Slate needs a flat-profile flashing kit, a wider side clearance than a tiled roof, and cut slates that are re-holed and hung rather than trimmed and bedded. Slate is thin, brittle and laid double-lapped with a head lap, so the flashing sits almost directly onto the surface and the whole interface depends on the lower courses being rebuilt correctly rather than on any depth of metalwork. Get the side clearance wrong and the slates crack at the cut edge. Get the head lap wrong and the roof leaks under wind-driven rain regardless of how good the kit is.
That makes slate the easiest covering to flash neatly and one of the easiest to get wrong, because the tolerances are small and nothing about the finished job looks difficult from the ground. This page covers the slate interface specifically. Kit selection across all coverings, and the order of work up the slope, is on choosing and fitting the right flashing kit. If your slate rooflight is already leaking, the diagnosis sequence is on flashing failures on pitched roof rooflights.
Why slate behaves differently to tile
Three properties of slate drive every detail at a rooflight, and none of them apply to a plain or interlocking tile.
It is thin. A natural slate is typically 4 to 8mm thick, against 12 to 15mm for a plain tile and up to 45mm of profile depth on interlocking concrete. The flashing has almost no depth to sit into, so it lies down on the surface. That is why the finished job on slate looks so tidy, with the metalwork nearly invisible from the ground, and it is also why any distortion in the flashing shows immediately.
It is brittle and it cannot flex. A concrete tile will tolerate being levered, packed and adjusted. A slate will not. It splits along its cleavage plane, and it splits from any point where it is stressed, particularly a cut edge or a nail hole under load. This is the single biggest cause of failure at a slate rooflight and it is entirely a workmanship issue.
It is laid double-lapped, and the lap is the waterproofing. Slate has no interlock and no profile to shed water sideways. Every slate covers the joint between the two below it, and there is a head lap, usually 75 to 100mm depending on pitch and exposure, where three thicknesses overlap. Interrupt the courses at a rooflight and the head lap has to be re-established on the way back up, course by course. On a tiled roof the interlock and the profile give you some slack. On slate there is none.
The lower course detail, which is where slate jobs are won
The apron below a rooflight discharges onto the courses beneath it, and on a slate roof those courses have been cut back to form the opening and then rebuilt. How they are rebuilt decides whether the interface holds.
Cut slates must be re-holed and hung, not bedded
Slates are fixed either by nailing at the head or by centre nailing, depending on pitch and slate size. When a slate is cut to fit alongside a rooflight, it loses one or both of its original nail holes. The correct answer is to hole the cut slate afresh, from the underside with a slater’s holing tool or a proper punch so the hole spalls on the face and the nail head sits down into the dish it creates, and hang it on new nails or on a copper or stainless slate hook.
The wrong answer, and the one we find constantly, is to lay the cut slate in without a fixing and hold it with mortar or a bead of silicone. It sits fine for a couple of years. Then wind uplift under the tail lifts it, or the mortar cracks, and either the slate slides or it snaps at the point where the mortar has it rigid while the rest is free to move. Slates held by adhesive rather than by fixings are a slate roof waiting to unravel from one point outwards.
Head lap has to survive the rebuild
Where courses are cut and reinstated around an opening, the batten gauge often gets adjusted slightly to make the courses meet the frame neatly. On a tiled roof you can lose 10mm of lap and nothing happens. On slate at a shallow pitch, losing 10mm of head lap in the courses immediately below the apron is enough to let wind-driven rain track up between the slates and past the lap. The rule is that the gauge does not change to suit the window. The window position is set to suit the gauge, which is one of the reasons we measure the actual course spacing on the roof before confirming a unit size.
The apron needs slate under it, not slate cut around it
The apron should land on a full, properly hung course. Where a fitter cuts a horseshoe of slate around the apron to make the courses line up visually, every one of those cuts is a stress point and a potential capillary path. Better to set the window so the apron discharges onto a clean course and accept that the cut slates fall at the sides where they can be holed and hung normally.
Side clearance, the detail nobody sees
The commonest slate rooflight fault in central Ipswich is slates cut hard up against the frame with no clearance. It looks better than a gap, so a fitter chasing a neat line does it deliberately.
It fails for two reasons. The soaker or side section needs physical room to sit in, and if the slate occupies that room the soaker is pushed out of position or omitted. And slate, timber and aluminium all move at different rates with temperature. A slate held rigid at its cut edge against an aluminium frame that expands on a warm day is being loaded across its cleavage plane. It cracks, usually within two or three seasons, and the crack runs straight down the exposed face.
Manufacturers publish the side clearance their kit needs, and it is typically in the region of 30 to 45mm each side on a flat-profile kit. That gap is not sloppiness. It is where the flashing lives.
| Detail | Correct | What we find | How it fails |
|---|---|---|---|
| Flashing profile | Flat-profile kit, metalwork lying on the surface | Standard profiled kit intended for interlocking tile | Metalwork stands proud, water tracks under the raised section |
| Side clearance | Manufacturer’s dimension, typically 30 to 45mm | Slate cut hard to the frame | Soaker omitted or displaced, slate cracks at the cut edge |
| Cut slates | Re-holed from the underside, hung on nails or hooks | Laid loose and bedded in mortar or silicone | Uplift lifts the tail, mortar cracks, the slate slides or snaps |
| Head lap below the apron | Original gauge maintained, 75 to 100mm lap | Gauge adjusted to make courses meet the frame | Driven rain tracks past a shortened lap |
| Fixings | Copper or 316L stainless on the coast, never bright steel | Galvanised or A2 grade 304 | Rust bleed down the slate, then loss of section and uplift |
| Underlay at the opening | Cut to pattern and formed into a collar | Slashed with a knife and left open | Water past the slate lands on the trimmers, hidden for years |
| Conservation roofs | Dressed lead, code 4 minimum, flush unit | Visible mill-finish aluminium kit | Consent refused, or an enforcement problem later |
Clearances and laps are typical. The survey confirms the actual slate size, gauge and pitch on your roof.
Fibre cement slate is not natural slate
A lot of the 1960s to 1980s stock at Chantry, Whitton and Rushmere St Andrew is covered in fibre cement slate rather than natural, and a fair amount of that stock is now at an age where it needs handling carefully.
Fibre cement takes the same flat-profile flashing kit and the same general approach, but it is more fragile in age than natural slate, particularly around the fixing holes. It also delaminates at the edges after thirty or forty years, which means the slates immediately around an opening may not survive being lifted and relaid at all. We allow for a proportion of replacement slates on any fibre cement roof over about thirty years old, and we say so at survey rather than discovering it on the day. Matching is a genuine problem, because the profiles and colours have changed repeatedly, and on some roofs the honest answer is that the replacements will be visible.
The other point on older fibre cement: anything laid before the mid 1980s may contain asbestos. Where the age or appearance suggests that, we stop and it goes to a licensed contractor for identification and handling before any rooflight work is programmed. That is not a delay we are willing to skip.
Conservation slate roofs in Suffolk and north Essex
Slate and conservation constraint overlap heavily here. The Victorian and Edwardian terraces across central Ipswich, much of Woodbridge, and the older stock at Sudbury, Hadleigh and Colchester are slated and a substantial share sits inside conservation areas or on listed buildings.
On those roofs the proprietary aluminium kit is usually the wrong answer visually even when it is technically sound. A conservation officer looking at a front or visible slope wants a flush unit with a slim dark frame, a central glazing bar, tall proportions closer to 1:2 than square, and dressed lead flashing at code 4 or heavier rather than a visible pressed kit. Lead can be dressed tight to slate very neatly, and on a slate roof it weathers to a colour that disappears in a way aluminium does not.
The practical position on consent is unchanged from any other covering. A listed building needs listed building consent wherever the rooflight goes, including a rear slope nobody can see. In a conservation area a rear slope not visible from a public highway is often acceptable, while a visible slope will usually need a conservation pattern unit. Article 4 directions can remove rights you would otherwise have. Speak to the conservation officer at Ipswich Borough, East Suffolk, Mid Suffolk, Babergh or Colchester before you order anything, because a refused application with a window already on site is an expensive way to learn.
Coastal slate
Slate roofs on the exposed coast at Felixstowe, Harwich, Dovercourt, Frinton and Aldeburgh get treated differently, and the changes are mostly about metal.
- 316L stainless throughout, not A2 grade 304. The molybdenum content resists chloride pitting. A 304 fixing within a mile of open sea shows rust staining within two or three winters, and on slate that staining runs down a pale grey surface where everybody can see it.
- Copper or stainless slate hooks rather than nails where the slate size and pitch allow, because a hook holds the tail down against uplift in a way a head nail does not.
- Lead at the apron rather than a pressed aluminium section on the most exposed slopes, because lead can be dressed tighter to an irregular slate surface than a pressed section can.
- Increased laps and mechanical fixing at the head, since driven rain on an east-facing slope arrives horizontally and the standard lap dimension assumes gravity is doing the work.
- Isolation between dissimilar metals. Aluminium against steel in a chloride environment sets up galvanic corrosion, and a lead apron against an aluminium frame needs the same thought.
Estuarine slate at Woodbridge, Wivenhoe, Manningtree, Shotley and Pin Mill sits on tidal saline water with real wind exposure across open water, but it does not carry open-sea salt loading. Upgraded fixings are sensible there. A full marine specification generally is not, and we are not going to sell it as though Woodbridge faced the North Sea. Inland at Stowmarket, Needham Market, Sudbury and Hadleigh, corrosion is not a factor and the constraints are conservation consent and irregular historic structure instead.
What a slate rooflight installation runs to
Slate itself does not add much to a straightforward job. A flat-profile kit is the cheapest kit there is, and a sound modern slate roof is quicker to open and close than clay pantile. The costs on slate come from three other places.
- Replacement slates. On a roof over about eighty years old, or any fibre cement roof over thirty, a proportion of the slates lifted will not go back. Matching reclaimed Welsh slate to an existing roof costs real money and we allow for it at survey.
- Conservation pattern units and dressed lead. A conservation unit with lead flashing runs £2,900 to £4,200 supply and fit against £1,900 to £2,650 for a standard 780 x 1180mm centre pivot with one rafter trimmed.
- Structure. Older slated roofs across Suffolk were frequently built with irregular rafter sizes and spacings, and many have been altered. Where two or more rafters are cut, where the roof is trussed, or where the opening lands near a valley, hip, ridge or eaves, an engineer’s calculation comes first and we build to that design.
Published ranges for every pitched unit type are on the pitched roof skylight installation hub, and figures across all nine services are on our costs page.
One thing we will not do. We do not lift and relay slate that has reached the end of its life just to get a rooflight in. If a survey shows nail sickness across the slope, with slates slipping and the fixings corroded through, the honest advice is that the covering needs addressing first and the rooflight goes in as part of that work. Fitting a new unit into a roof with two years left in it wastes your money and ours.
Common questions about rooflights in slate roofs
Can a rooflight go into any slate roof?
Structurally, most traditionally cut slate roofs take one comfortably with a single rafter trimmed. The constraints are pitch and condition. Standard units are certified between 15 and 90 degrees, and most Victorian slate roofs sit at 30 to 40 degrees, which is ideal. Condition matters more. If the slates are nail sick, slipping and corroded at the fixings, the covering needs dealing with before or alongside the rooflight. And a listed building needs consent regardless of which slope the unit goes on.
Why does the flashing kit for slate differ from the one for tiles?
Because the flashing has to sit down onto the covering rather than bridge across it, and slate has almost no profile. A flat-profile kit lies on the surface with short soakers. A kit designed for interlocking concrete tile has depth pressed into it to clear a raised interlock rib up to 45mm, and on slate that depth becomes a raised void with nothing filling it. Water tracks under the standing section. It is the same principle in reverse to fitting a flat kit on pantiles.
Should the slates be cut tight to the frame?
No, and this is the most common botched detail we find on slate. Manufacturers specify a side clearance, typically 30 to 45mm, because the soaker or side section has to physically sit in that space. Slate cut hard against an aluminium frame is also being loaded across its cleavage plane every time the frame expands in the sun, and it cracks at the cut edge within a few seasons. The gap looks wrong to a homeowner and is exactly right.
What about a conservation rooflight in a slate roof?
Slate is where conservation units look best and where they are most often required. The pattern sits flush or nearly flush with the slate plane, has a slim dark frame and a central vertical glazing bar, runs to tall narrow proportions, and is flashed with dressed lead at code 4 or heavier instead of a visible aluminium kit. Expect £2,900 to £4,200 supply and fit. On a listed building consent is required wherever it goes, and in a conservation area a visible slope will generally need this pattern rather than a standard unit.
My slate roof is fibre cement rather than natural. Does that change anything?
The flashing approach is the same, but the handling is not. Fibre cement over about thirty years old delaminates at the edges and cracks around the fixing holes, so a proportion of the slates lifted around the opening will not go back and matching them is genuinely difficult because profiles and colours have changed. We allow for replacements at survey. Anything laid before the mid 1980s may contain asbestos, and where the age suggests that, identification and handling goes to a licensed contractor before we programme any work.
Get a fixed price for a slate roof rooflight
Tell us roughly when the house was built, whether the slate is natural or fibre cement, whether the property is listed or in a conservation area, and which slope you are thinking about. We will come out, measure the actual course gauge and rafter positions, check the condition of the slate and the fixings, and give you one written fixed price with the unit, the flat-profile kit or dressed lead, both collars and any replacement slates itemised. That price is fixed, and every installation carries a 10-year workmanship guarantee.
Request a fixed quote for a slate roof rooflight in Ipswich, Suffolk or north Essex, or read how the kit is selected and fitted on our flashing kit installation page.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.