What Flashing Do You Need for a Slate Roof Rooflight?
A slate roof rooflight needs a flat-profile flashing kit, because slate is thin and flat and the metalwork has to sit down almost directly onto it. The apron dresses down over the slates below, short soakers, often dressed lead, interleave with every slate course up the sides, and the head flashing tucks under the slates above. A deep-profile kit made for pantiles or interlocking tile, fitted to slate, stands the metal proud of the flat covering and leaves gaps the slates cannot close, so it leaks. On slate the flashing shows more than on any other covering, so both the kit choice and the setting out have to be right.
This answer covers the slate flashing specifically. The full kit-by-covering logic is on choosing and fitting the flashing kit, and the wider slate junction detail is on the rooflight to slate roof interface.
Why slate takes a flat kit
Slate, whether natural Welsh, Cornish, or fibre cement, is a thin flat covering laid double-lapped in fine courses. It has almost no profile, under 10mm, so the flashing has nothing to bridge and can sit low, close to the slate surface. That gives the neatest finish of any covering, with the flashing almost invisible from the ground, which on the Victorian and Edwardian slate roofs across central Ipswich is exactly what you want. It is also the easiest covering to weather correctly, because there is no raised profile to leave a gap under.
The slate flashing has the same four parts as any kit, adapted to the flat covering:
- The apron dresses down over the slate course immediately below the unit, throwing water clear.
- Short soakers interleave with every slate course up each side. Because slate is thin, the soakers are shorter than on a tile roof, and they are often individual lead pieces rather than a continuous section.
- The head flashing tucks up under the slates above and dresses down, with the drainage gutter behind it.
- Dressed lead, code 4 or heavier, is common throughout on slate, especially on period and conservation roofs, because it forms tight to the fine courses and reads better than aluminium.
Why a tile kit fitted to slate leaks
The most common slate flashing failure is a kit made for a deeper covering fitted to slate. A pantile or interlocking-tile kit has a deep-pressed apron and tall side sections shaped to reach down into a raised profile. Fit that to flat slate and the deep metalwork stands proud of the covering, because there is no profile for it to drop into. That leaves a gap between the flashing and the slate all the way round, and thin slates cannot be dressed up to close it the way a shaped tile could. Water tracks into the gap, and no amount of mortar holds it out, because mortar on a slate roof cracks and lifts within a few winters.
The reverse of the pantile problem, in other words. On pantiles a flat kit is too shallow and leaves voids under the rolls. On slate a deep kit is too tall and leaves gaps above the slates. Each covering needs its own kit, and slate needs the flattest one.
| Element | Correct for slate | What goes wrong |
|---|---|---|
| Kit profile | Flat-profile kit, sits low on the slate | Deep tile kit stands proud, gaps all round |
| Soakers | Short, interleaved every course, often lead | Long tile soakers that lift the slate courses |
| Head flashing | Tucked under slates above, dressed down | Set too high, bridging the slates, water runs under |
| Gap filling | None needed, metal sits on the slate | Mortar used to close gaps, cracks in a few winters |
| Slate cutting | Slates cut to the metal, side clearance kept | Slates cut too tight to the frame, no clearance, they crack |
The slate-specific details
Side clearance
Slate is brittle. Cut the slates too tight to the frame with no side clearance and they crack under the covering’s own movement and under foot traffic during future roof work. The correct detail keeps a small clearance and lets the soakers weather it, rather than jamming the slate up against the metal. This is the single most common workmanship error on slate, and it is why the finish looks fine on handover and cracks appear a year later.
Lead, not aluminium, on period roofs
On a Victorian slate roof, and especially anything in a conservation area, dressed lead code 4 or heavier is the right weathering, not a visible aluminium kit. Lead forms tight to the fine slate courses, weathers to match, and is what a conservation officer expects to see. A shiny aluminium flashing on a period slate roof reads wrong from the ground and can be refused in a conservation area. The consent side is on conservation area rooflight rules.
Exposure on the coast
On exposed and coastal slate slopes at Felixstowe, Aldeburgh and Harwich, we dress lead at the apron rather than relying on a pressed section, increase the laps, and use 316L stainless fixings, because driven rain off the North Sea is pushed at the slope and standard inland laps let it track back. Slate itself is a durable coastal covering, but the flashing detail is where the exposure is won or lost.
We do not go up to reseal a leaking slate flashing. A slate rooflight leaking from the wrong kit or cracked slates is not resealed on the roof, it is put right by lifting the covering, fitting the correct flat kit with proper side clearance, and replacing any cracked slates with matching salvage. We do not offer repair or patching, and we never advise anyone onto a slate roof, which is fragile and dangerous to walk. The fix is a proper reinstatement, priced as one figure.
Related questions
If you are weighing slate against a tiled roof for cost, the labour comparison is on does fitting cost more on slate than tile. If your slate roof is due for re-covering, doing the rooflight at the same time is covered on whether to replace rooflights when re-roofing, and it makes particular sense on slate because the covering is fragile to disturb twice.
Common questions about slate rooflight flashing
What flashing do I need for a rooflight in a slate roof?
A flat-profile kit, because slate is thin and flat and the metal has to sit low onto it. The apron dresses over the slates below, short soakers, often dressed lead, interleave every course up the sides, and the head tucks under the slates above. On period and conservation slate roofs, dressed lead code 4 or heavier is used throughout rather than visible aluminium.
Why is my slate rooflight leaking when the flashing looks fine?
Usually because a deep tile kit was fitted to flat slate, so the metal stands proud and leaves a gap the slates cannot close, often filled with mortar that has since cracked. Or the head flashing was set too high and bridges the slates instead of tucking under them. Both are kit and setting-out errors. The fix is to lift the covering and fit the correct flat kit, not to reseal from above.
Can you use lead flashing on a slate rooflight?
Yes, and on period and conservation slate roofs it is the right choice. Dressed lead code 4 or heavier forms tight to the fine slate courses, weathers to match the roof, and avoids the visible aluminium that looks wrong on a Victorian roof and can be refused in a conservation area. Lead is also what we use at the apron on exposed coastal slopes because it forms tighter than a pressed section.
Get your slate rooflight flashed properly
Tell us it is a slate roof and roughly where the property sits, and we will confirm the slate type and specify a flat kit or dressed lead to suit, with proper side clearance and matching salvage for any cracked slates. You get one written fixed price, a 10-year workmanship guarantee, and installers with 25 years fitting rooflights behind them.
Request a fixed quote for a slate roof rooflight in Ipswich, Suffolk or north Essex, or read our published prices first.
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.