How close to the ridge can a rooflight go?
Close, but not right up to it. The working answer on a tiled or slated slope is that the head of the frame wants to sit at least two full tile courses below the underside of the ridge tiles, which on a concrete interlocking tile is roughly three hundred millimetres and on a plain tile or slate is rather less. Some positions allow tighter than that. None allow the frame to run into the ridge itself, because the flashing at the head needs sound covering above it to lap into.
This page covers what sets the limit, what sits behind the top course, what the planning rules say about height, and how the position is set out on site. The wider process for a slope is on roof window installation.
What sets the limit at the top of a slope
Water running down a roof has to get past the window, and it does that by being carried around the head and rejoining the drainage plane either side. That requires two things above the frame: a run of covering long enough to shed onto the head flashing properly, and a run of underlay long enough to form the drainage channel that takes water sideways.
Neither of those exists if the frame is tight to the ridge. There is nothing above to lap from, the apron has nowhere to terminate, and the underlay channel has no fall to work with. The limit is a weathering limit before it is anything else.
The head flashing and the space it wants
Every roof window arrives with a flashing kit, and the top piece of that kit is an apron or a head section that sits over the frame and under the covering above it.
The manufacturer publishes a minimum dimension from the top of the frame to the finished covering above, and it is typically expressed in tile courses rather than millimetres because the figure follows the covering. Two courses is the common instruction. On a deeply profiled tile the apron has to be dressed down into the troughs, and doing that over a partial course produces a bridge with nothing under it, which is exactly the condition that lifts in wind.
What is behind the top course of tiles
Under the ridge tiles there is a ridge board on a traditional cut roof, or the apex of the trusses on a modern one, and the rafters from both slopes meet it. The top tile course is usually cut, and it is bedded or dry fixed under the ridge system.
There is also usually a ridge ventilation detail on any roof built or re-covered in the last thirty years, either a dry ridge system with a ventilated roll or a proprietary vent. That detail is continuous along the ridge and it wants leaving intact. A frame pushed up into it interrupts the ventilation path along the top of the roof, which then shows up as condensation in the void rather than as water on the ceiling.
Permitted development and the ridge height rule
The planning position has its own version of the same constraint, and it is worth knowing because it is absolute rather than negotiable. Under permitted development a rooflight must not project more than one hundred and fifty millimetres beyond the plane of the existing roof slope, and no part of it may be higher than the highest part of the roof.
On a normal slope that second condition is satisfied automatically, because the frame sits below the ridge and projects only slightly. It starts to matter on a very shallow slope, where a unit set close to the apex with a standard rather than a recessed flashing can genuinely rise above the ridge line. On a shallow bungalow roof around Canvey or Rayleigh, that is checked with a level rather than assumed.
| Covering | Usual minimum below the ridge | Reason for the figure |
|---|---|---|
| Concrete interlocking tile | Two courses, around 300mm | Deep profile needs a full course to dress into |
| Plain clay tile | Two to three courses, around 200mm | Small double lapped units, soakers between courses |
| Natural slate | Two courses, around 250mm | Low profile apron, but slates are holed not nailed at the tail |
| Low pitch, any covering | More than the minimum | Slower run-off and greater wind driven rain risk |
Headroom, and why people want the window high
The reason this question comes up so often is that a high window is genuinely useful. In a room with a sloping ceiling, light entering near the apex travels further across the space and reaches the wall opposite, whereas a low window lights the floor immediately below it and little else.
There is also a standing headroom argument. In a converted loft the only part of the room where a full height person stands comfortably is the central strip, and a window set high in the slope puts the glass where somebody is already upright rather than where they would have to stoop. So the pull towards the ridge is real, and the job is to get as close as the weathering allows rather than to talk people out of it.
The other timbers near the top of a slope
A purlin is normally at mid height, which is not the issue near a ridge. What does turn up at the top of a cut roof is a collar, a horizontal timber tying opposite rafters together part way up, and on older roofs a collar can sit surprisingly high.
Collars are structural, holding the rafter feet from spreading, and they are not casually cut. Where a collar crosses the intended opening, either the window drops below it or the collar arrangement is redesigned as part of a wider loft conversion. On trussed roofs the equivalent obstruction is the web layout near the apex, which is fixed and not adjustable.
Where the ridge is genuinely too close
Some slopes simply do not have the run. A shallow pitched bungalow roof with two and a half metres from eaves to ridge, once you take two courses off the top and the eaves detail off the bottom, leaves a working band that will not accept a tall unit at all.
The same happens on rear additions and outriggers on Victorian terraces, where a lean-to slope abutting the main wall can be short and shallow at once. In both cases the honest answer is a shorter unit set centrally, or a pair of shorter units side by side, and the light is spread horizontally instead of vertically. Where the slope is under about ten degrees, the conversation moves to a kerb mounted unit on a flat roof instead.
Setting the position out on the roof
The position is marked on the actual roof rather than scaled from a drawing, and the sequence is always the same. The batten gauge of the existing covering is measured, because the course spacing is what the head dimension is counted in. The ridge line is established. The frame position is then set out downwards in whole courses, so the head lands on a course line rather than halfway across one.
That last point is what separates a tidy job from an awkward one. A frame that lands mid course means every tile above it is cut, and cut tiles near a ridge are the hardest ones to fix securely. Working in whole courses from the ridge down usually shifts the window by fifty or eighty millimetres from the drawn position, and it is worth the shift every time.
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