Why water sits on a flat rooflight, and what a proper fall does about it

Glass is smooth, so water has no reason to stay on it unless something holds it there. A pool that lingers for hours after the rain has stopped points to a surface that is lying almost perfectly level.

The word flat is the trouble. Nobody wants a flat rooflight to be dead level, yet that is what many end up as. Rain lands, finds no gradient, and spreads until it meets the frame. Some of it then creeps to the lowest corner and stays there, carrying a little dust with it. When it dries, it leaves a grey tide mark in a curve, often along one edge only. That single edge is the clue. It shows you where the low point of the pane really is, and it is seldom where anyone would have guessed when the roof was built.

Look at the pattern before you blame the glass. A ring of dried dirt in the middle of the pane means the glass itself is sagging slightly or the unit is sitting on a twisted base. A line along the bottom edge means the whole unit is tilting the wrong way, or not tilting at all. A scatter of small droplets that never join up is just surface tension and is normal on a clean pane in light drizzle.

Each of these starts below the glass, in the roof build. The pane is only reporting what it sits on.

1. A level upstand on a level deck. The builder sets a kerb, checks it with a spirit level, and is pleased when the bubble sits dead centre. The glass is then perfectly horizontal, which is exactly the wrong target.
2. A deck that has moved. Older timber joists sag a little over the years, and a flat roof extension built in the 1960s or 1970s may have settled under its own weight. The kerb follows the deck, and the glass follows the kerb.
3. Insulation laid to falls, glass laid flat. The roof itself may have a tapered insulation layer, so rain runs to the gutter or outlet. If the kerb then sits on top without taking up that angle, the rooflight ends up more level than the roof around it.
4. A unit that expects a slope it was never given. Manufacturers state a minimum slope for the glass on their data sheets. A unit fitted below it will hold water, however tidy the flashing looks.

One of those is usually dominant. The rest add to it. That is why guessing rarely works, and why a survey with a level and a straight edge tells you more than any amount of staring at the pane.

Standing water is not dramatic, but it is hard on everything it touches, and it changes how the room sounds and looks.

First, the glass. Water that dries in place leaves a mineral film, so the pane looks dirty within weeks. Coatings designed to shed dirt, such as self-cleaning glass, rely on rain sheeting across the surface, and they do little when the water simply stops. The pane needs a clear slope for the coating to do its job.

Second, the edges. Seals and gaskets are designed to shed water, not to sit under a shallow pond. Constant wetting at the lowest point wears the seal earlier than it should wear, and moss and algae take hold in the damp line at the frame. A rooflight that keeps a pool at its lower edge is one that will need replacing sooner.

Third, the sound. A pool of water on glass damps some of the drumming in light rain, which sounds like a benefit, but it is not one you can rely on. Heavy rain still strikes the pane, and the trapped water adds mass that can make the resonance lower and longer. The quiet you want comes from the glazing build, such as a laminated acoustic inner pane and firmly seated gaskets, not from a puddle. Our bedroom glazing guide covers what we specify.

The fix is a rooflight set to shed water by design. That is a matter of how the upstand is formed, and it is decided before the glass arrives.

In practice there are two routes. The first is a tapered upstand, where the top edge is cut or packed so the rooflight sits at a gentle slope, with the lowest edge facing the direction the roof falls. The second is a factory-made upstand, sometimes already cut to a pitch, which is bolted to a level deck and arrives with the slope built in. Either way, the glass ends at a fall that the manufacturer accepts, and the flashing is dressed to match.

The upstand also has to be tall enough to keep the roof membrane clear of any standing water on the roof itself. Height, insulation and finish are dealt with in our page on flat roof rooflight upstands, and the different approaches to the kerb are compared in what a builder's upstand is.

When we survey, we check the deck with a level before we talk about glass. The result goes into the written specification that comes with every quote. Where the pane needs to be sloped, it says so, with the upstand shape and the fall it is aiming for.

Is a little water on the glass after rain normal?

Yes, for a short while. A few beads or a thin film that clears within an hour or so is ordinary. Water that is still there the next morning, or a dried ring you clean off every month, means the pane has too little fall.

Will a new kerb cure it, or does the roof need rebuilding?

Often the roof can stay as it is. A new upstand formed to a slope can carry the glass to a proper fall on a deck that is level or only slightly out. If the deck has sagged badly or holds water elsewhere, the roof needs looking at, and we will say so at the survey.

Does the type of roof covering matter?

It changes the detail, not the principle. Felt, rubber and fibreglass each meet the upstand in a different way. See our notes on rubber roofs and felt roofs for how each is finished.

Would a dome or a frameless unit behave differently?

A dome is curved, so water runs off it without needing a fall in the kerb. Frameless flat glass behaves like any other flat pane and needs the slope. Our comparison of flat glass and domes goes through the trade-offs.

If water has been sitting on your rooflight, get a quote and we will survey the fall, then write the upstand and glazing into your quiet spec. The wider picture of drainage is in our falls and drainage guide, and the service itself is on the flat roof rooflight installation page.