The answer
No, not a steep one, but flat never means level. A flat rooflight needs a gentle fall so rain and dust run off the glass, and the roof around it needs a fall to the gutter too. The upstand and the angle of the unit give the first. The way the roof deck is built gives the second.
Why water needs somewhere to go
Rain on a perfectly level pane has no reason to leave. It sits, dries in a ring and leaves a mark. Dust collects in that ring, and over a season the glass looks tired. A gentle fall lets each shower carry the dirt away with it.
The same is true of the roof. A flat roof that holds water in puddles ages faster than one that drains, and a puddle against an upstand is the last thing a rooflight wants. So a flat rooflight is never installed in isolation. It is fitted into a roof that already sheds water, and it is built so that it sheds water too.
This is the reason the trade says flat rooflight and not flush rooflight. The glass is raised on its upstand and tilted a little. That small angle is doing real work every time it rains.
Cleaning shows the benefit. A pane with a good fall needs little attention beyond an occasional wipe from inside or a rinse from a ladder, because the weather does most of the work. A pane that holds water needs more, and it never looks as bright.
Where the fall comes from
There are two separate falls on a flat roof job, and people sometimes blur them together.
The first is the fall of the glass. The upstand is built to a shape that tips the unit a little, so water runs toward the lower edge and off. You will not notice this from inside the room. From outside, you will see the pane sitting at a calm, shallow angle.
The second is the fall of the roof itself. Flat roofs are built with timber firrings or tapered insulation laid over the joists, so the whole surface drains toward a gutter or outlet. The upstand then stands on that surface, and the roof covering is taken up its sides. Our guide to upstands and falls shows how the two layers fit together.
Get either one wrong and water lingers. Get both right and the roof sheds a storm quickly, leaving the rooflight clean.
What if the roof looks dead level?
Some older flat roofs have settled, or were laid with very little fall. That does not rule out a rooflight. We read the roof at the survey, check where water would run and agree how the opening should be built.
Where the roof has pockets that hold water, the upstand can be set so that the water has a clear way past the rooflight, and not into the back of it. Where the covering is due to be relaid, the sensible moment to improve the fall is while the roof is open. We tell you what we see and put it in the written quote, so there are no surprises on the day.
A skylight changes nothing about the view from below. Straight up is clear, and the sky does not care how shallow the angle of the glass is.
Milton Keynes is a city laid out on a grid from 1967, and its garages, porches and rear extensions carry flat roofs of every age. Some drain beautifully. Others have a low corner where the water gathers. Knowing which you have is the first job at the survey.
If a better fall is needed, there are sound ways to add one, from tapered insulation to new firrings under the covering. These are decided with you on site, and any extra work is written into the quote before you agree to anything.
Which rooflight types need most care
All flat rooflights need the same two falls. The difference is in the detail, and the table shows where the fall matters most for each kind.
A fixed pane is the simplest, and the easiest to keep clean. An opening unit has a hinge and a seal, and you want those seals clear of standing water. A walk-on unit has a surface that people stand on, so a wet film on the glass is a slipping risk, and the surface is textured to help.
| Type | Fall matters because | Know |
|---|---|---|
| Fixed pane | Dirt dries in rings if water pools | Easiest to clean |
| Opening unit | Seals must stay clear of standing water | Check the vent seals |
| Walk-on | Wet glass is slippery underfoot | Needs a textured surface |
Before you book
Four quick checks on your own roof help us give you a straight answer. If you cannot see the roof safely, photographs from an upstairs window will do, and a survey fills in the rest.
Worth looking at on your roof
- Does water stand on the roof after rain?
- Is there a gutter or outlet at the low edge?
- Is the covering due to be relaid soon?
- Which way does the roof face?
Next step
A free survey settles the question for your roof. The fall is read on site, the opening is sized, and you receive a written quote whose figure holds on the day. Fitting is tidy, the inside is left finished, and the workmanship is guaranteed for ten years in writing.
More on this subject is in our flat roof rooflights hub. If you are still deciding whether a skylight suits the roof at all, start with the answer on whether your flat roof suits a skylight.
A few more questions
Is a flat rooflight really flat?
No. Water drains because the glass is set at a shallow angle on an upstand. It looks calm from the garden and clean from inside.
Will a gentle fall show from inside?
No. The lining below is finished square to the room, so the fall is only visible from outside, and then only as a shallow angle.
Does a rooflight cause ponding?
It should not. The upstand is built so water has a way round it, and the roof around it should already drain. We check this before we quote.
