Do flat rooflights need to be sloped?

How it actually works

The rooflight sits on a kerb standing proud of the roof surface. Water crossing the roof arrives at the kerb and is directed around it by the fall of the roof, then away to the outlet.

The glass being level does not matter, because water does not need to run off the glass to leave the roof. It needs to get past the kerb.

Except for self cleaning

One caveat. Self cleaning coatings rely on rainwater sheeting across the surface rather than beading and sitting. On dead level glass they do much less than the sales sheet suggests.

Some manufacturers offer units with a built-in slope of a few degrees for exactly this reason, and on a rooflight you cannot reach to clean it is worth asking about.

Where the roof has no fall

Older roofs that have settled, or roofs laid without firrings. Water then ponds along the upslope face of the kerb and sits against the termination, which is where leaks start.

The fix is to build local falls into the roof around the opening with tapered firrings, so water is carried away from the kerb even though the wider roof is flat. What we will not do is set a unit into standing water.

Debris

A level rooflight collects leaves and grit where a sloped one sheds them, particularly under trees. That is a cleaning consideration rather than a performance one, but on an unreachable unit it is worth factoring in.

The unit does not, the roof does

Most flat roof rooflights are sold as flat, and they are fitted to a kerb. What must have a fall is the roof deck around them, and on better installations the unit itself sits with a slight pitch built into the kerb.

The design target for a flat roof is a minimum finished fall of 1 in 80, which in practice means designing to 1 in 40 so that deflection and workmanship tolerances do not leave a flat spot.

Why any fall at all

Standing water kills seals. A perimeter seal designed to shed water is not designed to be submerged. Sitting in a puddle after every shower works it loose over a few winters, and that is the commonest cause of a leaking flat rooflight.

Dirt. Water that sits and evaporates leaves everything it was carrying on the glass. A pane that never drains becomes permanently dull, and no cleaning schedule keeps up with it.

Self cleaning coatings need run off. The coating breaks down organic deposits and relies on rain sheeting off to carry them away. On a genuinely flat pane it does very little.

Pitched flat rooflights

Several manufacturers offer units with a modest built in pitch, typically around 5 degrees, achieved either in the frame or by building the kerb higher on one side.

They shed water and dirt considerably better than a truly flat unit and they look almost identical from the ground. Where a roof has minimal fall and correcting it fully is impractical, this is the sensible compromise.

The trade is that a pitched unit collects more direct sun through the middle of the day than a flat one, which is a point in favour of solar control glass on a south facing installation.

What to check on your roof

Look at it after rain. Where the water goes, and where it does not, tells you everything. Standing water anywhere near the proposed opening is a problem to solve before the rooflight rather than after.

Then check the outlet it drains to. A perfect fall discharging into a gulley full of leaves and silt produces exactly the same pond.

Correcting a poor fall

Locally around the kerb, using tapered insulation or firrings under a new deck section. It is normal work, it belongs in the quotation, and it is considerably cheaper than replacing a unit that has failed because it was sitting in water.

We look at the fall before anything else on a flat roof enquiry, which is why we will not price one from a photograph.

What a sloped unit is for

Drainage and self cleaning, not weathering.

A properly made flat rooflight is watertight whether it is horizontal or not. The seal does not depend on the pitch. What depends on the pitch is what happens to the water once it has landed.

On a truly horizontal pane, water sits and evaporates, leaving behind everything it was carrying. Over a few years that builds into a dulled surface that no longer looks like glass. On a sloped pane the water runs off and takes the dirt with it.

Self cleaning coatings depend entirely on this. They break down organic deposits and let rain sheet off, so on a horizontal pane with no run off they do very little.

How the slope is achieved

Either the kerb is built with a fall, so the unit sits at an angle, or a pitched glass unit is used on a level kerb.

The second is usually neater. The kerb stays square and level, which makes it far easier to build accurately, and the pitch is in the unit itself.

How much

Enough that water visibly runs rather than sits. Manufacturers publish a recommended minimum for their own products and it is worth following, because a token slope that disappears into construction tolerance achieves nothing.

Beyond a modest fall the returns level off, and a steeply pitched flat rooflight starts to look like a mistake from the garden.