No. Nothing is fitted over a valley. A valley is the drainage channel where two slopes meet, it carries the water off both of them, and putting an opening in it is not a detailing challenge to be solved: it is a guaranteed failure. What you can do is fit a rooflight beside a valley, on either slope, provided the unit and its side flashing stay clear of the channel by a workable margin. On most roofs that is achievable, and the question is where the unit moves to rather than whether it happens at all.
What a valley is doing while the rest of the roof does nothing
Every square metre of a pitched roof sheds water downwards at the same modest rate. A valley does not. It collects the discharge from two full slopes and concentrates it into a channel perhaps 300mm wide, then runs it at speed to the gutter.
In heavy rain the flow in a valley is many times what any other part of the roof handles, and it is moving fast enough to climb a lap rather than pass over it. That is why valleys are lined continuously in lead, GRP or a preformed section instead of being covered in the same slates or tiles as everything else. The lining is unbroken from the top of the valley to the eaves, and unbroken is the whole point.
An opening in that run introduces a joint into the one part of the roof least able to tolerate one. There is no flashing kit for it, because no manufacturer will publish a detail for a condition that cannot be made reliable.
The clear distance we work to alongside the channel
Working measurements start from the edge of the valley lining, not the centre line of the valley. The lining runs up under the covering on each side by a set amount, typically 100mm to 150mm depending on pitch and lining type, and the cut edge of the slates or tiles sits on top of it.
The rooflight’s side flashing then needs its own clear run of undisturbed covering between it and that cut line. In practice we set out for a minimum of around 300mm of full-width covering between the outside edge of the unit’s side flashing and the cut edge of the valley, and we prefer 450mm where the roof allows it. On a plain tile roof that is roughly two or three tile widths. On a stone or slate roof it is set by whole slates in the course.
Why the flow rate is the real constraint, not the geometry
People reasonably ask why a smaller margin will not do, since a side flashing is watertight in its own right. The answer is what happens above the unit rather than beside it.
A rooflight sheds every drop that lands on its glass to the sides and then down past its lower corners. That water leaves the flashing as a concentrated stream rather than a sheet, and it lands on the covering below. If the unit sits close to the valley, that stream joins the valley within a course or two, at a point where the valley is already running full. Water arriving fast and sideways into a channel that is already loaded is how a valley overtops, and it overtops sideways, under the covering.
Distance solves it by letting the discharge spread back into normal sheet flow before it reaches the channel.
Valley construction changes the numbers slightly
Not all valleys are built the same, and the type on your roof shifts the working distance a little.
| Valley type | Typical setting | Effect on the clearance |
|---|---|---|
| Lead-lined open valley | Older houses, stone and clay roofs | Widest channel, generous margin needed either side |
| GRP or preformed liner | Post-war and modern tiled roofs | Fixed width, clearance easy to set out from the moulding |
| Dry valley system | Modern concrete interlocking tiles | Upstands each side, unit must clear the upstand not the trough |
| Swept or laced valley | Handmade clay and some stone roofs | No liner, tiles curve through the angle, largest exclusion zone |
Swept and laced valleys are the strictest. There is no lining at all: the tiles themselves are tapered and swept through the change of direction, and the geometry only works if the courses run uninterrupted for a good distance on either side. On those roofs we hold the unit further back again.
Fitted too close, and how it announces itself
The failure is rarely immediate. It shows in sustained heavy rain with wind driving up the valley, which in this county means two or three occasions a year rather than every shower.
What appears inside is a mark on the ceiling below the valley, not below the rooflight, which is why the cause gets misread. The water has overtopped the valley lining a course or two up, tracked under the covering across the battens and appeared wherever the underlay first sags. By the time it is visible the timber has been damp intermittently for a season.
Putting that right is not an adjustment to the flashing. It means stripping back the covering, moving the unit, and often relining the valley. It is far cheaper to set out correctly at the start.
The valley rafter is not one you can trim
Structure sets its own limit before drainage does. Where two slopes meet, the valley rafter is a full-depth member running from the wall plate to the ridge or to a hip intersection, and it carries the jack rafters from both slopes bearing into it along its length.
It is not a common rafter and it cannot be cut, trimmed or notched to make room for an opening. Neither can the valley board or the sole plate beneath it on a boarded valley. That fixes the nearest possible position for a unit at one full jack rafter bay away from the valley rafter, and often more, because the jack rafters nearest the valley are short and take their support from it.
Where the roof is trussed the position is fixed harder still, and rooflights in a trussed roof covers what may and may not be touched.
Where the unit ends up instead
The usual outcome is that the rooflight moves up the slope, away from the valley, rather than sideways. Valleys are widest and busiest at the eaves and narrowest at the top, so the further up the slope you go the more clear covering there is between the unit and the channel.
The second option is the other slope entirely. A valley by definition has two, and the one you were not considering is frequently the better lit of the pair. The third is a narrower unit set in a single rafter bay, which buys the clearance without moving the daylight far from where it was wanted.
Occasionally the honest answer is that the position asked for cannot be served and the room needs its light from elsewhere. We will say so at the survey rather than after the covering is off.
Valleys that meet a hip, a dormer or a rear outrigger
Oxford terraces and villas with a rear outrigger nearly all have a valley where the back addition meets the main roof, and it is usually the most congested piece of roof on the house. It is short, steep at the junction, often lead-lined and frequently close to a chimney stack as well.
The same arithmetic applies, with less room to apply it. Where a valley runs into a hip the exclusion zones from both overlap, and the usable rectangle can vanish entirely on a small slope. Can a rooflight be fitted to a hipped roof? works through what is left in that case, and can a rooflight go near a chimney? covers the stack.
What the survey measures around a valley
We measure the valley lining width, the cut line of the covering on each side, the position and depth of the valley rafter, the jack rafter spacing, and the length of clear slope above and below the proposed opening. Those five numbers decide the answer, and none of them can be taken from a photograph.
You get a range by email once we know the roof and the covering, then a fixed price after the survey has confirmed the position is workable. The perimeter detailing is what keeps a roof dry, and next to a valley it is the whole job. That is the work our ten-year guarantee is written against.
Helpful reading on this
Costs, comparisons and the questions we are asked most.
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