What is the minimum pitch for a rooflight?

Fifteen degrees, and what the figure on the box is protecting

A roof window is weathered by a flashing kit that relies on gravity. Water crossing the tiles or slates has to run down the side flashings, off the apron at the cill and away, and it has to do that faster than wind can push it back up the slope. At 15 degrees the margin is already thin, which is why the flashing kits for shallow pitches use deeper side pieces and a longer head lap than the same kit for a 40 degree roof.

The other half of the figure is the sash. A centre-pivot unit at a shallow angle sheds rain slowly off the glass, so dirt stays put, and the gasket line at the head spends more of its life wet. Manufacturers publish 15 to 90 degrees for most centre-pivot models because that is where the drainage geometry and the hinge geometry both still work.

The narrower pitch band a top-hung sash is rated for

Top-hung units usually carry a tighter rating, commonly 15 to 55 degrees and on some models 20 to 65. The sash swings outward and upward on a hinge at the head, so on a steep roof the open sash ends up nearly parallel with the slope and the stay has to hold considerable weight against gravity. On a shallow roof the same sash barely lifts clear. Which one suits your roof is a separate question from pitch alone, and the roof window installation hub sets out how the choice is made against the room.

Below fifteen degrees the unit changes, not the roof

Nobody re-pitches a roof to take a window. Under 15 degrees the answer is a timber kerb built up off the deck, insulated on its outside face, with the roof covering dressed up and over it, and a made-to-size flat roof rooflight sitting on top. That is a different product with different sizes, a different weathering principle and a different price basis, described on the flat roof rooflight installation page.

It also changes the planning position. Permitted development for a house limits a roof alteration to 150mm beyond the plane of the existing slope, so a kerb-mounted unit on a visible pitched slope can push past that limit where an in-plane window would not.

Five degrees on the glass, and what a three degree unit looks like after a winter

Flat roof rooflights are never truly flat. The glass is set at a fall, and 5 degrees is the working minimum on most made-to-size units. Some manufacturers will supply at 3, and the difference shows within months: rain sheets off at 5 and crawls at 3, so dust from the road, pollen and the fine grit that washes out of a felt roof all stay on the pane and dry there. On a north-facing plane at 3 degrees you get algae in the corners by the second autumn.

The fall can come from the kerb being built taller at one end, or from a factory-formed sloping frame, or from the deck itself already falling. Which of those is used depends on what the existing roof does, and it is settled at survey rather than on the day.

Reading the pitch off your own roof without going up a ladder

You can get within a degree or two from inside. In the loft, hold a spirit level horizontally against a rafter and measure the vertical drop over a known horizontal run: 400mm of drop over 1000mm of run is roughly 22 degrees, 700mm over 1000mm is about 35, and 1000mm over 1000mm is 45. A phone with an inclinometer laid flat against the underside of a rafter gives the same answer faster, though it reads the rafter rather than the finished covering, and battens plus tile thickness make the outside face a fraction shallower.

At survey the pitch is taken off a digital level on the covering itself, at two or three points up the slope, because old roofs sag and the pitch at the eaves is often not the pitch at the ridge.

Where the pitch changes halfway up the slope

Central Cheltenham is full of mansards: a steep lower slope of Welsh slate you can see from Montpellier or the Bath Road, and a shallow upper slope hidden behind a parapet. The lower face is frequently 65 to 75 degrees and the top can be under 10. Two different pitches, two different products, one roof. The same split shows up on Regency and early Victorian terraces in Gloucester and Tewkesbury.

Shallow rear additions are the other Gloucestershire pattern that runs into the pitch limit. A two-storey rear addition on a Severn Vale terrace often carries a lean-to at 22 to 30 degrees, which is fine for a roof window, but the single-storey kitchen extension tacked on behind it is frequently at 10 degrees or less and needs a kerb.

What to send if you do not know your pitch

A photograph of the roof taken square-on from the garden is usually enough to get an indicative range going, because pitch is readable from a straight elevation shot and the covering is readable with it. Add a photograph from inside the room or the loft, and say roughly where you want the daylight. Email info@severnskylightsgloucester.co.uk and the reply will say which unit type your pitch puts you in before anybody books a survey.