Rooflights in Clay Tile Roofs

Two families, not one covering

Everything sold as a clay roof tile falls into one of two families, and the distinction is not cosmetic.

Plain tiles are small, flat or very slightly cambered rectangles, laid in short courses with a large amount of overlap. They cover a roof by sheer repetition, and a plain tile slope has a fine, close-grained appearance made of many small units. Pantiles are larger, formed into an S-section in profile, and laid with a single overlap and a side interlock. A pantile roof has a strong wave running across the slope and far fewer units doing the same job.

Between them sit interlocking clay tiles of various proprietary profiles, some flat, some Roman, some closer to a pantile, and each is its own case again. Handmade and machine-made versions of the same nominal tile also differ, the handmade ones being more variable in camber and outline.

The first question at any survey on a clay roof is therefore not what size unit you want. It is which of these is actually on the building, and whether it is a shape a manufacturer makes a flashing for.

Plain tiles are double lapped

Double lapping is the idea most people have never had explained, and it governs everything about an opening in a plain tile roof.

Each tile in a course does not simply overlap the course below. It overlaps the two courses below it, so that at any point on the slope there are two or three thicknesses of tile stacked over each other. The reason is the joint. Plain tiles are butted side by side with an open gap between them, and that gap would be a direct route into the roof were it not covered by the tile two courses further up. The tile immediately below covers nothing useful; the tile two below is the one doing the work. Waterproofing here is a stacking arrangement, not a seal.

Two consequences follow. The gauge is short, so the coursing is fine and there are many course lines to a slope. And any cut into the roof interrupts three thicknesses rather than one, so the edge of an opening exposes the ends of courses that were relying on tiles which are no longer there.

That is why a flashing designed for a single-lap covering cannot simply be dropped onto a plain tile roof. It expects one thickness of tile and one plane of contact. It gets three thicknesses and a stepped edge, sits proud, and leaves the cut ends of the intermediate courses doing a job they were never asked to do. The lead or the formed kit has to be interleaved into the courses, not laid over them.

Choosing a kit by size alone

Flashing kits are ordered by unit code and by covering type, and the covering type is where the mistake happens. Somebody selects the right unit for the opening, sees a choice between a flat covering kit and a profiled covering kit, picks whichever sounds closest, and the parts turn up on the day.

What arrives is either a slate-type kit with a shallow skirt that will not sit into a pantile’s corrugation, or a deep-flute profiled kit set for a wave the plain tile roof does not have, standing the whole perimeter off the covering. The unit itself fits the hole perfectly. The unit was never the problem. The perimeter does not weather, and it does not fail immediately, which is worse, because it fails in the first spell of wind-driven rain some weeks later when everybody has left.

The manufacturers publish profile-specific kits precisely because approximation does not work here. There are separate arrangements for plain tile, for pantile, for particular interlocking profiles and for low-pitch versions of the same, and they exist because each one meets a different surface. Choosing correctly means identifying the tile on the roof, its profile, the pitch and the gauge, and ordering against all of it.

This is settled at survey, at the same moment the unit is chosen, and it goes on the order together. Discovering it on the scaffold means either waiting for the right parts with the roof open or making the perimeter by hand, and only one of those is a good outcome.

Pantiles and the S-profile

A pantile is a large tile rolled into an S in cross-section, so that laid up a slope it produces an alternating pattern of raised rolls and low pans running down the roof. It laps the course below once, and it laps its neighbour sideways over the roll, which is what makes the side joint work without a second thickness of tile.

Water behaves accordingly. It collects in the pans and runs down them in defined channels rather than spreading across the surface, which is efficient and which also means the roof has clear routes that must not be blocked or diverted at an opening. A flashing that dams a pan sends water sideways under the tiles either side of it.

Setting out therefore has a horizontal constraint that neither slate nor plain tile has. The sides of an opening land somewhere in that repeating wave, and where they land changes the detail entirely. An edge cut down through a pan leaves an open channel arriving at the frame. An edge that follows a roll leaves a high point with a clean line to work against. Given the choice, the opening is positioned so its sides fall on whole tiles at the roll, and the internal reveal takes up any difference.

The tiles themselves are also less forgiving than they look. Older pantiles vary in the depth of the roll and in twist, so two nominally identical tiles can sit differently on the same batten.

Flashing a corrugated surface

The apron below a rooflight on a pantile roof is the detail that decides whether the installation is any good, because it has to leave the flat face of the frame and marry into a wave.

Done properly, it is dressed down into every pan and over every roll, following the profile closely enough that there is no gap under it for wind to drive water into, and no unsupported lead spanning a hollow where it will fatigue and split. Lead will do this because it can be worked cold and it stretches where a bossing tool tells it to. The proprietary equivalent is a formed skirt with flutes matched to the profile, which is why the profile has to be right rather than approximately right.

The sides need their own answer, because the cut has broken the side interlock that made the tile joint work. Depending on the profile and the pitch that means soakers worked in course by course, a secret gutter formed alongside the frame, or the manufacturer’s side sections used as designed. What it never means is a bead of something run along the junction.

The head is the more conventional part, taking water arriving from the slope above and carrying it out to either side, though it too has to terminate onto a corrugated surface rather than a flat one. On steep pantile roofs the head can be relatively straightforward. On shallow ones, where water moves slowly and wind has more purchase, it is the detail that wants the most attention.

Weathered clay and new tiles

Clay gets its colour from the clay body and from the firing, and both vary. Tiles from the same kiln differ from one another, more so where the firing was less even, which is why an old clay roof reads as a mottled surface of oranges, browns, reds and near-blacks rather than a single flat tone. That variation is a large part of why these roofs look the way they do.

Then decades of weather work on it. Lichen takes hold on the damp and shaded parts, moss gathers where a valley or an abutment holds water, soot and dust soften the face, and the exposed slopes fade differently from the sheltered ones. The result is specific to the roof and effectively impossible to buy.

A new tile of the same nominal pattern is even in colour, clean, and bright against all of that. It does not weather in quickly and it does not disappear. Set a run of new tiles around a rooflight and the opening acquires a pale halo which draws the eye straight to it, which is the exact opposite of the intention on a visible slope.

So the tiles taken off are the tiles that go back, sorted as they come down and set aside with the sound ones separated from the frost-damaged ones. Salvage makes up the difference, matched by pattern and by tone rather than by name, and any material that cannot be matched well goes to a slope out of view. Old handmade tiles are not interchangeable with machine-made ones even where the nominal size agrees, because the camber differs and they will not bed down together.

Where clay sits between the two

Set against the other two traditional coverings, clay occupies a middle position, and knowing which way it leans on any given roof is useful.

It is considerably more forgiving than stone slate. Clay tiles are a repeating module. The gauge is fixed, the courses do not change size as they climb, and the set-out for an opening can be worked out in advance and relied upon. There is no graded rhythm to re-establish afterwards and no sorting into size groups. The material is far lighter, so access and structure are ordinary questions rather than governing ones, and matching stock, while it takes care, is not the search that reclaimed limestone can be.

It is less predictable than Welsh slate, in three ways. The surface is not truly flat, since camber, twist and variation in the tiles mean a flashing meets a slightly restless plane rather than a smooth one. Old clay is porous and can be frost-damaged along its edges and around the nail holes, so a proportion of what comes off is not fit to go back. And there are two families with different rules, so the covering has to be identified correctly before anything is ordered rather than assumed from a photograph.

In practice that makes clay a covering where a well-surveyed job runs cleanly and a poorly-surveyed one goes wrong at the perimeter. The margin for a wrong assumption is narrower than it looks.

Concrete tile, the straightforward relative

Concrete interlocking tile deserves a mention here because so much of the county’s post-war housing carries it, and because it is the easy case against which the others can be judged.

It is manufactured to close tolerance, so tiles are genuinely identical. It is single lapped with a strong side interlock. Its profiles are a short and well-known list, and every rooflight manufacturer makes a flashing formed for those exact profiles, which means the kit fits as drawn instead of being persuaded. The roofs it sits on are usually trussed at regular centres with underlay beneath the battens and a pitch that does not vary along a street, so the structural questions are answered by looking rather than by investigation.

Colour is pigmented through the tile and fades with exposure, so a new tile is brighter than an old one and takes time to settle. On a plain estate roof that matters less than it would on a period slope, and the tiles taken off still go back wherever they can.

The honest summary is that a rooflight in a concrete tile roof is ordinary work, done well by anybody competent, and priced accordingly. A rooflight in an old clay roof is not the same job, and the difference is not in the glass. We install VELUX, Fakro, Keylite and Roto rooflights into all of these coverings, and the covering is established before the kit is ordered rather than after.