Do flat rooflights hold water after rain?
A flat rooflight sheds most of the rain that lands on it within a few minutes of the shower stopping, and then holds a thin film along the low edge and in the corners for a while afterwards. That is normal for glass laid at five degrees. What is not normal is water that is still sitting in a measurable depth on the pane an hour later, or water standing on the roof surface up against the kerb for weeks at a time. Those are two separate conditions with two separate causes, and it is worth being clear which one you are looking at before deciding anything. The wider detailing sits on the flat roof rooflight installation page.
Water on the glass against water on the roof beside it
Stand back and look at where the water actually is. If it is on the pane, the question is about the pitch of the glass and the state of its surface. If it is on the membrane alongside the upstand, the question is about the falls in the deck, and that is a different piece of work covered on the hub and in the section further down.
People report both as the rooflight holding water, and the two get conflated in phone calls constantly. A unit can shed perfectly and still sit in the middle of a lake.
Surface tension, and the film that will not run away
Water on glass does not behave like water in a gutter. It forms a continuous film held to the surface by adhesion, and that film only moves once gravity along the slope exceeds the force holding it. On a five degree pane the driving force is small, so the film thins slowly and then breaks into separate beads rather than draining as a sheet.
Those beads sit until they evaporate. On a mild breezy afternoon that is twenty minutes. On a still December morning at three degrees with the air already near saturation, they can be there all day. Nothing is wrong with the unit. There is simply no energy available to remove the water.
The low edge, the frame upstand and where the beading collects
Every framed unit has a small lip where the glass meets the frame at the bottom of the fall, formed by the bonding bead and the drainage profile. Water running down the pane arrives at that lip, and a band of it stops there.
A band 10 to 20mm deep along the low edge after rain is the designed behaviour, and it drains out through the frame rather than over it. A band that is 60mm deep, or one that sits along the top edge as well as the bottom, says the unit is not sitting to the fall it was built with, usually because the kerb is out of level or the deck has settled unevenly under one corner.
Drainage channels and weep holes inside the frame
An aluminium rooflight frame is drained, not sealed. The extrusion contains a chamber below the glazing rebate, and any water that gets past the outer gasket lands in it and is taken out through small weep holes at the low corners.
Those holes are typically 5 to 8mm and they block. Moss, pollen, lichen fragments and the general grit that settles on a low roof all end up in that chamber, and once the weeps are closed the chamber fills. Water then stands inside the frame instead of leaving it, and on a cold night it freezes and expands against the gasket. Clearing the weeps with a length of trimmer line once a year takes a few minutes and prevents a slow problem.
The tide line, and the dirt that arrives with the water
Rain lands with airborne dust in it and leaves that dust behind when it evaporates. Because the water always retreats to the same low band, the dust always deposits in the same low band, and within a season there is a visible line across the bottom of the pane.
On a shallow unit the line creeps upward, because each shower drains slightly less far than the last. This is the single most common reason people believe a rooflight is holding water when what it is actually holding is the record of water that has already gone. It cleans off with warm water and a soft brush. It comes back, and it comes back faster on a north facing unit under a tree.
Ponding on the membrane against the upstand
Standing water on the roof itself is the version that matters structurally. A pond that sits against a kerb through the winter keeps the bitumen or the single ply permanently wet at the one junction that has the most joints in it, and it puts a hydrostatic head against the base of the upstand rather than a passing film.
The cause is nearly always the deck rather than the unit: too shallow a designed fall, a joist that has crept, or a kerb positioned across the flow so it dams water arriving from up-roof. The figures for what a deck should be falling at are set out on the page covering the minimum fall for a flat roof with a rooflight. Where a roof already ponds at the position a customer wants, the deck is re-falled first, and that gets said at survey rather than discovered later.
How long a unit should reasonably take to clear
A rough working expectation, on a unit set at five degrees with clean glass.
| Conditions | Sheeting stops after | Pane visually dry after |
|---|---|---|
| Mild, breezy, 12 degrees | 2 to 4 minutes | 20 to 40 minutes |
| Still, overcast, 8 degrees | 5 minutes | 2 to 4 hours |
| Still, cold, 3 degrees, high humidity | 5 minutes | Often not until the next dry day |
| Below freezing | Film freezes in place | When the temperature rises |
Anything materially slower than this on a mild day, or beads that stay in the same fixed positions rather than moving, points at a surface that has picked up a film of grease or a residue from a cleaning product.
Coatings, and what they can and cannot do about it
A self-cleaning outer coating works in two stages: a photocatalytic layer breaks down organic dirt in daylight, and a hydrophilic surface lets the next rain sheet across the pane rather than bead on it. Sheeting water carries the loosened dirt off with it, which is why these units keep a cleaner tide line.
The limits are worth knowing. The coating needs ultraviolet light to work, so a unit permanently shaded by a wall or a mature tree gets little benefit. It needs regular rain, which Gloucestershire supplies. And it does nothing at all about a shallow pitch: a self-cleaning pane laid nearly level still holds its film. Specification options are on the energy efficient rooflights page.
Vale winters, still air, and glass that stays wet for days
Gloucester, Longford, Tewkesbury and the vale villages sit low beside a tidal river on ground that holds mist, and through a settled winter anticyclone the air there can stay at or near saturation for the better part of a week. Under those conditions a flat rooflight will be wet in the morning, wet at midday and wet again after dark, without a drop of rain falling.
That is regional climate rather than a defect, and it affects a unit in Quedgeley or Maisemore far more than one on the wolds above Northleach, where the air moves and the frosts are harder but drier. It is the reason a five degree rake is treated as a minimum here rather than a target, and why a slightly steeper set on a made-to-size unit is worth asking for when the room below can take the extra kerb height.
Next on this subject.
- Question Can a rooflight be fitted to a flat roof? Yes. A flat roof takes a rooflight readily, and on most houses it is a more predictable job than putting…
- Question What is a rooflight upstand? An upstand is the raised edge that lifts a rooflight frame clear of the roof surface, and on a flat…
- Question How much does a flat roof rooflight cost? No figure is published on this site, and the reason is that the same glass unit can sit inside two…
- Question Do flat rooflights need to be sloped? Yes. A flat rooflight is laid to a slope, and most manufacturers set a minimum of about five degrees on…