What is the minimum pitch for a rooflight?

Fifteen degrees, and where the number comes from

The figure is not a rule of thumb. Each manufacturer tests its units and its flashings as a system and publishes the pitch band that system is warranted across, usually fifteen to ninety degrees for a standard centre-pivot or top-hung window with a tile or slate flashing.

Two things set the lower end. The first is the rate at which water leaves the head of the unit and gets back onto the covering. The second is how far up the slope wind can drive water underneath a lap before the lap runs out. Both get worse as the slope flattens, and fifteen degrees is where the test results stop being comfortable.

The number belongs to the unit and the kit together. Fitting a fifteen-degree-rated window with a flashing kit intended for a steeper installation puts you outside what either was tested for, and it is the sort of detail that never shows until a wet, windy February.

Ten degrees, and what the low-pitch kit actually adds

Where a maker publishes ten degrees, the kit that gets you there normally does three things. It sets the unit slightly higher out of the plane of the covering, so the head flashing has more depth to work with. It extends the underlay collar and the drainage apron further up the slope, so water driven under the covering is carried past the opening rather than to it. And it deepens the side laps.

What it does not do is change the physics. A unit at ten degrees is working harder than the same unit at thirty-five, and it will show soiling and water marking that the steeper one never does.

Water is not the only reason there is a floor

People assume the minimum exists purely to stop water getting in. Weathertightness is the headline, but the glass has requirements of its own.

Self-cleaning coatings work by letting rain sheet off the surface and carry loosened dirt with it. That needs run-off velocity. On a shallow slope the water crawls, sits, and evaporates in place instead of running, leaving the mineral content behind as a bloom. The coating is not faulty. It is being asked to work outside the conditions it was designed around.

Drying time matters too. Glass that stays wet for long periods grows a green film at the edges far sooner, particularly on a slope with trees over it.

Lead and flashing need a fall to work at all

Every weathering detail on a pitched roof is a lap held apart by gravity. Water arrives at the head of the unit, is thrown outward over the head flashing, lands on the covering below and continues down. Take the fall away and each of those handovers becomes a place where water can sit rather than move.

The side details are the same story. Soakers and side flashings depend on water having a consistent downhill direction. At a shallow pitch, capillary action and wind pressure start to compete with gravity, and a lap that is generous at forty degrees is marginal at twelve.

Snow, and what sits on a shallow slope

A steep roof sheds snow. A shallow one holds it, and held snow melts from underneath where the building is warm. Meltwater then runs a short distance and refreezes at the colder head of the unit, which is the classic mechanism behind water appearing indoors during a thaw rather than during rain.

It is not common in Oxfordshire, but it is not rare either, and it is one of the reasons manufacturers are conservative about the low end of the band.

How to measure the pitch before anyone quotes

You can take a usable figure from inside the loft without going near the roof. Hold a spirit level horizontally against the underside of a rafter, mark a run of 300mm along it, then measure the vertical drop from the end of that run up to the rafter. The rise over the run gives you the tangent, and a phone with an inclinometer app laid against the rafter soffit will confirm it in seconds.

Two cautions. Measure against a rafter, not a purlin or a ceiling tie, and take the reading on the slope the unit will actually sit on. Extensions, outriggers and rear catslides frequently run at a different pitch from the main roof of the same house, and the shallow one is usually the one somebody wants a rooflight in.

What the options are below the minimum

Three honest routes exist, and which one applies is a survey question.

• Use the low-pitch kit, where the roof is between ten and fifteen degrees and the chosen model is published for it.
• Change the product. A flat rooflight bedded on a timber kerb is designed for slopes near horizontal, drains at the perimeter, and is rated for the conditions a roof window is not. Our page on upstands, kerbs and warm roofs covers how the kerb is built.
• Change the slope the unit sits on. Where there is a steeper slope in the same room, moving the opening is often cheaper and always more durable than forcing the shallow one.

Raising a pitched-roof window on a site-made timber frame to fake a steeper angle is the fourth route, and we do not offer it. It puts the unit outside its tested configuration and leaves a wedge-shaped void behind the flashing that nobody can weather convincingly.

Where the guarantee sits on a marginal pitch

Our workmanship is guaranteed for ten years, and that guarantee is only worth having if the installation is inside what the manufacturer published. So on a slope in the ten to fifteen degree band we specify a model rated for it, order the low-pitch kit with the unit, and record the measured pitch on the survey and on the quotation.

Where the roof is below the published band for every unit that would suit the room, we will say so and price the flat-roof alternative instead. That is a shorter conversation than the one that follows a unit fitted where it should not have been. There is more on the upper end and the full working range on our page about how steep a roof can be for a rooflight.

Fifteen years installing rooflights around Oxford, the pitch measured at survey rather than estimated from the pavement, and a fixed price once we have been on the roof.