What is the minimum pitch for a rooflight?
There is no single figure, because the answer changes with the type of unit. A roof window fitted into a slope usually wants fifteen degrees. Several models are rated from ten, but only with a specified underlay and flashing arrangement. A kerb mounted flat rooflight will sit happily on a roof laid at one or two degrees, provided that roof drains. The number that matters is the one printed by the manufacturer of the unit you are actually buying, and it is a condition of the warranty rather than a recommendation.
This page covers where those figures come from, how to measure the pitch of your own roof without getting on it, and what the sensible route is when a slope falls between the two families of product. The other end of the range, roofs that are too steep rather than too shallow, is dealt with on how steep a roof can be for a rooflight.
Three minimums, one for each family of unit
Rooflights divide into two groups and the groups obey different physics. A unit set into the plane of the covering depends on the slope to carry water past it. A unit sitting above the covering on an upstand depends on height instead. The first needs pitch. The second needs somewhere for the water to go.
| Unit type | Usual minimum | What the minimum is protecting |
|---|---|---|
| Roof window in a slope | 15 degrees, some rated from 10 | The flashing apron and the underlay laps |
| Conservation cast frame | Commonly 15 to 20 degrees | A slimmer frame with less apron depth |
| Kerb mounted flat unit | About 1 degree of roof fall | Standing water against the upstand |
| Glass in a flat unit | Often 5 degrees, or 1:80 | Soiling rather than water ingress |
Where the fifteen degree figure comes from
Fifteen is not a round number chosen for tidiness. It is roughly the pitch at which water running down a tiled or slated surface still carries enough momentum to clear the apron of a flashing rather than creeping sideways beneath it. It is also the pitch below which most tile manufacturers stop warranting their own covering, so the window and the roof reach their limits at about the same place.
Two things change as a slope shallows. Water slows down, which means it spends longer against every lap it crosses. And wind driven rain arrives closer to perpendicular, so it drives into a joint rather than glancing across it. Both work against a detail that is dressed and lapped rather than glued.
The ten degree versions, and what has to be true
The low pitch ratings are genuine, and they come with conditions attached that are easy to skim past. Typically the covering has to be a flat interlocking tile or a slate rather than a deep profile pantile, a specific low pitch flashing kit has to be used rather than the standard one, and the underlay has to be dressed with an increased head lap above the frame.
Read the condition, not the headline. A window advertised as suitable from ten degrees, fitted with the standard kit onto a bold profile tile, is outside its rating even though the pitch on its own is inside it.
Measuring the pitch of your own roof
This is done from the loft in about two minutes. Hold a spirit level horizontally against the underside of a rafter, mark a level run of five hundred millimetres out from the rafter face, then measure vertically from the end of that run back up to the rafter. Rise divided by run gives the tangent, and any phone will turn that into degrees.
From outside, a photograph taken square on to the gable and measured on screen is good to a degree or two, which is enough to know which side of a limit you are on. Where the figure lands within two degrees of a minimum, it is measured properly on the slope at survey rather than assumed, because two degrees is the difference between two entirely different products.
Below ten degrees, where a kerb takes over
A slope under ten degrees is treated as a flat roof for this purpose, whatever it happens to be covered in. The answer is a kerb mounted unit: a timber or insulated upstand built off the structure, the covering dressed up the sides of it, and the glazing landing on top. Height replaces slope as the thing keeping water out of the building.
That approach is set out in full on flat roof rooflight installation, and the upstand itself on what a rooflight upstand is. On a shallow tiled slope the kerb is usually built square to gravity rather than square to the roof, so the glass lies close to level and the frame sheds cleanly on all four sides.
Flat units still want a fall on the glass
A flush rooflight is rarely fitted dead level, and the reason is dirt rather than water. Rain leaves a level pane slowly and leaves its dissolved solids behind as it goes, so a truly level unit dries with a grey bloom across it within a single season and needs washing far more often than one laid to a fall.
Self cleaning coatings depend on a moving film of water, which makes them least effective exactly where the fall is least. Where the wish is for a flush, level look from the garden, a tapered kerb delivers the fall on the glass while the frame line stays parallel with the roof surface.
Essex slopes that sit close to the limit
Shallow roofs turn up in predictable places across the county. Interwar rear additions behind the Chelmsford and Colchester terraces often run at ten to fifteen degrees where a lean-to has been extended once already. Bungalow extensions from the sixties and seventies around Canvey, Rayleigh and South Woodham Ferrers frequently sit at eight to twelve. Garage and side return roofs almost everywhere are shallower than they look from the pavement.
The older stock is easier. Victorian main slopes commonly run forty to fifty degrees, and interwar semis thirty to forty, which puts both comfortably inside every roof window rating on the market.
What happens when a unit is fitted below its rating
Nothing, for a while, and that is the difficulty. A window set at eight degrees with a fifteen degree kit will keep an ordinary autumn out and will let go on the first night of sustained rain driven from the wrong direction, usually two or three winters after handover.
The wet mark appears at the head rather than the sill, because water that has crept under an apron runs to the lowest point it can find inside the frame before it shows. By then the covering has bedded down around the flashing and putting it right means opening the roof again. The manufacturer’s position on a unit fitted outside its rated pitch is also unambiguous, which is worth knowing beforehand.
Settling the pitch before anything is ordered
Pitch is the first measurement taken at survey and it settles the shortlist before anyone discusses glass, finish or opening type. Sunspire measures it on the slope rather than reading it off the original drawings, because rafters deflect, rear additions get rebuilt, and the figure on a plan is often a degree or two adrift from the timber that is actually up there.
For an emailed enquiry, the loft measurement above and one photograph of the roof from the garden are enough to say which family of unit applies. Everything else a slope decides, from rafter spacing to which flashing kit is ordered, is covered on the roof window installation page.
Tell us about your roof
We start with the building, the covering and the slope. Then we tell you what will suit it, and what it will cost, as a fixed written number.
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