Why a rooflight must not sit level
Water that cannot run off stays put. On a level pane it collects in a film, carries dirt to the lowest edge, leaves a ring of grime when it dries, and sits against the seals for longer after every shower. On a roof, the water that lies against a kerb has more chance of finding a weakness than water that is moving away.
A flat roof is not truly flat for the same reason. The deck is laid to a gentle slope, and the covering follows it so rain reaches a gutter or an outlet. A rooflight is another part of that roof, and the same principle applies to it twice: the glass needs a fall of its own, and the roof covering around the kerb needs a fall that carries water past it.
The fall is often too small to see by eye. So we measure it on the roof, with a level and a straightedge or a digital inclinometer, instead of judging it by looking.
Three places the fall can come from
Different makers solve the problem differently, and the fall on your roof can be supplied by any of these.
- Built into the unit. Some rooflights have a glazing frame shaped so the glass sits at a slight slope even when the base is level. The kerb can then be level.
- Formed in the kerb. Other units are flat in themselves, and the kerb is built higher on one side than the other so the unit tilts when fixed to it.
- Taken from the roof. Where the roof deck already slopes, a kerb built level with the deck gives the unit the same slope, as long as it is within the maker's range.
The maker's installation sheet says which of these a unit expects and what range of fall it will accept. A unit fixed outside its stated range is outside its stated conditions, so the sheet is read before the kerb is built, not after.
The kerb is the one part of a flat rooflight job that can be set to match both the unit and the roof, so it is built after the fall is measured.
Measuring the fall on your roof
The measurement is simple and is done on the roof itself. A long straightedge is laid on the deck along the direction water runs, with a level on top, and the gap under one end is measured. The same is done across the roof.
As an example, a roof that drops 25 mm over a 1 metre run has a fall of 25 mm per metre. Over a 3 metre roof the same slope drops 75 mm from top to bottom. A fall of 25 mm per metre is 1 in 40, a figure flat roof designers use as a design fall so that the finished roof still drains once the deck has settled. These are illustration figures to show how the reading works, and your roof will have its own.
| Reading | What it shows | Next step |
|---|---|---|
| Falls along the length | Water has a path off the roof | Record the direction |
| Level across one section | A dead-flat area near the opening | Form the fall in the kerb |
| Dips near the opening | Ponding risk beside the kerb | Correct the build-up there |
The readings go onto the measured sheet with a note of which edge of the roof is the low side. The kerb height on the low side and the high side is then worked out from them.
The fall around the kerb
A kerb is a small wall standing in the path of the water. On the up-slope side, rain meets the kerb and has to divert round it. If the roof around the kerb is dead level or dips, a pool forms against the kerb on that side.
The remedy is a tapered build-up behind the kerb, sometimes called a cricket or saddle, a small raised area shaped like a shallow ridge. It sends the water either side of the kerb and onward. It is part of the roof covering, built with the insulation or the deck below it, and it is worked out during the survey so that the right materials are ordered.
Without it, ponding behind a kerb shortens the life of the covering and puts the dressing under constant moisture. The fall is therefore part of the roof detail and not an optional extra.
How the kerb is built and how high it stands are covered in the upstand height guide.
What we do if the roof is too flat
If the readings show an area with too little fall, there are a few ways to deal with it, and the survey chooses between them.
- Form the fall in the kerb, when the unit takes it that way.
- Add a tapered insulation layer under the new covering, which gives the whole area a fall.
- Re-lay the deck to a fall locally around the opening, if it is soft or has dipped.
The choice depends on the covering you have, the size of the opening, and the height of the kerb the unit needs. Sizing comes first, and the page on flat rooflight size shows how the opening is worked out.
The wider job is described on the flat roof rooflight installation page. Where an existing roof has a covering that must be dressed round the new kerb, the process for each material is in the EPDM page and the fibreglass page.
faq
fall questions
Can I tell if my flat roof has enough fall by eye?
Usually not, because the slope is shallow. A level and straightedge on the deck shows it, and standing water after rain is the clearest sign of too little.
Does a flat rooflight need a fall if the roof already has one?
The roof's fall may be enough for the unit if it falls within the maker's range. The reading is compared with the installation sheet before the kerb is built.
Is a pitched rooflight a different matter?
Yes. A roof window on a pitched roof relies on the slope of the rafters, and each model has a rated pitch range from its maker, covered in the roof window pitch answer.