Flashing kits by roof covering

How the kit works

Four elements. An apron at the bottom, carrying water from the window onto the covering below. Soakers up each side, interleaved with the covering course by course. A head flashing at the top, taking water from the covering above around the window. And an underlay collar, connecting the roofing membrane to the window frame so anything that gets past the covering is carried away.

The underlay collar is the one nobody sees and the one that saves you when driving rain gets under a tile.

Flat coverings: slate and plain tile

Both lie flat, so a flat profile kit sits tight to them. Slate is the most forgiving covering to detail into. Plain clay tile is nearly as good, though the tiles are small so more of them need cutting.

Profiled coverings: pantile and interlocking

Concrete interlocking tiles and clay pantiles have a roll, so the covering surface is not flat. These need a deep profile kit, with sections shaped to bridge the roll of each tile.

Fit a flat kit to a profiled tile and there is a gap under every roll. It will not leak on the day. It leaks in the first driving rain from the wrong direction, which on the Essex coast is a matter of weeks.

Combination kits

Where two or more windows sit side by side, a combination kit weathers the space between them properly. Two single kits butted together is not the same thing and it will leak in the gap.

Low pitch kits

Below about 15 degrees a standard kit is outside its working range. Manufacturers make low pitch versions with deeper laps for down to around 10 degrees. Below that a roof window is the wrong product and a kerb mounted rooflight is the right one.

Locally

Colchester’s older stock is largely clay plain tile and pantile. The post-war and later estates are almost all concrete interlocking. That means a deep profile kit is the more common requirement here, and it is exactly the one people economise on.

Why there is no universal kit

A flashing kit has to seal the junction between a rectangular frame and a roof surface that is not flat. The shape of that surface differs entirely between a slate, an interlocking tile and a pantile, so the flashing that seals against it has to differ too.

Kits are supplied matched to the covering and they are not interchangeable. Using the wrong profile is the most common installation fault we find on other people’s work, and it is invisible once the covering is back.

That invisibility is exactly why it happens. The kit is a separate order line, it costs money, and nobody can see afterwards which one was fitted.

Flat profile: slate

Slate is flat, so the flashing can lie directly against it. The arrangement is an apron at the sill, soakers or side sections interleaving with the courses up each side, and a head flashing at the top with the slates above lapping over it.

Slate is the most cooperative covering in this trade. Individual slates can be lifted and replaced without disturbing their neighbours, and the pitches are usually steep enough that gravity does much of the weathering work.

One thing to know: a replacement slate in the middle of a slope cannot be nailed, because you cannot reach the head. It is held with a slate hook or a tingle, and a visible metal tingle is what a proper repair looks like rather than a shortcut.

Deep profile: concrete and clay interlocking tiles

The covering on most housing built from the 1930s onward, and the one where using the wrong kit does the most damage.

An interlocking tile has a raised profile with a channel running down it. A deep profile flashing is formed to dress into and over those channels, sealing against the tile shape rather than lying on top of it.

A flat kit fitted to a profiled tile leaves open voids under every channel where it crosses the tile. Water finds them within a season, and because it enters below the flashing it appears inside as a leak that seems to come from nowhere and defeats attempts to trace it.

Manufacturers make different depths for different tile profiles, so it is not simply flat against deep: the specific tile matters.

If you take one thing from this guide, it is to ask which kit is specified for your covering, by name and part number. A quotation that cannot answer that has not been written from a survey.

Plain clay tiles

Small tiles laid in double lap, so every point on the roof has two thicknesses over it. Common on older property across this area.

The detail generally uses soakers interleaved with the tile courses rather than a single formed apron, which is more work than fitting a proprietary kit to an interlocking tile.

The practical difficulties are about the tiles rather than the flashing. They have to be lifted individually over an area larger than the opening, because the double lap means you cannot simply remove a rectangle. Old clay is brittle and a proportion breaks whatever care is taken. And matching a weathered handmade tile is genuinely difficult.

The usual answer is to take sound tiles from a rear or hidden slope, use those around the new opening, and put the replacements where they cannot be seen.

Pantiles

The S shaped profile, common in parts of East Anglia and on agricultural buildings across this county.

They need their own deep profile, formed to the specific shape, and they are unforgiving of a universal kit. The interlock is a single overlap rather than the double lap of plain tiles, so a gap under a flashing is a direct route into the roof.

Pantile roofs are also frequently at shallower pitches than plain tile roofs, which means less water sheds under gravity and more of the weathering falls on the flashing. That combination makes the correct profile more important here than almost anywhere.

Metal, fibre cement and other coverings

Profiled metal sheet, on agricultural and light industrial buildings, needs a purpose made detail matched to the sheet profile.

Fibre cement slates look like slate from the ground and behave differently: more brittle at the edges, and matching a weathered one is harder than people expect. A flat kit is generally right and the handling is not.

Corrugated fibre cement sheet, on older garages and outbuildings, may contain asbestos. Where we suspect it, work stops and it is tested rather than guessed at.

Thatch is a specialist trade with its own detailing, and it is not one we work in.

The two components that are not the flashing

The underlay collar. Every tiled or slated roof has a membrane beneath the covering, and some water always gets past tiles in driving rain. That water runs down the membrane, and if the membrane simply stops at the opening it funnels straight in.

The collar is dressed and lapped so the membrane above drains onto it, it carries the water around the opening, and it discharges onto the membrane below. Get the laps the wrong way round and the roof delivers water into the opening rather than past it.

Invisible once the covering is back, and where a great many leaks originate.

The insulation collar. Nothing to do with weathering and fitted at the same stage. It closes the gap between the frame and the structure, and without it the reveal runs cold enough for room air to condense on it.

The sequence

Underlay collar first, dressed and lapped. Then the flashing kit. Then the covering reinstated over the flashing, never under it.

Water runs down the tiles onto the flashing and off it. Reverse that and it runs under the flashing and into the roof. Reversing the sequence is a guaranteed leak and it happens more often than it should on rushed work.

Roofs with no underlay

Common on any pre war property that has never been stripped: tiles or slates hung on riven battens with the rafters visible from inside.

Modern weathering assumes a membrane to lap into. Where there is none, the detail is built around the opening instead. Entirely workable, more time, and it should be in the quotation rather than discovered on the day. On a period property in this area it should be anticipated.

Recessed and combination kits

Recessed kits set the window lower into the roof plane, reducing how far it stands proud. On a property where profile matters, for planning reasons or simply for appearance, that is worth asking about.

Combination kits join two or more windows side by side or one above another under a common flashing. They are purpose designed and not something to improvise: the junction between two units is the hardest detail on the roof and water arrives there from both sides.

On a replacement, always fit a new one

Reusing a flashing that has been on the roof for the same twenty five years as the window that just failed is a false economy, and on an exposed roof it is where the next leak comes from.

The kit is size specific in any case, so a replacement of a different size needs a new one regardless.

Where a quotation is noticeably cheaper than the others on a replacement, this is one of the first places to check.

Slate

Flat profile kit. Slate is thin, flat and regular, so the flashing sits tight against it with no voids underneath.

Soakers interleave with the courses up each side, an apron at the sill, a head flashing at the top with the slates above lapping over it.

Replacement slates in the field of a slope cannot be nailed, because their heads are under the courses above. They are held with slate hooks or tingles, and a visible metal tingle is what a correct repair looks like rather than a bodge.

Plain clay tile

Flat profile kit again. Small, flat, double lapped tiles lie close to the batten line and the fine coursing means cuts fall in manageable places.

The practical difficulty is matching. Old plain clay weathers to a range of colours with lichen, and new tiles beside them are unmistakable. Take sound tiles from a hidden slope for use around the rooflight and put new ones where nobody looks.

Pantile and profiled interlocking tile

Deep profile kit, and this is where the commonest error is made.

A flat kit cannot follow the tile form. It bridges the rolls and leaves the troughs as open channels running straight under the flashing, and driven rain finds them.

The deep profile apron is formed to dress down into the troughs and over the rolls.

Cut tiles lose part of their interlock, so they need mechanical fixing and support from the flashing rather than the engagement that was removed.

Metal and unusual coverings

Standing seam, corrugated sheet and single ply on a low pitch all need their own detail rather than a standard kit, and frequently a bespoke flashing made by the covering manufacturer.

Where no kit suits, the answer is lead dressed on site by somebody who can do it. That is a leadworking skill, it takes time, and it should be priced rather than substituted with a kit that nearly fits.

The variants worth knowing about

Combination kits weather two or more units as one continuous system, with a fixed gap between openings that has to be set out before carpentry starts.

Recessed kits set the window lower into the roof plane, which on slate and plain tile works particularly well and can make a proposal acceptable on a designated building.

What sits below all of them

The underlay collar, dressed and lapped so water on the membrane passes around the opening. Collar first, then flashing, then covering over the flashing.