Short answer
Water sits on a flat rooflight when nothing tilts it towards an outlet. The glass may be set too level, the roof beside it may fall the wrong way, or the route to the drain may be blocked or too shallow. Where the puddle forms tells you which, and a look at the rooflight, its upstand and the falls across the roof settles the rest.
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Where the puddle forms
Start with a wet-day look from inside the room. A pool on the glass itself says something different from a pool at the base of the upstand, and both differ from one out on the roof a metre away. Rain does not wander across a flat roof. It runs downhill by the shortest route and stops where the surface stops falling. Marking the edge of the pool with a strip of tape on the inside of a window, or noting it against a roof feature, gives the survey a starting point.
The glass has its own part to play. Rainclear, a flat glass rooflight maker, recommends a minimum 5 degree slope on the kerb to minimise water pooling. A pane set dead level on a level kerb has no such help, so water has little reason to leave it.
| Where it sits | What it can point to | Looked at on survey |
|---|---|---|
| On the glass | Too little slope on the kerb | Kerb angle and set-out |
| Along the upstand | Roof falling towards the rooflight | Falls across the roof |
| Beside the rooflight | A slow or blocked outlet | Outlets and gutters |
The fall under and around the glass
A flat roof is built with a gentle fall so that water runs to an outlet. A fall that was never quite enough, a covering that has dipped between joists or an upstand built out of square can each send water the wrong way. Any of them can leave the lowest point of the roof beside the glass instead of at the drain.
A kerb can also sit across the line the water used to take, so that the water now stops against it. For that reason the roof around a rooflight gets as much attention as the rooflight, and a puddle at the upstand needs looking at before anyone talks about new glass.
Makers publish the pitches their products suit. The dome in VELUX's range suits roofs of 0 to 15 degrees, and the low-pitch roof window in the same range is meant for 10 to 20, so the pitch of your roof is one of the first figures worth taking.
Standing water and dirty glass
Water that cannot leave does something to the pane as well. As a pool dries it can leave a ring of whatever it was carrying, such as grit or pollen, and that ring shows from the room below.
Self-cleaning glass depends on rain running off. Pilkington describes its Activ coating as working in two stages: daylight loosens the dirt, then rain washes it off. Roof glazing is on the list of uses it gives. On a pane that holds a pool, the rain has nowhere to carry the dirt, so the coating only does its work once the water has a way to move.
What happens next
Keep off the roof and the glass while you look. A wet flat roof is slippery, and the view from a window or from the ground, or a photograph taken from an upstairs room, tells you most of what a first look needs. Rooflight glass is not made to be stood on unless it is a walk-on unit.
The survey is free, and it covers the rooflight, the upstand, the falls and the outlets. You get a fixed written quote afterwards. The answer may be a new rooflight set on a kerb with the right slope, or the roof around it may be where the problem begins, and the survey tells you which.
If you are weighing options, our guide to choosing a flat roof rooflight compares the types, and the page on putting a skylight into an existing flat roof covers the permissions. Where the work is notifiable, we handle the Building Regulations notification with local authority building control or a registered building control approver. Ring 01792 929062 or book through the online form. Browse the whole flat roof rooflight range for the types we fit.