Guide · Roof Window Installation
Rooflights in plain clay tile roofs
A plain clay tile roof is the oldest and fussiest covering a rooflight goes into, and it is also the one where a good job is most obvious from the ground. The tiles are small, they are laid three deep, and the whole surface has a slight movement to it that a modern covering does not. Cut into that badly and the eye picks it up immediately, even from the far side of the road.
Essex has a lot of it. The Victorian and Edwardian terraces in Chelmsford, Colchester and Maldon, the older cottages in Thaxted, Coggeshall and Saffron Walden, and the interwar suburbs where plain tile was the standard before concrete took over. This page covers what makes double lap tiling behave differently at an opening, how the tiles are cut and re-laid, and where an old brittle roof changes the method. It sits under roof window installation.
Double lap, and why plain tiling behaves differently
A plain tile has no interlock. It weathers the joint between two tiles by putting a solid tile over it in the course above, which means every point on the roof has at least two thicknesses of tile over it and the joints have three. That is double lap tiling, and it is the reason a plain tile roof is heavy and the reason it is forgiving.
At an opening the consequence is direct. Because the covering is layered rather than jointed, the flashing can be dressed low and tight into the courses and the tiles above will simply lie over it. There is no deep trough to bridge and no rib to sit on. The trade-off is that far more tiles are disturbed for the same size of opening, because each one is small and each one is holding down the two below it.
The gauge of a plain tile roof
Plain tiles are commonly 265mm by 165mm, and with the usual 65mm headlap the batten gauge works out around 100mm. That is a course of tiles every four inches up the slope, which is why a roof of this kind uses roughly sixty tiles to the square metre.
For a rooflight it means the vertical setting out is fine-grained and forgiving. Where a concrete interlocking roof forces the window up or down by a whole 330mm course, a plain tile roof gives you a position every 100mm. It is one of the few respects in which the fussier covering is the easier one to work into.
Hand-made, machine-made and the tolerance between them
Machine-made plain tiles are regular, flat-faced and consistent in size. Hand-made tiles, which is what you find on the older village stock across north Essex, vary in length, thickness and camber, and they are usually laid with a more open, textured surface as a result.
That variation changes the work. Around a rooflight the courses have to be brought in level and true against the frame, and with hand-made tiles that means selecting and gauging rather than simply laying. It is slower, it is skilled, and it is the reason a plain tile job carries more labour than the same opening in a modern covering.
| Tile type | Where it turns up in Essex | What it means at an opening |
|---|---|---|
| Machine-made clay | Interwar suburbs, 1920s to 1950s | Consistent gauge, straightforward cutting |
| Hand-made clay | Thaxted, Coggeshall, Saffron Walden, older cottages | Selection and gauging, slower re-laying |
| Reclaimed clay | Sympathetic re-coverings anywhere | Condition varies tile by tile, order surplus |
| Concrete plain tile | 1960s to 1980s re-coverings | Heavier, more brittle at the nib, same lap |
Why the flashing sits low and tight
The flashing kit for plain tiling is the shallowest one the manufacturers make. The apron has a short skirt, the side pieces are thin and flat, and the whole assembly is designed to disappear under the courses rather than sit on top of them.
It works because the covering is flat. Water arriving at the side of the unit runs onto a soaker or a side flashing and straight back onto the tile below, with no channel to find. The individual pieces and how they overlap are covered on soakers, aprons and head flashings; what matters here is that a deep profile kit fitted to a plain tile roof stands proud, looks wrong and collects debris.
Cutting plain tiles around an opening
Clay cuts differently from concrete. It is harder, more brittle and more inclined to chip at the edge, and a tile that has been cut badly shows a pale broken face against a weathered surface.
Cuts are made off the roof with a disc and water, and the cut edge is always placed into the joint or under the flashing, never left exposed to view. Where a raking cut is needed at a corner, the tile is scored and cut in one pass rather than nibbled. A supply of spares matters more than on any other covering, because a proportion of clay tiles will break during the strip regardless of how carefully they are handled.
Tile-and-a-half units and keeping the bond
Plain tiles are laid half-bond, with each joint centred over the tile below. At a verge, a hip or the side of a rooflight, that bond has to be maintained or the joints stack up in a line, and a stacked joint is a direct route for water through two courses.
The solution is the tile-and-a-half, a wider tile made in the same pattern that resets the bond at the edge. Alternate courses take one. It is a standard component, it is available in most patterns, and using cut halves instead is the shortcut that produces a line of thin, unfixable slivers against the frame.
A line of half tiles down the side of a rooflight is the mark of a covering that was refitted rather than re-laid.
Nibs, nails and how a plain tile is held
Each plain tile has one or two nibs moulded on the back that hook over the batten, and holes for two nails. The general field of an old roof is often nibbed only, with nailing every fourth or fifth course, which was the practice for a very long time.
Around an opening the rules tighten. Perimeter tiles are nailed, cut tiles are nailed or clipped because a cut has usually taken a nib with it, and the tiles under the head flashing are fixed so they cannot creep. On an old roof where the nail holes are already worn oval, that sometimes means a tile is better replaced than re-hung.
Camber and cross camber, and the reason tiles are not flat
Plain tiles are made with a slight longitudinal camber, and hand-made tiles usually have a cross camber too. It is deliberate. The curve means the tiles bear on each other at the edges rather than lying dead flat, which keeps a small air gap open, sheds water off the tail and stops capillary action drawing water up between the layers.
At a rooflight that camber has to be respected. Bedding a course down flat to make it sit neatly against a flashing closes the gap and can draw water sideways. The courses are laid to their natural bearing and the flashing is dressed to them, not the reverse.
Sourcing matching tiles for an Essex roof
Clay weathers to a colour that new clay does not have, and on a Victorian roof the difference between a hundred year old tile and a new one of the same pattern is obvious. There are three routes, and which one applies depends on the roof.
Re-lay the originals wherever they survive the strip, which is the default and usually accounts for most of the tiles. Take sound weathered tiles from a hidden slope or the back of a chimney stack and put new ones there instead. Or source reclaimed tiles of the right size and character. On a prominent front slope in a conservation area, the third route is often the only acceptable one.
Reclaimed tiles, and what to check before using them
Reclaimed clay is not automatically good clay. A tile that has spent a century on a north facing slope in a damp position may be laminating at the tail, and one that has been frost damaged rings dull rather than sharp when tapped.
The checks are quick: sound the tile, look at the tail for delamination, check the nibs are intact, and hold it against the existing covering for size, because plain tile dimensions varied by maker before standardisation. Ordering a generous surplus is sensible, since a working yield of well under a hundred per cent is normal and the shortfall always appears at the worst moment.
The head of the opening on a double lap roof
At the head, water from the whole slope above arrives at the top of the unit. On plain tiling the head flashing is taken up under two courses rather than one, because the covering itself is two deep, and the course immediately above the flashing is usually cut to length so it seats properly.
That extra course of overlap is what makes a plain tile head detail quietly reliable. It also means the strip has to go a little higher up the slope than the flashing itself suggests, which is worth knowing when somebody asks why more of the roof came off than expected.
Setting out from the eaves course
Plain tiling starts with a doubled course at the eaves, using a short under-eaves tile, and every course above is gauged from there. When an opening is formed part way up a slope, the setting out for the new courses is taken from the existing gauge above and below rather than measured fresh.
Where an old roof has drifted, and many have by ten or fifteen millimetres over a full slope, the drift is absorbed over several courses away from the window rather than corrected in one jump beside it. Correcting at the frame leaves a visibly tapered course right where the eye is drawn.
Moss, lichen and a roof that has aged into place
Old clay carries a biological surface, and on the shaded slopes of the Blackwater villages and the wooded parts of mid Essex it can be substantial. It is part of how the roof reads, and stripping it off around an opening leaves a clean rectangle that will take years to blend back.
The working approach is to disturb as little as possible: lift the tiles, keep them the right way up, keep them in order, and put them back where they came from. Around the flashing itself, where the tiles are cut anyway, some loss of surface is unavoidable and it weathers in within a couple of seasons.
Where a conservation frame changes the detail
On a plain tile roof in a protected setting, the unit itself is usually a conservation rooflight, with a slim cast frame, a central glazing bar and a flush profile that sits within the plane of the covering rather than above it.
That flush setting changes the work at the perimeter. The frame is set lower, the soakers are dressed closer, and the tiles are cut to a tighter line because there is no proud upstand to hide behind. It is more exacting and it is the reason those units take longer to fit than a standard roof window in the same opening.
Working on a brittle old plain tile roof
A hundred year old clay roof has a limited number of times it can be walked on. Crawl boards, roof ladders and a strip carried out from a scaffold rather than from the tiles are the ordinary countermeasures, and they are why access appears as a real line in a quote for this kind of covering.
It also affects sequencing. Everything that involves being on the covering is done in one visit rather than three, the opening is formed and weathered the same day, and the roof is closed and watertight before anyone leaves. A plain tile roof left open is a great deal of small loose components in the wind.
What the finished opening should look like from the garden
Courses that line through on both sides of the unit. A flashing that reads as a thin dark line rather than a raised frame. No stacked joints and no slivers. Tiles that match the rest of the slope in colour and surface, with any new ones placed where they are least visible.
That is the whole test, and it is worth applying before the scaffold comes down. If the job passes it from the garden it will almost certainly pass the more important test, which is being dry in ten winters. The perimeter is where the workmanship lives, which is why it is the workmanship that carries the ten year guarantee.
Keep reading
More on roof window installation
Rooflights in a trussed roof and what cannot be cut
Read the guide →Soakers, aprons and head flashings explained
Read the guide →Rooflight reveals and why splaying them matters
Read the guide →Two rooflights side by side on one slope
Read the guide →Rooflights on a shallow pitch below 15 degrees
Read the guide →
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