Does a flat rooflight need a slope to shed water?

Why the glass needs a fall

A pane of glass that lies perfectly level holds on to the water that lands on it. Rain collects in the middle, dust settles into a ring at the edge of the puddle, and when the water dries it leaves a mark that is hard to clean. In winter, standing water freezes and thaws on the surface and sits around the seals longer than it should. A small fall lets the water run to one edge and away, and it carries a good deal of the dirt with it.

That fall does not turn a flat rooflight into a pitched one. It is only a gentle slope across the glass, usually too slight to see from the room below, and the rooflight still reads as part of a flat roof.

Where the figure comes from

There is no single number that applies to every unit. Each maker states the minimum fall for their own rooflights in the installation instructions, and the figure can differ between a framed unit, a frameless one and a walk-on one, and between single, double and triple glazing. The instructions are what the guarantee and the fitting have to follow, so we work from the sheet that comes with the unit and not from a rule of thumb.

You will sometimes see a roof described as flat when it has a fall of its own, built into the joists or the insulation to carry water to the gutter. That fall is for the roof covering. It helps, but it does not replace the fall the rooflight needs, because the rooflight sits on a kerb that is level to its own frame unless it is built otherwise.

If the maker's figure and the roof's own fall point the same way, the job is simpler. If they do not, the kerb has to correct for it, and that needs to be known early.

Two ways the fall is achieved

The first is a sloping kerb. The carpenter builds the upstand higher on one side than the other, so the frame sits at an angle and the glass inherits the fall. The roofer then covers the kerb as normal. This is common with large units and with frameless glass, which lies directly on the kerb. The sloping kerb is drawn on the plan, and the heights at each side are given to the builder in writing.

The second is a unit with the angle built in. Some rooflights are made with a frame that is thicker on one side, or with an upstand that is part of the unit, so they sit at the right fall on a level base. The kerb is built to the maker's instructions as a level one, and the slope comes with the product. This keeps the carpentry simple, but the unit has to be ordered with that option, and the choice is fixed when the order goes in.

Which method suits a job depends on the rooflight type, the roof construction and the look you want at the edges. The details of heights and build are in the kerb and upstand guide.

What it means for the roof beneath

The roof can stay flat. The rooflight's fall is a feature of the unit and its kerb, and it does not ask the roof deck to be tilted. What the roof does need is a covering turned up the kerb to a proper height, and a way for the water that runs off the glass to leave the roof. The glass sheds onto the surrounding membrane, and the roof's own fall takes it to the outlet.

Water running off the low side of the rooflight lands on the roof near the kerb, so it is worth keeping that area clear and checking that there is an outlet or gutter to take it. The covering type matters too: the way EPDM, GRP and felt are dressed against the kerb differs, and the guide to each roof covering sets out how.

Agreeing it on the drawing

The fall is a design decision, and it is cheapest to make before the roof is built. We send the kerb or opening size, the upstand height and the fall for the unit chosen to your builder or architect in writing, ahead of the deck going on. That gives the carpenter a drawing to build to and lets the roofer plan the covering around it.

If the roof is already finished, the same checks apply, but the kerb may need to be altered, and fitting a rooflight to a finished roof explains what that involves. The price of altering a kerb is picked up in the written quote, which also covers the unit; our page on flat rooflight costs explains how each part is built up.

What to check before you order

• Ask for the maker's installation sheet for the unit in question, and read the stated fall.
• Confirm whether the fall comes from the kerb or from the unit itself.
• Check that the drawing shows the kerb heights on each side and not just one overall figure.
• Check that the outlet or gutter beneath the low edge can take the water that leaves the glass.
• Make sure the roofer has seen the drawing before the covering is laid.

These checks take little time and they prevent the kind of correction that has to be done with a roof half finished. For the full range of products, see our flat roof rooflights page.