Rooflights in concrete interlocking tile roofs

The deep profile kit

The important point. These tiles have a roll, so the covering surface is not flat, and the flashing has to bridge that roll at every course.

A flat profile kit on a profiled tile leaves a gap under every single roll. It looks acceptable when finished, it does not leak on the day, and it leaks in the first driving rain arriving from the wrong direction.

This is the most common defect we are called to correct, and it is entirely avoidable.

The tile module

Interlocking tiles are large, so openings tend to align to the tile module rather than to any dimension you choose. That is worth knowing when positioning a unit: shifting the opening 100mm may mean cutting far more tiles than shifting it 300mm.

Standard rooflight sizes and common tile modules are broadly compatible, which is not a coincidence.

Cutting

A cut interlocking tile loses its interlock, which is the thing making it weathertight. Cut tiles are supported and lapped so they do not depend on it.

Concrete tiles cut cleanly and are less brittle than old clay, so breakages are fewer.

Matching

Concrete tiles fade with age, so replacements are visibly newer for a few years before they weather in. On a visible slope worth mentioning.

Weight

Concrete tiles are heavy, and roofs carrying them are structured for it. Forming an opening removes structure that was carrying real load, so trimming is done properly rather than approximately.

The commonest roof we work on

Concrete interlocking tiles cover most housing built in this country from the 1930s onward. They are heavy, profiled and durable, and they take a rooflight very well provided one thing is right.

The deep flashing kit

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

A flat kit, designed for smooth slate, leaves open voids under every channel where it crosses the tile. Water gets under them within a season, and because it enters below the flashing it appears inside as a leak that seems to come from nowhere.

This is the most common installation fault we find on other people’s work in this part of Essex, it is invisible once the job is finished, and it is exactly where a cheap quotation saves money because the kit is a separate order line.

Ask any quoting installer which kit is specified for your covering by name and part number.

Working with the tiles

Concrete interlocking tiles are among the easier coverings to work with. They are robust, they lift cleanly, and the work goes faster than on a fragile clay roof.

On a roof of fifty or sixty years some tiles will be at the end of their life and a proportion will break on removal. Matching a discontinued profile and a weathered colour is not always possible, and where an exact match cannot be found the answer is to take sound tiles from a rear or hidden slope and put the replacements there.

We allow for a small number of replacements on any roof of that age rather than treating it as a surprise.

Pitch, which usually goes with the covering

Roofs with these tiles typically run between 25 and 35 degrees. That is comfortably above any manufacturer’s minimum, and it has two consequences worth knowing.

Water sheds more slowly than on a steep slate roof, so more of the weathering job falls on the flashing kit rather than on gravity. And the reveal is longer and shallower, which blocks a surprising amount of the light the window collects.

Splaying the reveal, cutting it back at an angle at head and sill rather than boxing it square, opens the beam out and makes a modest unit feel considerably larger. It costs a little more in plastering and it is the best value decision available on a roof of this pitch.

The structure below

Houses with this covering span the change from traditional cut roofs to trussed rafters, which happened around the 1970s. That distinction decides more about the price than the choice of window does, and a photograph looking up inside the loft answers it without a visit.

The covering most modern estates have

Large format concrete interlocking tiles, usually with a pronounced profile, laid single lap. They are the standard covering on post war and modern housing across the area.

Two characteristics matter for a rooflight. The profile has real depth, so a unit stands proud of the covering more than it would on slate. And the tiles interlock, so removing one is not like lifting a slate.

The flashing profile

A deep profile kit, not a flat one. This is the most common error on this covering.

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

The deep profile kit is formed to be dressed down into the troughs and over the rolls, so there is no continuous void.

Cutting interlocking tiles

Interlocking tiles lose part of their interlock when cut, so cut tiles at the opening need mechanical fixing and support from the flashing rather than the engagement that has been removed.

Where a cut would leave a narrow sliver, the correct move is to widen it and use a broader piece, or adjust the opening position slightly so the cuts fall better. That is a decision made on the roof.

The structural point specific to these roofs

Concrete tiles are heavy, and roofs carrying them are frequently trussed rafter roofs at 600mm centres.

A truss cannot be cut without a designed repair. So a unit narrow enough to sit between two trusses, touching neither, avoids the structural question entirely, and going taller rather than wider is the way to get more glass without an engineer.

Matching is easier than on clay: concrete tiles are current products and replacements are obtainable, though new ones will be lighter in colour until they weather.