Rooflights in natural slate roofs
The defining fact about a slate roof is that a slate has no nib. It is a flat sheet of split rock held on by two nails driven through holes punched in it, and once the slates above it are laid those nails are unreachable. That one difference from every tiled roof in the county sets the whole method of putting a rooflight into slate. Slates come off by cutting their nails blind with a steel ripper. Slates go back on with clips rather than nails, because there is nowhere to swing a hammer. Every slate cut down beside a frame has lost one of its two fixings and needs a new hole or a clip. Handled well the result is the neatest rooflight detail available on any covering, because a slate is five to eight millimetres thick and the flashings can be interleaved until nothing shows. Handled badly a slate roof unravels for a metre in every direction. The wider sequence is on the roof window installation page.
A slate is holed rather than hung, and that governs the method
A plain tile or an interlocking tile hooks over a batten and gravity does the rest. A slate does nothing of the kind. Two holes are punched near the head or at the centre, two copper or aluminium nails go through them into the batten, and the slate is then covered by the courses above.
Everything else follows. There is no way to lift a single slate out of the middle of a roof without dealing with its nails, and there is no way to nail it back once its neighbours are down. The covering is therefore worked strictly in sequence from the top of the working area downwards, and reinstated slates in the finished area are held mechanically instead. A slater working around a rooflight opening is managing a chain of dependencies rather than swapping components.
Head nailed or centre nailed, and the gauge each gives
Where the holes sit changes the arithmetic of the whole roof. Head nailing puts the fixings about 25mm down from the top edge. Centre nailing puts them at the mid point of the slate, just above the head of the slate two courses below.
The gauge, meaning the exposed depth of each course, comes out of a formula. For a centre nailed slate the gauge is the slate length minus the headlap, divided by two. For a head nailed slate it is the slate length minus the headlap and minus another 25mm, divided by two. So a 500mm slate at a 100mm lap gives a 200mm gauge centre nailed and about 187mm head nailed. Head nailing suits smaller slates and steeper pitches. Centre nailing suits larger, heavier slates and exposed sites, because the nail is closer to the middle of the slate and there is less lever arm for the wind to work on. Which one the roof uses is established at survey, because it determines where the frame head can sit in the course rhythm.
The imperial sizes still on Gloucestershire roofs
Welsh slate was sold in named sizes and most of the county’s slate roofs still carry them. The names sound archaic and the dimensions are entirely practical.
| Name | Nominal size | Typical headlap | Gauge, centre nailed | Where it turns up |
|---|---|---|---|---|
| Duchess | 610 by 305mm | 100mm | 255mm | Larger Victorian roofs and Cheltenham terraces |
| Small Duchess | 560 by 305mm | 100mm | 230mm | Common on better vale housing |
| Countess | 508 by 254mm | 100mm | 204mm | The workhorse across the county |
| Viscountess | 457 by 229mm | 100mm | 178mm | Smaller terraces and rear slopes |
| Ladies | 406 by 203mm | 90 to 100mm | 153 to 158mm | Forest of Dean cottages and outbuildings |
| Randoms, graded | Mixed lengths, laid diminishing | Varies by course | Changes up the slope | Older Forest and valley work |
The size matters at an opening because it fixes the gauge, and the gauge decides how much slate the head flashing has to tuck under. On a Ladies slate at a 155mm gauge there is a course boundary every 155mm up the slope, which gives a lot of choice about where the frame head lands. On a Duchess at 255mm there is far less.
Headlap, and the 100mm that becomes 115 on an exposed slope
Headlap on slate is the vertical distance a slate overlaps the head of the slate two courses below, and it is the number that makes the roof watertight. One hundred millimetres is the general figure on a moderate pitch in a sheltered position.
It goes up as the pitch goes down and as exposure increases, and both apply on the western slopes of the county. Around Coleford, Cinderford and Mitcheldean, driving rain arrives from the south west with the Forest offering little shelter, and a roof there will often be found running 115 or 125mm rather than 100. It goes the other way on a very steep slope, where 90mm can be adequate. The consequence for a rooflight is that the interleaved soakers down each side lap each other by the same figure the slates do, so an increased headlap means deeper soakers and more lead per course. That is a material quantity rather than a guess, worked out from the measured lap.
Taking slates off with a ripper, and the nails you cannot see
The tool is a slate ripper: a long flat steel blade with a hooked, sharpened notch at the end and an offset handle. It slides up under the slate, the hook is located on the nail shank, and a sharp downward blow with a hammer on the handle shears or draws the nail.
You are working blind. The nail positions are known from the coursing but the nails themselves are invisible, and a copper nail shears cleanly while a corroded iron one may not shear at all. Where the nail will not give, the slate breaks. That is why an experienced slater strips a slate roof around an opening from the top of the working area down, releasing whole courses in order rather than fishing individual slates out of the middle, and why the breakage allowance on an old slate roof is treated as a real number in the estimate rather than as an optimistic zero.
Nail sickness, and the roof that unravels around an opening
Nail sickness is the condition where the slates are sound but the ferrous nails holding them have corroded away, so the covering is essentially resting on itself. It is common on late Victorian and Edwardian roofs across Gloucester and Cheltenham where iron nails were used, and it is the single most important thing to establish before quoting a rooflight into an old slate roof.
The signs are visible from the ground with binoculars: slipped slates, a scatter of lead tingles from previous work, and a wavy line to the courses. From inside the loft the nail heads are gone and the slate heads sit loose on the battens. On a nail sick roof, disturbing a metre square area does not stay a metre square. Slates above the working zone start to move because the ones that were holding them by friction have been taken away, and the honest answer at survey is that the slope needs stripping and re-slating as its own piece of work, with the rooflight opening formed as part of it. Saying that before the scaffold goes up is far better than discovering it on the first morning.
Tingles, and the slate that cannot be nailed back
When a slate has to be placed into an area where the courses above are already laid, there is no access to a batten and no way to nail it. The traditional answer is a tingle: a strip of lead or, better, copper, about 25mm wide, nailed to the batten in the joint between two slates in the course below and turned up over the tail of the new slate to hold it.
Copper is preferred because lead tingles fatigue and unroll after a decade or two, which is why a roof scattered with drooping lead tabs has usually been worked on more than once. Around a rooflight, tingles are needed on the last course or two put back above the head flashing, because those slates are being fed in under work that is already complete. Planning the sequence so as few slates as possible need tingles is part of the craft. Some are always unavoidable.
The cut slate beside the frame, and re-holing it
Each side of the opening, slates are cut down to width against the frame. A slate cut in half lengthwise usually loses one of its two nail holes, and a narrow strip loses both.
The fix is straightforward and it has to actually be done. The cut slate is re-holed, punched from the back so the hole spalls outward on the face and forms its own countersink for the nail head. Where the remaining width is too narrow to take a hole with sound slate around it, a slate-and-a-half is used in the alternate course instead so the cut lands somewhere holdable, or the piece is held with a copper clip. A nail head standing proud on a slate face lifts the slate above it and cracks it within a few years, which is why the countersink from punching in the right direction is not a nicety.
Slate-and-a-half, and keeping the bond at a vertical edge
Slating is laid in a broken bond exactly as plain tiling is, with the perpend joints of each course centred over the slate below. A rooflight frame creates two new vertical edges in the middle of that bond.
Slate-and-a-half units, half as wide again as the standard size, are the tidy answer. Used in alternate courses at the edge of the opening, they let the bond continue right up to the frame with every piece wide enough to carry two holes. They are made in the standard sizes, so a Countess slate-and-a-half is 508 by 381mm. On a reclaimed roof they can be made on site by selecting oversized slates from the sort, which is one reason a slater wants the full stack sorted by width before starting rather than pulling from a random pile.
Dressing an edge rather than grinding it
A slate cut with an angle grinder leaves a square, pale, dusty edge that reads as bright grey against a weathered roof and stays visible for years. A slate cut with a slate cutter or a zax, worked from the back, leaves a slightly bevelled edge with the arris broken on the face, and it looks like the rest of the roof from the day it goes up.
Around a rooflight this matters because the cut edges are all in one place, framing the window at eye level from the garden. The straight run of a dozen cut slates down each side is the most visible piece of slating on the whole roof. Dressing them by hand takes longer, and it is the difference between a rooflight that disappears into a slate slope and one that announces itself. Grinding has its place for a very hard slate or an awkward shape, and the cut edge is then dressed back afterwards.
Welsh, Spanish, Brazilian and fibre cement, compared honestly
What the roof is made of changes how it behaves under a ripper and how well a new slate will match.
Welsh slate from Penrhyn, Ffestiniog or Nantlle is the county’s traditional material, delivered here by rail and river in the nineteenth century. It splits thin, holds its colour indefinitely and lasts well over a century. Spanish slate covers a large share of roofs laid since the 1970s, and the good grades are excellent while the poorer ones carry pyrite inclusions that rust and bleed orange, or delaminate at the head. Brazilian slate is generally consistent and slightly thicker with a distinctive even colour. Fibre cement slate is a manufactured board, dimensionally perfect, fixed with a nail and a rivet or hook at the tail, and it takes the same flat flashing kit as natural slate. Matching a Welsh slate with a Spanish one on the same slope shows, and the courses around a rooflight are exactly where it shows most.
A thin covering takes the flat flashing group
Natural slate is 5 to 8mm thick, which puts it in the flat covering group for manufactured flashing kits, alongside fibre cement slate and flat faced concrete tile. Side flashings are interleaved soakers rather than a continuous formed length, because there is not enough thickness anywhere on the roof to hide a continuous flashing without it standing proud.
The upside is that a slate roof gives the flattest, tidiest result of any covering. The apron lies almost flat, the soakers vanish entirely, and the head flashing tucks under a slate that is barely thicker than the lead. The relationship between covering profile and kit group is set out on soakers, aprons and head flashings explained, and the general behaviour of the metal itself is on lead flashing and weathering detail.
Recessed installation, and the covering it suits best
A recessed flashing kit sets the frame roughly 30 to 40mm deeper into the roof so the glass sits closer to the plane of the covering. On a deep profiled tile the recess is largely swallowed by the tile depth and the visual gain is modest. On a 6mm slate the whole depth of the recess is visible gain, and the window goes from an object sitting on the roof to something set into it.
The condition is room in the gauge. The head flashing on a recessed unit has further to travel before it gets under the course above, so on a small slate at a 155mm gauge there may not be enough. It is checked against the measured gauge and the pitch, not assumed. Where a slate roof is in a conservation area and the aim is a genuinely flush appearance, the conversation usually moves to a slim steel conservation unit instead, covered on the conservation rooflight installation page.
Slating over boards, and the deck the nails go into
A proportion of the county’s slate roofs, particularly steeper Forest of Dean work and better Victorian building in Cheltenham, are slated over a solid timber deck rather than over open battens, sometimes with battens over the boards and sometimes with the slates nailed straight through.
Two differences follow at an opening. The board is often bracing the roof plane, so the rectangle cut out of it wants noggins between the rafters to carry the cut edges, and the slater has a fixing surface everywhere rather than only on batten lines, which actually makes reinstating cut slates easier. Where the slates are nailed directly into the board, the ripper still works but there is no gap behind the slate, so the blade has to be introduced carefully. The membrane detail over boarded roofs is on underlay and sarking around a pitched rooflight.
Cheltenham mansards, and the slope the pavement can see
Central Cheltenham runs to stucco, sash windows and Welsh slate, and a great deal of it is mansard: a steep lower slope at 65 to 75 degrees facing the street, and a shallow upper slope hidden behind a parapet.
Slating a near vertical mansard face is its own discipline, with reduced laps, heavier fixing and often a slate-and-a-half at every course end. A rooflight in that face is highly visible and generally the harder consent conversation on a listed terrace. The shallow upper slope behind the parapet is invisible from the street and is where the useful daylight usually comes from, but at 5 to 15 degrees it sits below the minimum pitch a manufactured roof window is rated for, so the unit becomes a kerb mounted one rather than an in-plane window. Those two facts together explain most of the rooflight arrangements on Lansdown and Pittville roofs.
Random slating, and courses that change depth up the slope
Not every slate roof is a uniform module. Older Forest and Stroud valley work is often random slated: slates of mixed lengths laid in diminishing courses, largest at the eaves, reducing towards the ridge, with the gauge changing course by course.
Where that is the case, every soaker beside the opening is a different depth, cut on the scaffold rather than taken from a bundle, and the head flashing is sized to whatever the gauge happens to be at that height in the slope. Reinstating the courses afterwards means matching the diminishing rhythm so the finished roof reads correctly rather than showing a rectangle of uniform coursing around the window. It is slower, it is a real cost difference, and it is said at survey rather than absorbed quietly.
What the survey measures on a slate roof
The order goes out against measured facts, and on slate there are more of them than on any covering except stone.
- Slate size measured on a sample lifted at a verge or a hidden slope.
- Whether the roof is head nailed or centre nailed, established from the sample.
- Measured gauge over ten courses, and the headlap it implies.
- Nail material and condition, which decides whether nail sickness is in play.
- Slate origin and thickness, and whether matching material exists.
- Pitch off a digital level on the actual slope.
- Whether the roof is slated over boards, and their thickness.
- Existing tingles, slipped slates and previous work, counted rather than noted vaguely.
- Whether the coursing is uniform or diminishing.
- Exposure, and whether the headlap has been increased above 100mm.
Those ten answers decide the flashing kit, the soaker schedule, the slate order, the breakage allowance and the programme. A slate roof gives the best looking rooflight in the county when the work is done in the right order, and the right order is decided before anybody puts a ripper under the first course. The route from an enquiry to a figure is on the costs page.
Next on this subject.
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