To replace a roof window in a slate roof, the working area is stripped back, the old unit is removed, the new frame is set and collared, and a low-profile slate flashing kit is fitted. The slates are then cut accurately and re-hung with their head lap maintained and no nail hole exposed. Slate is the least forgiving covering to work, so allow for breakage and matching reclaimed slate.
The frame side of the job is the same as any pitched-roof fit. What makes a slate roof its own case is the covering: how it is laid, how it breaks, and how the flashing has to sit against it. Get those three right and a replacement reads as if the window was always there. Get them wrong and it reads as a patch from the street for the next fifty years.
The slate-specific steps
Slates are laid with a head lap, typically 90mm to 100mm, and each slate is nailed twice. That interlocking, double-nailed arrangement is why you cannot simply lift a few and drop them back.
- Strip a working area. Slates come off around the opening, further out than the window itself, because you need room to re-hang with the lap maintained. They are set aside carefully for reuse.
- Cut with a slate cutter, not an angle grinder. A grinder chips the edge and scorches the surface, and that damage shows for the life of the roof. Slates that have to be trimmed to fit around the flashing skirt are cut cleanly by hand.
- Fix cut slates properly. Where a slate has to be face-fixed because its nail hole now falls in the wrong place, it is held with a copper or lead tingle, a small strip clip, not a bead of sealant. Sealant fails and stains. A tingle lasts.
- Re-hang with the lap kept. Every re-laid slate goes back with its head lap intact and no nail hole left open to the weather. That is what keeps a slate roof watertight, and it is fiddly, slow work around an opening.
Breakage, and matching reclaimed slate
Old Welsh slate is brittle. A hundred-year-old slate will break if you look at it wrong, and some will break simply being lifted off. This is not poor handling, it is the material, and any honest quote for slate work builds in a breakage allowance rather than pretending every slate survives.
Broken slates have to be replaced with slates that match. Dropping a modern fibre-cement slate into the middle of a Victorian natural-slate roof leaves a visible patch that catches the eye from the pavement. So on slate work we source matching reclaimed slate of the right size, colour and thickness. That sourcing takes time and the reclaimed material costs more than new, which is why matching-slate reinstatement adds to both the price and the working hours. It is the difference between a repair-looking result and a proper one.
The flashing kit is different on slate
Flashing has to match the roof covering, and slate takes a slate or low-profile kit. This kit is designed for coverings up to about 8mm thick, which covers natural slate, fibre-cement slate and plain clay tiles. The flashing sits tight to the covering with a shallow skirt.
Fit a deep profiled-tile kit onto slate by mistake and you leave a gap under the skirt you can see daylight through, which is an obvious leak path. Fit the low-profile kit correctly and the skirt beds down onto the slate with nothing for wind-driven rain to get behind. Our roof window flashing kit explained page runs through all four kit families and why the match matters.
Pitch is the other slate trap. Suffolk terraces often carry natural slate at 25 to 30 degrees, which is shallow for a roof window. Most units are rated down to 15 or 20 degrees, so they are within range, but at low pitch driven rain gets further up under the covering. The flashing lap and the underfelt collar have to be right, because a shallow slate slope is where any corner-cutting shows up first.
What it means around Ipswich
Ipswich has a lot of natural slate, particularly on the Victorian and Edwardian terraces off Norwich Road, Spring Road and around the town-centre conservation area. Many of those roofs carry loft-conversion windows fitted in the 2000s that are now failing together. Replacing one is not just a window swap, it is slate work, and the terraces where these windows sit are also the ones most likely to need full scaffold to reach a second-floor slope. In the conservation area a recessed, flatter external profile is sometimes what gets approved, which changes the kit again.
None of this is a homeowner job. Slate work at height on a stripped roof is skilled and hazardous, and we would never suggest anyone climb up to attempt it. Where a slate roof window has failed, the answer is a proper replacement, not a patch, because on a unit old enough to be failing the timber and flashing beneath the slate have usually been wet for years too.
Related reading
For the full detail on working slate around a roof window, see our roof window installation on a slate roof knowledge page. For the general fit sequence, read how roof windows are installed, and for how long the job takes, roof window replacement time. All of it sits under our main VELUX window installation guide.
Get a fixed price
Slate work rewards a proper survey, because the pitch, the slate type and the access all move the figure. Send a photograph of the roof from the garden and one of the window from inside, and we can give you an early sense of the job. Then we survey, confirm the flashing kit and the slate match, and give you one written price, fixed, backed by a 10-year workmanship guarantee from installers with 25 years fitting roof windows.
Request a fixed quote for slate-roof roof window replacement in Ipswich, Suffolk or north Essex.
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.