Guide · Roof Window Installation
Rooflights in natural slate roofs
Natural slate is the thinnest covering a rooflight goes into and, detailed properly, the neatest. A slate is four or five millimetres thick, dead flat and laid double lap, so a flush unit set into a slate slope reads almost as part of the roof. The catch is that slate is unforgiving. It cannot be bent, it splits along its cleavage if it is handled roughly, and a fixing hole in the wrong place ruins the slate rather than adjusting it.
In Essex, slate arrived by rail in the Victorian period and it went onto the terraces and the rear additions across Chelmsford, Colchester, Maldon, Braintree and Brentwood, along with a great many station-era commercial buildings. This page covers the fixing methods, the setting out, the flashing depth and the handling that a slate roof asks for. It sits under roof window installation.
Why slate takes the lowest profile kit of all
Every flashing manufacturer publishes a kit for flat coverings up to about 8mm thick, and slate is the reason it exists. The apron has almost no skirt, the side pieces are little more than a folded upstand and a flat wing, and the assembly is meant to lie under and over the slates rather than around them.
The result is a very shallow detail with no gaps to close. It also means there is nothing to hide behind. On a deep tile the flashing has bulk and any small irregularity in the cut disappears into the profile. On slate everything sits in one plane and the eye reads the line straight away.
Head nailing, centre nailing and the difference at an opening
Slates are holed and nailed either near the head or at the centre, and British practice varies by slate size and pitch. Centre nailing puts the fixing roughly halfway up the slate, which reduces the leverage the wind can apply to the tail and is standard on larger slates and lower pitches. Head nailing puts the fixing near the top, is used on smaller slates and steeper pitches, and gives a longer unsupported tail.
At a rooflight, the difference decides where a cut can be made without losing the fixing. A centre nailed slate cut down its width may keep one hole and lose the other, which usually still works. A head nailed slate cut short at the head loses both. Knowing which method the roof used, which the survey establishes, is what determines whether a course can be cut in or needs a different approach at the edge.
Slate sizes, and the gauge they produce
Slate is sold by size, and the size sets the batten gauge through the lap. The gauge is the slate length minus the lap, halved, and it produces a much finer course spacing than a tile roof of the same area.
| Slate size | Typical lap | Approximate gauge | Common use |
|---|---|---|---|
| 500 x 250mm | 75mm | Around 212mm | General domestic, most Essex terraces |
| 500 x 300mm | 75mm | Around 212mm | Larger houses, wider coverage |
| 400 x 250mm | 75mm | Around 162mm | Smaller roofs, rear additions |
| 600 x 300mm | 100mm | Around 250mm | Exposed positions, lower pitches |
The lap is not a fixed number. It increases as the pitch drops and as exposure rises, so a slate roof on open ground towards the Dengie or the Blackwater will have been laid with more lap than the same slate on a sheltered street in town.
Welsh, Spanish and the slates found on Essex roofs
Original Victorian work in this county is overwhelmingly Welsh, brought in by rail, and it is a dense, fine-grained slate that has often outlived the battens holding it up. Spanish slate dominates modern supply and re-coverings, and quality varies considerably by quarry, with the better grades performing well and the poorer ones prone to rust spotting from iron inclusions.
For a rooflight the relevance is matching. A slope of aged Welsh slate does not take new Spanish slate without the difference being visible, so the working method is the same as on clay: re-lay the originals, take sound slates from a hidden slope if a shortfall appears, and put anything new where it cannot be seen.
Cutting and holing a slate by hand
A slate is cut with a slate cutter or a zax, drawn along the line so the cut edge is bevelled slightly on the underside. That bevel is not decoration. It means the cut edge sits down onto the slate below rather than perching on a sharp arris, and it throws water clear of the joint.
Holes are punched or drilled from the underside so the impact breaks out on the face, leaving a small countersink that the nail head sits into. A nail head standing proud of the slate is a slate that will not lie flat, and on a course beside a flashing that is exactly where it shows.
Tingles, and refixing a slate that cannot be nailed
Once an opening is formed there are always a few slates that have to go back where a nail cannot reach: the ones directly under a course that is already fixed, or a slate replacing one that broke during the strip.
The traditional answer is a tingle, a narrow strip of lead or copper nailed to the batten below, taken over the tail of the slate and turned back on itself to grip it. Done properly it is durable and almost invisible. Done as a long flapping strip it is the first thing that lifts in a gale, so the length and the turn matter more than they look like they should.
The side abutment on a slate roof
At the sides of the unit, water is carried by soakers, one per course, folded into an L and laid so each is lapped by the slate above it. There is no continuous flashing doing the work; it is a stack of individual pieces, each one weathering a single course.
Because a slate roof has a fine gauge, that means a great many soakers over the height of an opening. Proprietary kits supply the equivalent as a formed side section, which achieves the same drainage in one piece. Either way, the covering above holds the weathering down, which is why the last few slates go on after the flashing and not before.
Getting the head detail flat under thin slates
The head flashing on a slate roof has to disappear under two courses without lifting either of them. A slate that is riding on a fold or a nail head will rock, and a rocking slate at the head of a rooflight is directly in the path of everything coming down the slope.
The way through is to keep the fold heights low, dress the material properly rather than folding it once and hoping, and check the courses lie dead flat before nailing off. It is slow, careful work and it is the reason a slate opening takes longer than the same opening in an interlocking tile.
On slate the test is simple: run a hand up the courses beside the unit and nothing should rock.
Delamination, and reading the condition of an old roof
Slate fails in two ways and they look different. Delamination shows as the slate splitting into thin layers at the tail, usually starting on the shaded slopes, and it means the slate itself has reached the end of its life. Nail sickness is the other, where the slate is sound but the iron nails have corroded and slates begin slipping one at a time.
The distinction matters before an opening is agreed. On a nail sick roof, disturbing an area to form an opening frequently loosens the slates around it, and the honest answer is that the slope is closer to a re-covering than it appears from the ground. That is a statement about the building, and it is far better made at survey.
Fibre cement slate, and how it differs in practice
A large number of Essex roofs that read as slate from the road are fibre cement. They are laid in the same double lap pattern at a similar gauge, and they take the same flat profile flashing kit, so the detailing is closely comparable.
Three things differ. They are fixed with a nail and a disc rivet at the tail rather than nails alone. They cut with a disc rather than a slate cutter. And they are more consistent in thickness, which makes the setting out around an opening slightly easier. What does not differ is the requirement for a flat, low flashing, because the covering is just as thin.
Random widths and diminishing courses
Some older slate roofs, particularly on the more rural stock, were laid in random widths, with slates of varying width sorted and set out to keep the bond. A few carry diminishing courses, where the slate size reduces towards the ridge.
Both make a rooflight opening a setting out exercise rather than a measurement. The courses either side of the frame have to arrive with a proper half bond and no stacked joints, using the widths available, and on a diminishing roof the gauge itself is changing as you go up. It is worth naming this at survey because it adds real time to the day.
Slate on a Victorian rear addition in Chelmsford
The classic job in this county is a rooflight into the slated slope of a two storey rear addition on a Victorian terrace, lighting a bathroom or a stairwell. Those roofs are usually steep, often 40 degrees or more, with small rafters at close centres and a boarded or battened deck.
Steep pitch helps in several ways. Water leaves quickly, the flashing has an easy job, and the unit reads almost vertically from inside so the view is of the street or the garden rather than the sky. The constraints are usually access and the narrowness of the addition, which caps how wide a unit can be trimmed between the rafters. Where the internal wish is for more glass, two units on one slope is often the workable answer.
Torching, and what sits under an old slate roof
Many original slate roofs were laid without any underlay at all. Instead the underside was torched, a haired lime mortar buttered between the battens to close the gaps and stop wind and snow driving through. After a century most of it has fallen into the loft.
Forming an opening in a torched roof means dealing with that material, which is heavy, dusty and loose. It also means the roof has no secondary weathering layer at all, so the work introduces a modern membrane locally around the unit and laps it in as far as the strip allows. That local membrane is a genuine improvement to the slope, and it is covered in more depth on underlay and sarking around a pitched rooflight.
Fixing the window frame to the rafters through a thin covering
The unit is carried by the structure, not by the covering, and on a slate roof the structure is often lighter than on a tiled one, because slate weighs less than tile and the rafters were sized accordingly. Rafters of 100mm by 50mm at 400mm centres are common on Victorian additions.
That changes the trimming. Doubling a slender rafter and putting in trimmers above and below is straightforward carpentry, and the detail is covered on trimming rafters for a pitched roof rooflight. What is specific to slate is the depth: with a thin covering and a boarded deck, the frame sits low, and the bracket positions are chosen to suit rather than left at the factory setting.
Salvaging and re-laying the slates you take off
Slates come off in reverse order and go back the same way, and the practical trick is to keep them in stacks that record where they came from. Slates weather at different rates on different parts of a slope, and a slate returned to roughly its original position blends in a way that a randomly picked one does not.
Expect losses. Old nails rust into the slate, and a proportion will break at the hole no matter how carefully they are eased off. Allowing for that at the ordering stage, with matched replacements ready, avoids the situation where the roof is open and the slate merchant is closed until Monday.
What a good slate opening looks like close up
Flat courses that do not rock. Cut edges bevelled and turned down. Soakers or side sections invisible, with only a thin dark line showing where the covering meets the frame. Bond maintained on both sides with no stacked joints and no slivers. No new slate visible on the elevation that faces the street.
Slate rewards this because it holds the finish for a very long time. A well laid slate slope routinely runs to eighty or a hundred years, and an opening detailed to the same standard has no reason to be the part that fails first. As with every covering, it is the perimeter rather than the glass that decides that, which is why the workmanship carries a ten year guarantee.
Keep reading
More on roof window installation
Rooflights in a trussed roof and what cannot be cut
Read the guide →Soakers, aprons and head flashings explained
Read the guide →Rooflight reveals and why splaying them matters
Read the guide →Two rooflights side by side on one slope
Read the guide →Rooflights on a shallow pitch below 15 degrees
Read the guide →
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