Can you fit a rooflight over a valley?
Not over the valley itself. A valley is a drainage channel carrying water from two roof planes into one line, and putting a rooflight across it means putting glass in the middle of the busiest run of water on the roof. Nothing about that ends well. What is entirely achievable is a rooflight beside a valley, on either slope, set far enough back that the channel keeps working and the flashing has room to be dressed properly.
This page sets out what a valley is doing, how much clear distance a unit needs from it, what the timber underneath looks like, and where the window goes instead. It sits under roof window installation, which covers the wider process on a pitched slope.
What a valley is actually doing
Where two roof planes meet at an internal angle, the water from both arrives in the same line and runs down it to the gutter. On a rear addition joining a main roof, or on an L shaped Victorian plan, that single channel can be carrying the run-off from a large area of tile.
The flow is concentrated, it is fast, and in heavy rain it runs deep enough to stand a few centimetres up the sides of the channel. A frame set into that line would sit in moving water every time it rained, with wind pushing the flow up against the apron. No flashing kit on the market is designed for it, and no manufacturer warrants it.
How much clear distance a unit needs
The controlling figure is the width of the flashing on the valley side of the frame, plus the width of the valley channel itself, plus enough sound covering between them to lap properly.
In practice that means the side of the structural opening wants to sit at least three to four tile courses clear of the centre line of the valley, which on a typical concrete interlocking tile is somewhere around six hundred to nine hundred millimetres. On a slate roof, where the cut pieces either side of a window are already narrow and fiddly, more is better. Squeezing a frame closer leaves a triangle of cut tile between the window flashing and the valley that is too small to fix and too small to drain.
Valley types found on Essex roofs
The construction of the channel changes how much room is needed beside it, and four types turn up repeatedly across the county.
- Lead lined valleys on Victorian and Edwardian stock in Chelmsford, Maldon and Colchester, laid over valley boards with the tiles or slates cut to a margin either side.
- GRP and fibre cement valley troughs, the common modern replacement, supplied in lengths and lapped down the slope.
- Mitred valleys with lead soakers, where plain tiles are cut on the angle and closed with soakers behind, common on interwar plain tiled roofs.
- Swept and laced valleys, formed entirely in tile with no metal at all, found on better quality plain tiled roofs in the north of the county.
A swept or laced valley is the most demanding to work beside, because the tiling is a continuous piece of craft across the angle and a new opening interrupts a set-out that runs the full length of the slope.
Where the window goes instead
Almost always further up the slope, further along it, or both. The useful move is to treat the valley as a boundary and place the opening centrally within the plane it belongs to, rather than nudging it towards the corner because that is where the room is.
Where the room below genuinely sits under the valley, the answer is usually to bring the light in from higher on one of the two planes and splay the internal reveal hard towards the corner that needs it. A splayed lining spreads daylight sideways across a ceiling far more effectively than moving a frame six hundred millimetres ever will.
Two units, one on each plane
A pair of windows, one in each slope either side of the valley, is often the best answer available and it is a deliberate looking arrangement rather than a compromise.
Each unit gets its own flashing kit, its own head drainage and its own clear margin, and the valley runs untouched between them. Inside, the two openings light the corner from both directions, which removes the shadow that a single window always leaves on the wall it is nearest to. On an L shaped Victorian rear addition this is a common outcome, and the two units usually end up at different heights because the two planes rarely have matching eaves.
Internal gutters and flat roof valleys
Flat roofs have their own version of the same problem. A tapered insulation scheme forms falls that run towards outlets, and where two falls meet the line between them is a valley in everything but name.
A kerb set in that line dams it. The unit wants to sit on a plane that drains past it on both sides, with the tapered scheme designed around the kerb rather than into it. Parapet roofs make this tighter still, because the water has only one way off the roof and there is no spare fall to give away. The kerb detail behind all of this is covered on flat roof rooflight installation.
Valleys that are already struggling
A survey around a valley often turns up conditions worth knowing about before any glass is ordered: a lead valley with a stepped joint too high up the slope, moss holding water at the head of the channel, a valley trough lapped the wrong way, or cut tiles that have slipped and narrowed the margin.
None of those stops a rooflight going into the slope alongside. They do change the order of work, because there is no sense setting a new frame into a plane that is draining badly beside it. Sunspire fits new rooflights and replaces units at the end of their working life, and the sensible sequence is to get the roof draining first, then set the glass into it.
What the survey measures at a valley
Five things get recorded, and they settle the position before anything is ordered. The centre line of the valley and its construction. The clear distance from that line to the nearest point of the proposed opening. Where the valley rafter and the jack rafters actually run, which is confirmed from inside the loft rather than assumed from the tiles. The pitch of each plane, since two planes meeting at a valley are often not the same pitch. And the direction of flow, so the head drainage of the new unit discharges into sound covering rather than towards the channel.
With those five figures the answer is usually straightforward, and it is normally yes with the frame in a slightly different place from where it was first drawn.
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