Guide · Roof Window Installation
Rooflights in concrete interlocking tile roofs
Concrete interlocking tile is the most common roof covering in Essex, by a wide margin. Every estate built from the 1950s onwards is under it, most of the interwar semis were re-covered in it during the 1970s and 1980s, and the majority of enquiries that come in with a photograph attached are a photograph of one. It is also the covering that makes the biggest difference to how a rooflight is weathered, because the tiles are deep.
Depth is the whole story here. A concrete interlocking tile has a rolled or troughed profile that can stand 30mm or more above the plane of the batten, and a flashing designed for a flat covering simply bridges across the troughs and leaves an open channel straight into the roof. This page covers how the profile is read, how the tiles are cut and re-laid around an opening, and what a survey measures before anything is ordered. It sits under roof window installation.
What an interlocking tile actually is
A plain tile sheds water by being laid three deep, so that every joint has solid tile under it two courses down. An interlocking tile does something different. It carries a moulded rib and channel along each long edge, and the tile beside it locks into that channel, so the side joint is weathered by geometry instead of by overlap.
That single change means the tile can be laid single lap, one course overlapping the next rather than two. Fewer tiles, less weight per square metre in some patterns, faster laying and a lower minimum pitch. It also means a cut tile at the edge of an opening may have lost the very interlock that was weathering its joint, which is where the detailing comes in.
The profile depth, and why it dictates the kit
Manufacturers class their flashing kits by profile depth, and the classes are not interchangeable. The common split is a flat or low profile kit for coverings up to around 16mm deep, and a deep profile kit for anything from there up to about 45mm or 55mm depending on the maker.
The number that matters is measured from the top of the batten line to the crown of the tile, not from the underside. Getting it wrong in either direction causes trouble. Too shallow and the apron cannot reach down into the trough. Too deep and the flashing stands off the tile and lets wind driven rain travel under it. This is one of the very few figures on a job where a tape measure at survey removes all of the argument.
Reading which tile you have from the ground
Most concrete tiles can be identified from a decent photograph taken square on from the garden, and the survey confirms it up close. A single deep roll along one edge with a flat pan is one family. A double roll with a shallow pan is another. A flat-faced tile with a small interlock at the edge, often described as a plain-tile look, is a third and takes a different kit again.
The tell that catches people out is the flat-faced interlocking tile. It looks like a low profile covering from the ground, and it is often thicker at the interlock than it appears. Measuring rather than assuming is the only reliable method.
Redland, Marley and the common Essex patterns
The coverings that come up again and again in this county are the deep rolled tiles that dominated post-war housing, the double roman patterns on the 1930s re-coverings, and the flatter granular-faced tiles used across the 1980s and 1990s estates at Great Notley, South Woodham Ferrers and the Harlow and Basildon expansions.
Two things follow. First, the tile is usually still available or has a documented successor, so matching is realistic. Second, the flashing manufacturers publish which of their kits suits which pattern, so once the tile is named the kit stops being a judgement call and becomes a lookup.
Cutting a concrete tile without breaking the course
Concrete tile cuts cleanly with a disc cutter and it cuts badly with anything else. The cut is made off the roof, with water suppression, and the cut edge is placed so it is covered by the flashing rather than left in view.
The practical difficulty is not the cutting, it is what a cut leaves behind. A tile cut down its length has lost either its interlock or its nib, and sometimes both. A tile that has lost its nib needs mechanical fixing. A tile that has lost its interlock needs the flashing to weather that edge instead. Neither is difficult, but both need doing rather than hoping.
The tile-and-a-half, and the bond at the opening
Interlocking tiles are laid in a broken bond, with the joints of one course sitting over the middle of the tiles below. Cut an opening into that and the courses either side arrive at the frame at different points across the tile width, which looks wrong and, more importantly, can leave a joint sitting directly against the flashing upstand.
Most patterns are made in a tile-and-a-half width for exactly this reason. Introducing one at the right course shifts the bond so that joints land clear of the opening. It is a small ordering detail that is easy to sort at survey and awkward to sort with the roof open.
Batten gauge, and how it fixes the window height
The batten gauge is the spacing between tiling battens, and it is set by the tile length, the required headlap and the pitch. On a typical deep profile tile the gauge sits somewhere in the region of 320mm to 345mm, and it is fixed across the whole slope.
The consequence is that a roof window cannot be placed at an arbitrary height. It is set so the courses land cleanly against the head and the sill flashings, which usually means the vertical position moves by up to a course from wherever it was drawn. Where the internal position is critical, because a wall or a beam is in the way, that constraint is worth raising early rather than discovering it on the day.
On an interlocking roof the tile gauge, not the drawing, has the final say on where the window sits.
Setting the window so the courses land cleanly
The setting out starts from the existing courses and works towards the window, not the other way round. Once the frame height is fixed, the tiles below the sill are cut to sit under the apron with the right clearance, and the course above the head is set so the flashing tucks under it with its full lap.
A common shortcut is to set the window first and then force the tiles to fit. That produces a very thin cut course somewhere, and thin cut concrete tiles break, slip or lift. Anything less than about a third of a tile is a weak course and is better avoided by moving the window a fraction.
Where the deep profile lets water travel sideways
Water on a flat covering runs down the line of steepest descent. Water on a deep profile tile runs down the troughs, and it can also be pushed across the crowns by wind. At the side of a rooflight, that means the flashing has to be dressed down into each trough so the water it collects is delivered into the channel rather than left sitting on a rib.
This is why the deep profile aprons are made from a pliable material with a deep skirt: they are formed by hand into the shape of the tile in front of them. A rigid flashing that spans the profile is the single most common failure seen on concrete tile roofs, and it usually shows itself in the second or third winter.
Fixing brackets and the depth of the frame
Roof windows are fixed to the rafters or trimmers through brackets, and the brackets have alternative positions to suit how deep the frame sits relative to the covering. On a deep profile roof the frame usually sits in the upper position, so the flashing works from the right plane.
Set it too low and the head flashing has too much to make up. Set it too high and the unit stands proud of the slope, which affects both the look and, on a permitted development job, the 150mm projection limit that keeps the work out of a planning application.
Weathering the head on a deep profile
The head is the hardest part of a deep profile detail, because water is arriving from the whole slope above and has to be taken across the top of the unit and out to both sides without ponding. The kit supplies a head flashing with a formed gutter to do this, and it works when the tile above it is cut and seated so it holds the flashing down flat.
Where a course lands badly, the temptation is to trim the flashing. That is the wrong end of the problem. Moving the course, or introducing a cut course above the window, keeps the flashing intact and keeps the drainage geometry the manufacturer designed. The detailed treatment of the individual components is covered in soakers, aprons and head flashings.
Clips, nails and the wind fixing schedule
Modern British practice fixes far more tiles than older practice did, and every tile in the perimeter zone around an opening counts as a perimeter tile. In practice that means the tiles immediately around a rooflight are clipped or nailed even where the general field of the roof is not.
Cut tiles need particular attention because they are lighter and have lost part of their interlock. A cut tile beside a flashing that is neither clipped nor mechanically held is the tile that ends up in the garden after a February gale, and the resulting open joint is directly beside the unit.
Interwar semis and post-war estates across Essex
The county gives you two quite different concrete tile jobs. The interwar semis around Chelmsford, Brentwood, Rayleigh and Hornchurch mostly have cut roofs with rafters and purlins, so an opening can be trimmed conventionally and the width is a carpentry question.
The post-war and later estates are trussed, and there the width is set by the spacing of the trusses rather than by the tile. That usually caps a single unit at around 550mm wide unless an engineer designs a solution, which is covered in detail on rooflights in a trussed roof. The covering may look identical from the road; what is holding it up is not.
Colour matching a tile that has weathered thirty years
Concrete tiles change appearance as they age. The pigment in the surface layer weathers, moss and lichen take hold on the north facing slopes, and a tile that started as a fairly bright red settles into something browner and duller.
A new tile of the same reference will therefore not match. Around a rooflight this rarely matters much, because the tiles closest to the unit are largely covered by the flashing, and the honest approach is to re-lay the original tiles wherever they survive the strip and keep new ones for the positions that are hidden. Where a whole course has to be new, taking sound tiles from a less visible slope and putting the new ones there is the usual answer.
Mortar bedded verges and hips near the opening
Where an opening sits close to a verge or a hip, the bedding becomes part of the job. Older verges are bedded in mortar, which after thirty years is often loose and hollow, and disturbing the courses nearby can bring a length of it away.
The same applies at a hip, where the hip tiles are bedded over a hip iron and the courses run in at an angle. A window placed within a tile or two of either detail asks for more re-laying than one placed in open field, and it is a reason to shift a unit sideways at the design stage if the internal position allows it.
What the survey measures before the kit is ordered
Six things, and they take about twenty minutes. The tile pattern and its profile depth. The batten gauge and where the courses currently land. The pitch, taken with a level rather than estimated. What is under the covering, rafters or trusses, and at what spacing. The condition of the underlay across the working area. And access, because a rear slope over a conservatory is a different day’s work from a front slope off a driveway.
Those six answers turn a range into a fixed price and turn a flashing kit into a part number. They also settle whether the job is one unit or, quite often on a trussed roof, two units side by side. Sunspire quotes a range by email once the basics are known, then a fixed written figure after that survey.
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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