Can you fit a rooflight over a valley?
Not over the valley itself. A valley is the channel formed where two roof slopes meet at an internal angle, and it is doing the same job as a gutter: collecting water off both planes and carrying it down to the eaves. Putting an opening into that channel, or setting a frame across it, places a joint in the middle of the wettest strip on the whole roof. What is achievable, and what people usually mean when they ask the question, is a rooflight on the slope beside a valley, set far enough back from the centre line that the side flashing and the valley never compete for the same run of covering. The sequence for that job is set out on the roof window installation page. What follows is the geometry a valley imposes on it.
What a valley is carrying, and why nothing is set into it
A valley collects the run-off from two catchments and concentrates it into a strip a few hundred millimetres wide. On a modest L-shaped Gloucester house, that can be forty square metres of roof draining through a channel narrower than a floorboard. In heavy rain the water in a valley is moving fast and running deep at the bottom of the slope, and in a downpour it will surge above the level of the tiles either side of it.
Everything about the construction assumes that the channel is continuous from the top of the valley to the gutter. Interrupt it with a frame, a kerb or a flashing upstand and the water has to go around the obstruction, which means it has to climb the sides. That is how water ends up under the covering rather than on top of it.
The clear distance from the valley centre line
The working figure is a minimum of 300mm from the centre line of the valley to the nearest edge of the rooflight frame, and 400mm is a more comfortable number on anything below 30 degrees or on a covering that throws water sideways. That distance is not arbitrary. It has to contain the valley’s own width, the tilt fillet, the cut tiles or slates raking into the valley, the side soaker or side flashing of the rooflight, and a full-width run of undisturbed covering between the two so that neither detail is dressed into the other.
Below that, the two details start to overlap and the installer is left inventing a junction that no manufacturer draws and no kit supplies. Any junction that has to be invented on a scaffold is a junction with a short life.
Open lead valleys, and the tilt fillet that runs unbroken
Most valleys on Gloucestershire’s older stock are open lead: code 4 or code 5 sheet laid over valley boards, in lengths of no more than about 1.5 metres so it can expand, each length lapped 150mm over the one below. Under the edges sit tilt fillets, triangular timbers that lift the last course of covering so water is thrown back into the channel rather than being drawn sideways under the tiles by capillary action.
The tilt fillet is the detail that a nearby rooflight most often compromises. It runs the full length of the valley and it has to stay unbroken. Once a frame is close enough that the side flashing has to be dressed over or around the fillet, the fillet has stopped doing its job at exactly the point where the water is deepest.
Swept and laced valleys on Cotswold stone slate
The wolds villages do it differently. On a Cotswold stone slate roof through Painswick, Northleach or Bibury, the valley is often swept or laced: the slates themselves are cut and turned through the angle in a continuous curve, with no lead at all. It is a skilled piece of roofing and it is beautiful, and it is also the least forgiving thing on the roof to work next to.
A swept valley is set out course by course, tapering as it goes, and every slate in it relates to the one below. There is no way to lift a section and put it back. Working near one means taking the courses back on the plain slope only, keeping the sweep untouched, and the practical result is that the clear distance grows: on a swept valley in diminishing courses, 500mm from the centre line is a sensible starting point. Conservation officers take a close interest in these, and the conservation rooflight page covers what usually gets approved.
Valley rafters, valley boards and the timber underneath
The structure follows the same rule as the covering. A valley rafter is a heavier section than the common rafters because it carries the ends of every jack rafter running into it from both slopes. It stays whole. So do the jack rafters within about a metre of it, because their bearing is at the valley and shortening them removes the support from a fan of timbers at once.
On an interwar cut roof at Longlevens or Charlton Kings, the valley rafter is usually a 200mm or 225mm deep timber, sometimes doubled, and there are often valley boards laid across the jacks either side to carry the lead. The first full rafter bay clear of all that is where an opening can start, and it is worth going up into the loft to find it rather than measuring from outside.
L-shaped rear additions, where the valley lands where the light is wanted
This is the common Gloucestershire version of the problem. A Victorian or Edwardian terrace in Gloucester, Tewkesbury or Stroud has a two-storey rear addition one room wide, and where its roof meets the main roof there is a valley. The room underneath, usually a kitchen or a back bedroom, is dark precisely because it sits in the angle of the L with walls on three sides.
So the daylight is wanted in the one place the roof will not give it. The honest answer is to move the target: a unit further out along the addition roof, away from the valley and closer to the gutter, will still light the room, and on a run of slope of 1.8 to 2.4 metres there is usually room for a modest unit set high with the flashing lap it needs above it.
A pair of units on the two slopes rather than one across the join
Where the room genuinely straddles the angle, the better result is normally two smaller units, one on each slope, both set the required distance back from the valley. That gives daylight from two directions at two different times of day, it keeps every weathering detail standard, and it needs no structural design if each opening sits in a single rafter bay.
It also looks right from the garden. A pair set symmetrically either side of a valley reads as deliberate, whereas a single unit crowding the angle reads as an afterthought. Where two units are close enough to be seen as a pair, the spacing between them is worth setting out properly, and that is covered under roof window installation.
Snow, leaves and how a valley behaves in winter
A valley is also where debris collects. Leaves from an overhanging tree, moss washed down from the slope above and grit off an old concrete tile all end up in the channel, and in a valley that has partly silted up the water rises further than the design ever assumed. Snow does the same thing more dramatically: it slides off both slopes into the valley, packs there, and then melts from underneath.
Any rooflight near a valley has to be positioned for that condition rather than for a light shower. It is the main reason the clear distance is set generously rather than at the minimum, and the reason a frame is never placed where it would form a dam across the flow.
Measuring the usable slope before anything is ordered
The survey works out the answer with a tape rather than a guess. From inside, the valley rafter is located and the first clear bay identified. From outside, the centre line of the valley is marked and the distance measured across the slope to the ridge, the eaves and any other feature, which gives the rectangle actually available. Only then does a size code get chosen.
Send a photograph of the roof from the garden showing the valley, a photograph of the room below, and a rough idea of where you want the daylight to land. That is usually enough to say whether the position works, what size the slope will carry, and whether a pair is the better answer, before anybody puts a ladder against the wall.
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