Fitting a VELUX into a slate roof means using a slate-profile flashing kit, stripping back and re-hanging the slates by hand to the correct head lap, and allowing for breakage in old brittle slate. Slate is the least forgiving covering to work with and the fussiest to get right, because every slate is nailed twice and the lap has to be maintained around the new opening with no nail hole left exposed to weather. Get the kit, the gauge and the lap right and a slate installation is watertight for decades. Get any one of them wrong and it reads as a patch from the street. Never go onto a slate roof yourself to check it. That is survey work from proper access.
Ipswich has a lot of natural slate. The Victorian and Edwardian terraces off Norwich Road and Spring Road, and the streets around the town-centre conservation area, are largely Welsh slate laid in the second half of the nineteenth century and the early twentieth. Much of it now carries loft-conversion roof windows fitted between 2000 and 2010 that are failing together. When those come out for replacement, or when a first window goes in, the slate work is the part that separates a clean job from an obvious one.
The slate flashing kit
Slate takes a low-profile flashing kit, designed for coverings up to about 8mm thick. That family covers natural slate, fibre-cement slate and plain clay tiles. The skirt sits tight to the slate with a shallow dressing, because there is no deep contour to dress into as there would be on a pantile. This is the correct kit for an Ipswich slate terrace, and it is a specification error we see repeatedly on other people’s work: a profiled tile kit fitted to a slate roof leaves a gap under the raised skirt that you can see daylight through, and driven rain follows.
Where the window is being set into a sensitive period roof, a combination or recessed kit is often the better answer. It sets the window deeper into the roof plane for a flatter external appearance, which conservation officers frequently ask for. It costs more and takes longer, and on a Victorian slate roof it genuinely looks right. The full family breakdown is on our flashing kit explained page, and the specifics of taking an old unit out of slate are covered on replacing a VELUX in a slate roof.
Batten gauge and where the window lands
Slate is hung on battens set to a gauge, and the gauge is dictated by the slate length and the head lap. On a roof window installation the flashing skirt and the top and bottom aprons have to marry into that gauge, so the window position is not entirely free. It has to sit in a sensible relationship to the batten courses above and below, otherwise you end up with an awkward part-course of slate that is hard to weather and looks wrong.
At survey we set the window position against the existing gauge, not just against where the light is wanted inside. On a like-for-like replacement the position is fixed by the existing opening, which makes this simpler. On a new opening there is some freedom, and we use it to land the head and sill of the window on clean batten lines wherever the internal layout allows.
Head lap and pitch, the two numbers that matter
| Figure | Typical value | Why it matters at the window |
|---|---|---|
| Head lap | 90mm to 100mm | The overlap of a slate over the one two courses below. It must be maintained around the opening or water tracks in. |
| Fixing | Each slate nailed twice | Nail holes must not fall in exposed positions near the skirt; face-fixed slates need copper or lead tingles. |
| Terrace pitch | 25 to 30 degrees | Shallow end for roof windows. Lower pitch lets driven rain travel further up under the covering. |
| Minimum window pitch | 15 or 20 degrees | Most roof windows are rated to 15 degrees, some to 20. A 25 degree Suffolk terrace is inside the rating but near the demanding end. |
| Cutting | Slate cutter, not a grinder | An angle-ground edge chips and reads as a repair for the life of the roof. |
Shallow pitch, the Suffolk terrace problem
Natural slate on Suffolk and north Essex terraces is often laid at 25 to 30 degrees. That is a lower pitch than you find on a tiled semi, and it is at the shallow end of what a roof window is happy with. Most units are rated down to 15 degrees and some to 20, so a 25 degree roof is inside the rating, but a shallow pitch changes how hard the details have to work.
At low pitch, wind-driven rain travels further up under the covering before gravity pulls it back out. That puts more load on the flashing lap at the head of the window and on the underfelt collar behind it. The underfelt collar connects the roofing underlay to the frame so water passing the slate drains over the head of the frame and back onto the covering below. On a shallow slate roof this is not a nicety, it is the difference between a dry window and a slow leak that shows up in the first hard easterly. We increase the lap and we fit the underfelt collar properly rather than stapling felt over the frame, which is the usual shortcut.
Do not go up to look. A slate roof is brittle, slippery and unforgiving, and a cracked slate you stand on today is a leak next winter. If you have a window you are worried about, or a slate that has slipped near it, photograph it from the ground and send it with your enquiry. We assess from proper access at survey. We never advise anyone to get onto a roof, and we do not offer to reseal or patch a slate window in place, because on slate a patch buys a season and a proper reinstatement ends the problem.
Breakage allowance and reclaimed slate
Old Welsh slate is brittle. A slate that has been on a roof for a hundred years will break if you look at it wrong, and stripping back a working area to fit a window means handling slates that are well past their easy-handling years. We allow for breakage in the quote rather than pretending every slate will come off and go back cleanly, because on a Victorian roof a proportion will not.
The replacements matter as much as the allowance. Dropping a modern fibre-cement slate into the middle of a natural Welsh slate roof leaves a patch that reads from the street for the next fifty years, lighter or greyer than the slate around it and flatter in texture. We source matching reclaimed slate so the reinstatement disappears into the roof. That adds for materials and for the slower, more careful working, and it is the right call on any period property, especially inside a conservation area where the roof is part of what is being protected.
Slate installation spec, at a glance
- Flashing: low-profile slate kit, or recessed combination kit for conservation settings.
- Head lap: maintained at 90mm to 100mm around the opening, no exposed nail holes.
- Cutting: slate cutter only, never an angle grinder.
- Face-fixed slates: copper or lead tingles, never a bead of sealant.
- Breakage: allowed for in the price; matching reclaimed slate sourced for reinstatement.
- Underfelt collar: fitted to drain over the head of the frame, lap increased on shallow pitch.
- Coastal slate: 316L stainless fixings within a mile of open sea, so the flashing does not stain within two or three winters.
Coastal slate and the Felixstowe factor
Slate roofs near the coast carry an extra demand. On the east and north-east facing slopes at Felixstowe, Harwich and the villages behind them, salt-laden air off the North Sea deposits chloride on every fixing and flashing lap, and the wetting and drying cycle concentrates it rather than washing it off. A grade 304 stainless fixing will show rust staining down a slate roof within two or three winters there. We use 316L stainless throughout on coastal slate work, because the molybdenum content resists chloride pitting, and we increase the flashing laps and mechanically fix the terminations. Our Felixstowe page sets out how the specification changes as you move towards open sea.
Inland, on the terraces of central Ipswich, corrosion is not the issue. There the questions are the shallow pitch, the age and brittleness of the slate, and the conservation constraints that come with the town-centre area. The slate detail matters everywhere, the fixing specification changes with the salt.
Where slate work fits the wider job
Fitting a window into slate is one interface within the whole installation. The frame still has to be set square, both collars still go on, and the internal lining still gets made good, all of which follow the same sequence as any other covering. That full sequence is on our VELUX installation process page. The specific meeting of rooflight and slate, where the skirt marries the slate courses, is covered in more technical depth on the rooflight and slate roof interface page, which is worth reading if you want the detail behind the photographs.
Common questions about slate roof installation
Can a roof window go into a slate roof at 25 degrees?
Yes. Most roof windows are rated down to 15 degrees and some to 20, so a 25 degree Suffolk terrace is inside the rating. What the shallow pitch changes is how hard the weathering details have to work. Driven rain travels further up under the slate at low pitch, so we increase the flashing lap at the head and fit the underfelt collar properly rather than stapling felt over the frame. Inside the rating is not the same as forgiving, and slate at the shallow end rewards careful detailing.
Will you break my slates taking them off?
Some, and we say so honestly. Century-old Welsh slate is brittle and a proportion will not come off and go back cleanly however carefully it is handled. We allow for breakage in the quote and we source matching reclaimed slate for the reinstatement, so the finished roof does not carry a lighter modern patch around the window. Working slowly and by hand keeps the breakage down, but pretending it will be zero on a Victorian roof would not be honest.
My slate roof is in a conservation area, does that change things?
It can. Conservation areas restrict anything visible from a public highway, and a standard roof window projecting above the slate plane may not be acceptable where the roof is seen from the street. A recessed combination installation, which sets the window deeper into the roof for a flatter profile, is often what gets approved. Listed buildings need listed building consent in their own right. We tell you which category your property is in at survey and specify accordingly.
Should I get up and check the slates around my old window?
No. A slate roof is slippery and brittle, and standing on it to inspect a window is how a sound slate becomes a cracked one and a dry roof becomes a leaking one. Photograph the roof and the window from the ground, or from a window on the floor below, and send the images with your enquiry. We inspect from proper access at survey. We do not advise anyone onto a roof and we do not patch slate windows in place.
Get a fixed price for slate roof work
Send a photograph of the roof from the garden showing the slate, one of the window from inside, and the data plate if it is a replacement and you can reach the plate safely from indoors. That is usually enough for us to tell you whether the slate takes a standard or a recessed kit and roughly what the job costs. Then we survey from proper access, confirm the gauge and the pitch, and give you one written price. Slate reinstatement with matching reclaimed slate is stated on the quote, and every installation carries a 10-year workmanship guarantee behind installers with 25 years fitting roof windows into slate.
Request a fixed quote for a slate roof installation, or go back to our VELUX window installation hub for the full picture.
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.