The short answer
A flat rooflight will hold a little water on the glass if it sits dead level, but a well-specified one is set to shed it. Most flat glass rooflights are made with a slight slope in the glass, or are fitted on a kerb with a built-in fall, so rain runs off in a minute or two and takes the dust with it. A puddle that stays for days is a sign that something about the unit or the roof around it is wrong.
There is a second question inside this one, which is water collecting on the roof beside the rooflight. That is a separate matter, and a more important one, so both are covered below.
Water on the glass
Rain landing on a level pane has no reason to leave it. It spreads, sits, and evaporates, leaving a rim of dust behind. A pane with even a small slope drains continuously, and the water carries dirt to the edge before it can dry. This is why manufacturers of flat glass rooflights quote a minimum slope or a finished fall for their units, and why the figure in the data sheet matters more than the word "flat" in the product name. We check the figure for the exact unit before it is ordered, and write the kerb and the unit into the quote together.
A thin film of water after a shower is normal and harmless. Glass is not damaged by it, and the seals are made to be wet. What it does show up is the dirt: tide marks, a grey bloom of pollen and dust, and green growth in the corners of a shaded roof under trees. Self-cleaning glass coatings help with this, and our answer on what glass goes in a flat rooflight covers what they do and where they stop.
Water on the roof around it
This is the one to take seriously. A kerb rises above the roof surface, and the roof is laid to a fall. If the rooflight sits where water would naturally run, the kerb acts as a dam. Water banks up on the high side, finds no easy way round and ponds. Standing water is hard on any waterproof covering, because it holds dirt, freezes in winter and sits against the junction where the covering meets the kerb.
The cure is in the design. The kerb position is chosen with the fall in mind, and where needed a small sloping saddle, often called a cricket, is built behind the high side to push water round both ends of the kerb. It is simple to form during the build and awkward to add later. The relationship between fall and kerb height is covered in our guide to flat rooflight upstand heights and falls.
Ponding that was there before
Some roofs hold water in places already, before any rooflight goes in. Flat roofs settle: decks deflect between joists, old coverings sag, and the fall that was laid on day one is gone in places. A rooflight added in a hollow gives the water nowhere to leave, and it is better to know at survey than to find out in the first winter.
When we survey a roof we look for the signs: stained or dirty patches where water has dried, algae in lines, blistering of the covering and soft spots in the deck. If the roof ponds, there are choices. The fall can be corrected with tapered insulation as part of a new covering, the opening can be moved up the roof to a drier spot, or the roof can be left alone and the rooflight sited clear of the wet zone. The right answer depends on the roof, and the written quote says which.
Cleaning and what to expect over the years
Even a well-sloped pane gathers a film of dirt over time, and the glass on a flat roof tends to need a wash more often than a roof window on a steep pitch, because there is less gradient to wash it for you. A soft brush, clean water and a long pole from the ground are usually enough where the roof cannot be reached safely. Avoid abrasive pads and strong solvents, which can mark coatings and attack seals.
Leaves are the other thing to watch. A rooflight below a tree collects them on the high side of the kerb, where they hold water against the junction. Clearing them once or twice a year, ideally from a safe position, keeps the drainage path open. Guttering or an outlet nearby needs the same attention, since a blocked outlet causes ponding across the whole roof and not just beside the rooflight.
Frost is worth a thought too. Water trapped in a joint will freeze and expand, so a unit that drains freely is also a unit that deals better with a hard winter on the Devon uplands or in the Exe valley.
What shows on the inside
Water on the roof is not the only way moisture turns up in the room below. Condensation on the inner face of the glass is a different problem, caused by warm, damp room air meeting a cold surface. It is more likely in a bathroom or a kitchen with poor extraction, and it is reduced by a glazing unit with a warm-edge spacer and good insulation value, and by ventilation that works. It is not a leak, and it does not point to a fault in the roof.
The finish at the opening matters here too. The lining is insulated behind its faces, so the plaster stays warm enough not to stain. A lining boxed in without insulation behind it can show a tidemark on the ceiling that looks like a leak and is not. That detail is recorded on the finish schedule, so you can see what goes behind the plaster as well as what shows.
What to look for on your own roof
- Water still sitting on the glass a day after rain stopped, which suggests a unit set too flat.
- A band of dirt or moss along the high side of the kerb, which suggests water is banking up there.
- Puddles near the kerb that dry in a ring, which suggests the surface has lost its fall.
- Staining on the ceiling lining that appears in condensation weather and not after rain.
If you are planning a flat rooflight and want the fall checked before anything is cut, see our flat roof rooflight installation page, or ask for a quote and we will survey the roof first.