Short answer: The flat rooflight minimum fall is whatever the manufacturer states for that product, and the figure differs between makers and between models. That fall, or flat skylight pitch, can come from an angled kerb, from the slope built into the roof, or from a unit that has the fall built into its own frame. We quote a number only from the data sheet for the unit being fitted, never from memory.
Why standing water on glass is a problem
Despite the name, no flat roof is truly flat. It looks level, but the deck is laid to a gentle fall so water can run to the gutter or outlet. A flat rooflight sits in that roof, and it has to cope with whatever the glass collects. The same goes for a flat roof skylight on a terrace extension or a garage conversion.
Water lying on glass does harm in several ways. It leaves a tide mark of dirt where it dries, and the glass goes dull far sooner than a pane that sheds. In winter, a pond on the pane can freeze, and in a damp spell algae and moss take hold. Where the glass meets the frame, standing water sits against the seal for long periods, which is far harder on the seal than rain that runs off. Few people mind a little rainwater on the glass for an hour. They mind it for a week.
Where the fall comes from
There are three ways a flat rooflight ends up with a fall, and which applies depends on the roof and the unit.
An angled kerb. The kerb can be built with one side higher than the other, so the glass sits at a slope even when the deck beneath is nearly level. This is common when the roof was laid to a very shallow fall. The kerb then does the work the roof does not.
The roof itself. On a roof laid to a good fall, a standard level kerb can place the glass at the same angle as the deck. If the roof falls steeply enough, the rooflight inherits the slope without any special work. The roofer's design has already set the fall long before the rooflight arrives, which is another reason to bring us in early on a new extension.
A unit with fall built in. Some rooflights are made with a sloped frame, or a pitched glass profile, so the surface drains whatever the kerb below does. A pyramid or a lantern sheds water because of its shape. A flat-glass unit with a built-in fall uses the frame to tilt the pane.
When the choice is between a flat rooflight and a roof lantern for a rear extension, how each sheds water is part of the decision, and it is dealt with in our flat rooflight or roof lantern guide.
The figure belongs to the manufacturer
Ask three people for the minimum fall of a flat rooflight and you may well get three answers, because the number is not a universal rule. It is the manufacturer's stated minimum for that product, set out in the installation instructions and on the data sheet. A unit with a self-draining frame may allow a shallower roof than one that relies on the kerb. A unit fitted below the stated minimum can fall outside the warranty and can perform worse in the weather.
For that reason this page gives no single figure. We could name one, and it would be right for one product and wrong for another. What we do is read the stated minimum for the rooflight on your quote, then compare it with what your roof offers. If the roof falls short, the fix is on our side: change the kerb, choose another unit or alter the roof's fall before the covering goes on.
- Find the stated minimum fall on the maker's data sheet
- Measure the roof's actual fall at the opening
- Decide whether the kerb, roof or unit supplies the difference
- Confirm the choice before the unit is ordered
How we check a roof at survey
We measure the fall where the rooflight will sit, not at the edge of the roof. A level or an inclinometer across the deck shows what the roof actually does, which may differ from the plan. Old flat roofs on Victorian back additions often sag between walls, and a roof can pond in one place while it drains well in another. Newer extensions are usually laid to a deliberate fall, but a deck can still settle once the building is occupied.
If the covering is felt, EPDM or GRP, the way it turns up the kerb is a separate question, covered in our guide to felt and rubber roofs. The kerb's height and insulation are in the kerbs and upstands guide. Here the question is only the angle of the glass.
Once the survey is done, the unit and the kerb are named on a written quote with the price fixed. Where the job is notifiable we take care of the Building Control paperwork, and the installation is backed by a ten-year written guarantee on the workmanship. Other choices for a flat roof are laid out on the flat roof rooflight installation page. To have your roof measured, get a free quote.
Fall and drainage, answered
Is a flat roof skylight with no slope at all acceptable?
Only if the manufacturer permits it for that model. Most expect some fall, and fitting a unit against its instructions risks ponding and a weakened guarantee from the maker.
Can you add fall to my roof for a rooflight?
Often we can, with an angled kerb or by changing the roof's fall around the opening. Which is better depends on the deck, the covering and the unit, and we confirm it at survey.
Will rain wash the glass clean on its own?
A rooflight with a good fall rinses much of the dirt away. It will still need an occasional wipe, particularly under trees or on a roof near the coast, where salt dries on the surface.