Slate roofs and flashing kits explained

When a roof window sits in a pitched roof, there is a gap all the way round it between the window frame and the roof covering. The flashing kit is the set of pre-formed pieces that bridge that gap and shed water down and away from the opening. It is not a single item. A kit is made up of an apron at the bottom that lies over the covering below the window, side sections that run up each edge, and a top or head section that tucks under the covering above, so that water running down the roof is carried around the frame the way a stone in a stream splits the current.

The pieces overlap in the direction of flow, always upper over lower, so that gravity does the sealing rather than the mastic. A good flashing kit relies on lap and geometry, not on a bead of sealant that will harden and shrink in a few summers. That is the single most important idea in the whole subject. Water is managed by shape and overlap. Anything held out only by a smear of silicone is a detail waiting to fail.

The kits are made specifically for the window they surround, sized to the exact frame dimensions, and they are made in different versions for different roof coverings. This is where slate parts company from tile, and where the choice of kit stops being a formality and starts mattering.

Slate is flat. A natural slate is a thin sheet, roughly four to nine millimetres thick, laid in overlapping courses so that the finished surface sits very close to the plane of the roof. A profiled concrete or clay tile, by contrast, has a deep wave or interlock and stands proud of the roof by anything up to a hundred and twenty millimetres. Those are two completely different surfaces to seal against, and they need flashing pieces of a completely different shape.

A flashing kit made for profiled tiles has deep, corrugated side skirts designed to dress down into the troughs of the tile pattern. Lay that same kit against flat slate and the corrugations have nothing to grip. There are gaps under the skirt where the profile expects a tile trough and finds flat stone instead, and those gaps are exactly where wind-driven rain gets in. The reverse is just as bad. A flat slate kit laid against deep interlocking tiles cannot dress down into the troughs and leaves the metalwork bridging over voids.

Slate also has almost no forgiveness in it. A tile can be nudged, twisted a few degrees, or lifted and re-hung to close a gap. Slate is nailed twice through pre-holed heads and laid to a fixed gauge, and once a course is down it is down. The flashing has to be planned into the coursing before the slates go back, not squeezed in afterwards. That is why replacing a window in a slate roof is a covering job as much as a glazing one, and why the slating around the opening comes off and goes back in a controlled sequence.

Most roof-window flashings on the UK market follow the coding system that the largest manufacturer set, and the trade tends to speak in those codes whether the window is that brand or another. You will hear an installer or a merchant ask which flashing you need before they will even price the job, and the answer is a short group of letters. It is worth understanding what they mean, because ordering the wrong one is the commonest way a slate installation goes wrong on paper before anyone even gets on the roof.

The two that matter on a slate roof are the standard slate flashing and the recessed slate flashing. Both seal against the same flat covering. The difference is depth. The standard version sets the window so its frame sits at the normal height above the roof, which is the straightforward choice and the one most slate roofs get. The recessed version drops the whole unit lower into the rafters so the glass sits almost flush with the slate, giving a much flatter, quieter line across the roof. On a prominent slate elevation, particularly one that is seen from the street or the garden, that flush recessed line is often worth the extra depth of work in the roof structure to achieve it.

The combined flashings matter when you are putting more than one window in. Two or three roof windows in a row look far better, and seal far more reliably, tied together under a single run of flashing with a shaped gutter piece between them than they do as separate kits fighting each other for the same slate course. If a design calls for a bank of windows in a slate slope, the combined flashing is planned in from the start.

Open a slate flashing kit and you get a small stack of pre-formed and folded pieces plus a set of gaskets and trims. Each has a job.

• The apron. The base piece, a wide aluminium skirt that lies over the slate immediately below the window and carries the whole flow off the bottom of the frame onto the roof. It is dressed down onto the slate so it follows the surface.
• The side sections. Two lengths that run up the left and right edges of the frame, lapping over the slates beside the window and turning side water away from the joint. On a slate kit these are relatively shallow because the slate they sit on is flat.
• The head section and cover. The top piece, which tucks up under the slate courses above the window so that water coming down from higher on the roof passes behind the frame and out to the sides, never reaching the top of the glass.
• The underfelt collar or drainage gutter. A pliable piece that connects the roof underlay, the membrane beneath the slate, to the window frame, so that any water getting past the outer covering, and a little always does over the life of a roof, is caught on the felt layer and drained back out below the window rather than into the roof space.
• The foam and brush gaskets. Compressible seals that close the small gaps between the metalwork and the covering against wind-driven rain and blown snow, without relying on rigid sealant.

The underfelt collar is the piece most people never see and the one that quietly saves the ceiling below. A roof is a two-line defence. The slate is the first line and the underlay is the second, and the flashing kit has to tie into both. A kit fitted so that the apron is perfect but the collar is skipped or bodged will look faultless for a year and then show a damp stain the first time a storm drives water under a slate. Every slate flashing we install is dressed into the underlay layer as well as the covering, because that second line is not optional.

Slate has to be laid steep enough to throw water off before it works back up under the laps, and that minimum pitch is steeper than tile. The flashing kit and the roof window both have their own workable pitch ranges too, and the three have to agree. Here are the figures to hold in your head, as general industry guidance rather than a rule for any one roof.

• Natural slate, standard sizes: generally laid no shallower than about 25 to 30 degrees, with a head lap of 75 to 100mm.
• Natural slate, large formats: larger slates with an increased lap can go down towards 20 degrees on an exposed roof.
• Fibre-cement slate: often rated down to around 15 to 20 degrees depending on the product and lap.
• Roof windows and their flashing kits: typically work across roughly 15 to 90 degrees, so the covering is almost always the limiting factor, not the window.

The practical point is that if a slate roof is pitched steeply enough to be watertight in the first place, it is comfortably within the range the window and flashing can handle. The problems come at the shallow end. A slate roof laid close to its minimum pitch is already working hard to stay dry, and dropping a window into it concentrates the flow and demands that the flashing and the underlay collar are detailed properly, because there is little margin left in the covering itself. On the very shallowest slate roofs a standard kit may not be enough and the opening has to be planned with extra lap and a wider apron zone. That is a survey decision, taken on the roof with the pitch measured, not guessed from the ground.

Head lap matters as much as pitch. The slates around the opening have to keep their full head lap right up to the flashing, which means the coursing above and below the window has to be set out so that a joint between two slates never lands where it would channel water straight at the frame. Setting that out correctly is the difference between a slate roof that looks as though the window was always meant to be there and one where the courses run into the frame at awkward angles.

South Essex has a great deal of slate. The Victorian and Edwardian terraces and villas along the Southend cliffs, through Leigh, Westcliff and Clifftown, were roofed overwhelmingly in natural Welsh slate, brought in by rail once the line reached the coast. Much of that slate, or its like-for-like replacement, is still up there. A roof window going into one of those roofs is going into genuine natural slate laid to traditional laps, and it wants a proper slate flashing, dressed and coursed by someone who has handled slate rather than only tile.

There is a planning point that bites specifically on slate roofs in this area, and it catches people out. The Leigh Cliff and Leigh conservation areas carry an Article 4 Direction that removes the normal permitted-development rights for changes to roofing materials and windows. On most houses a conforming rooflight is permitted development and needs no application. Inside those Article 4 zones it is not, and a planning application is required for work that would be waved through a few streets away. The Clifftown Victorian estate and the Leigh Old Town fishing-cottage core are conservation areas as well, where the appearance of the roof from the street is taken seriously.

This is exactly where a recessed slate flashing earns its keep, because a window sitting flush and low in the slate reads far more sympathetically on a historic roof than one standing proud of it. It is also why we make the Building Control notification, and flag the planning position, as part of the job. New and enlarged rooflights are notifiable under Building Regulations regardless, and inside an Article 4 area the planning side has to be squared away before anyone lifts a slate. Always treat the permitted-development position as something to confirm with Southend-on-Sea City Council for your own address rather than assumed, because the line between a permitted rooflight and one that needs consent runs right through these streets. If your project is a new opening in a pitched slate slope, our pitched roof skylight installation work covers the whole sequence, from setting out the coursing to notifying Building Control.

Slate is dark, and a dark roof runs hot in the sun. A south-facing slate slope on a Southend terrace, with the open estuary horizon and no shade above it, can reach a surface temperature far above the air on a clear summer afternoon. People reasonably worry that the roof itself is cooking the room below. The slate does get hot, but the heat that actually reaches you through a window arrives as sunlight through the glass, not conducted through the stone, and that is the part the specification controls.

This is where the choice that matters is the glass, not the covering. A roof window in a south-facing slate slope is a solar collector aimed almost square at the summer sun, and the number that decides how much of that solar energy ends up as heat in the room is the G-value of the glazing. Clear double glazing lets through around 0.6, or sixty per cent, of the sun’s energy. A solar-control unit brings that down towards 0.3 or below, roughly halving the heat load, while keeping most of the daylight. On a south-facing slate roof over a loft bedroom or a converted top floor, that is the difference between a room that is usable through a July afternoon and one that is not. North-facing slate slopes have no direct sun to control and want clear glass and all the light they can get. The flashing keeps the water out. The solar-control glazing keeps the heat out. They are two separate decisions and a slate roof with slopes facing different ways may want different glass in each window.

You do not need to become a slater to make sure the flashing on your slate roof is specified properly. A few direct questions to whoever is quoting will tell you most of what you need to know.

1. “Is this a slate flashing kit, not a tile one?” The single most important question. A flat slate kit for a flat slate roof. If the answer wanders towards a profiled tile flashing, stop and ask why.
2. “Standard or recessed?” On a visible slate elevation, and especially in a conservation area, ask whether a recessed flashing to sit the window flush is possible. It changes how the roof looks for as long as it stands.
3. “How does the kit tie into the underlay?” You want to hear about the underfelt collar or drainage gutter, the second line of defence. If the answer is only about the outer metalwork, that is a gap in the plan.
4. “What is the pitch, and is my slate steep enough for it?” A measured pitch, taken on the roof, not an eyeballed guess. It sets the whole detail.
5. “What is the G-value of the glass on the south-facing slopes?” A real number should come back. Energy ratings weighted towards winter do not describe how a south-facing slate roof behaves in August.
6. “Who notifies Building Control, and have we checked the Article 4 position?” New rooflights are notifiable, and in the Leigh conservation areas the planning question has to be answered first. We handle both.

Slate rewards care and punishes shortcuts. The window is the easy part. The flashing, the coursing and the underlay collar around it are where a roof either stays dry for decades or starts staining a ceiling within a year, and on the older slate roofs of the Southend cliffs there is history in the covering worth respecting. If you are weighing a roof window for a slate slope and want it specified properly, covering and glass together, you can ask us for a specification, or read how the same care applies to a factory roof window installation in any pitched roof.