or droplets on the inside of the outer pane, in the sealed cavity, is not the same subject at all. No amount of cleaning touches it because it is inside the glass.

That is a failed edge seal. The desiccant in the spacer bar has taken up all the moisture it can, and humid air is now moving in and out of the cavity with temperature changes. It cannot be dried out and it cannot be put right in place. The sealed unit is replaced, and on many rooflights that is a straightforward swap without touching the frame or the kerb, which is worth knowing before anybody suggests replacing the whole rooflight.

When water on the glass does mean something

Three findings are worth acting on. A pool that persists into dry weather, because that indicates the pane or the frame is deflecting. A wet area on the covering against the kerb that never dries, because that means the discharge from the glass has nowhere to go. And water on the inside of the room in cold still weather, which usually points at condensation rather than rain and is covered on do flat rooflights cause condensation.

Standing water on the roof surface itself, as opposed to on the glass, is a wider subject with its own rules about depth and duration; that is set out on snow and standing water on a flat rooflight. We install and replace rooflights across Oxford and up to twenty five miles by road, and our workmanship carries a ten-year guarantee.