How close to the ridge can a rooflight go?
Close, but not as close as most people expect, and the number is set by the covering rather than by the ridge. On a standard tiled or slated roof the head of the frame wants a minimum of about 300mm of undisturbed covering above it before the ridge detail starts, and on a deep profiled tile or a stone slate roof that grows to 400mm or more. The reason is simple: the head flashing of a rooflight has to be dressed underneath a full, whole course of covering that laps over it. If the only thing above the flashing is the ridge tile itself, there is nothing for the water to run onto. The broader positioning question sits on the roof window installation page, and what follows is the top of the slope in detail.
The head flashing needs a whole course above it
Every manufactured flashing kit is drawn on the same assumption. The apron at the cill sits over the covering below. The soakers or side flashings interleave with the courses either side. The head flashing goes under the course above, so that water running down the slope crosses onto the metal and is carried out sideways past the frame.
That top course has to be a full course, properly fixed, with its own headlap onto the course above it. A cut strip of tile bedded in mortar is not a course, and it is the single most common shortcut visible when an old unit near a ridge is taken off. The 300mm figure is really just the depth of one plain tile course plus the lap the flashing itself needs, expressed as a distance anyone can measure.
The ridge detail, and the fixing zone underneath it
Above that course sits the ridge itself, and it takes up more room than it looks. A bedded ridge tile is set on a mortar bed that spreads 40 to 60mm down each slope. A dry-fix ridge system uses a roll or a ridge batten with a plastic union under every tile, and the batten is screwed down into the rafters through the top of the roof, which means there is a fixing zone at the apex that cannot have a frame in it.
Dry-fix has become standard on new work and on any re-roof of the last fifteen years, and it is common across the Quedgeley, Abbeydale and Bishops Cleeve estates. It is a better detail than mortar, and it is also less tolerant of anything encroaching on it, because the roll has to be dressed continuously onto whole tiles both sides.
Ridge ventilation, and the path that stays open
Most modern roofs and a good number of upgraded older ones ventilate at the ridge. Air enters at the eaves, travels up the 25mm gap above the insulation between the rafters, and leaves through a ventilated ridge, ridge vent tiles or a proprietary dry ridge roll.
A rooflight sitting too high blocks that path in two ways. The frame and its trimmers fill the top of the rafter bay, and the insulation dressed around the frame usually closes the gap behind it. On a cold roof that produces a stagnant pocket at the apex directly above the window, which is where the timber gets damp and where mould shows up on the sarking. Keeping the frame clear of the top of the bay, and taking a counter batten over the head of the frame so air can cross to the bays either side, is part of the reason the clearance is set where it is.
The ridge board, the purlin, and which one you meet first
Inside a traditional cut roof there is a ridge board at the apex, usually a 32mm or 38mm plank on edge, which the rafters are nailed against. It carries very little load and it is not the obstruction. The obstruction is usually the purlin further down, or on an interwar roof the collar tying opposite rafters together at around two thirds height.
What that means in practice is that the highest a unit can go is often decided from inside rather than outside. On a hipped interwar semi at Charlton Kings with a purlin at mid-height and a collar above it, the clear zone at the top of the slope may be less than a metre of vertical run, and the head clearance to the ridge is then the smaller of two constraints rather than the governing one.
What the figure comes to on each covering
| Covering | Minimum from head of frame to ridge line | Why |
|---|---|---|
| Slate, small to medium format | 300mm | Short courses, so one full course plus the flashing lap is a modest distance |
| Clay plain tile, double lap | 300 to 350mm | Two overlapping courses are involved above the flashing, not one |
| Concrete interlocking tile | 300mm | Long tiles, but a single lap, and a standard kit designed for it |
| Clay pantile | 400mm | Deep roll, sideways water travel, and a wider head flashing to accommodate |
| Cotswold stone slate | 400 to 500mm | Thick slates, diminishing courses, and the smallest slates sit right at the ridge |
| Dry-fix ridge over any covering | Add 50mm | The ridge batten fixing zone has to land on whole tiles |
The smallest slates sit at the top of a Cotswold roof
On a stone slate roof through Painswick, Bibury, Northleach or Chipping Campden, the slates diminish up the slope, so the courses nearest the ridge are the shortest and thinnest on the whole roof. A short course gives less lap for a head flashing to hide under, and the gauge is changing continuously, so a flashing set that would work halfway up the slope does not necessarily work near the apex.
This is one of the reasons a conservation rooflight on a Cotswold roof is normally placed in the middle third of the slope rather than high. It also reads better from a distance, because a unit tucked under the ridge on a heavy stone roof looks squeezed in a way that a unit centred in the slope does not. The finishes and sizes are on the conservation rooflight installation page.
Head height in the room, and the handle you can no longer reach
The other half of the question is what a high unit does inside. The convention is a cill at about 900mm above finished floor and a head at about two metres, which puts the glass in the field of view of someone standing and gives a view out rather than just a view of sky.
Push the frame up towards the ridge and the head can end up at 2.6 metres or higher in a loft room with a steep pitch. The sash then needs a rod or an electric opener, the blind needs to be operated the same way, and cleaning the inside face becomes a job with a ladder. None of that is a reason to avoid a high unit where the roof demands it, but it changes the specification, and electric rooflight installation covers the powered options.
Trussed apexes, where the top of the bay is a nail plate
On the trussed estates the apex is where the two top chords meet a punched metal nail plate, often with the top of the central web plated in at the same point. The plate is typically 100 to 200mm across and it is structural. Nothing gets cut there, nothing gets notched, and screws do not go into it.
Between two trusses there is a clear 550 to 570mm and the standard narrow units drop straight into it, but the head trimmer has to land on sound chord timber clear of the plate. That normally pushes the head of the frame a little further from the ridge than the covering alone would require. Where a truss is involved at all, the position is worked out in the loft with the plates visible before any size code is chosen.
Setting the head height from inside first
The order of decisions matters. Mark where you want the head of the glass to be from inside the room, standing where you will actually be standing. Transfer that up the slope. Then check the three constraints in turn: the covering clearance to the ridge, the internal timber, and whether the reach is still sensible.
If the position you want is inside the clearance, the usual answers are a shorter unit set slightly lower, or a wider one in the same bay pattern. It is very rare that the ridge clearance kills a project. It is common that it changes the size code, which is why it gets checked before an order rather than on the morning the covering comes off.
Next on this subject.
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