Interlocking or plain tiles: what is the difference?

A plain tile is small. It hangs from a batten by two nibs, or is nailed through holes, and the roofer lays it in staggered courses so that each tile sits over the joint between two tiles below. The covering is double-lapped: at any point on the slope there are two layers of tile, which is what keeps the rain out. Plain tiles are many to the square metre, and the finished slope has a fine, slightly rippled grain.

An interlocking tile is much larger. A groove or rebate along each side locks it to the next tile, and the headlap beneath the tile above gives the cover. Because each tile covers more of the roof, fewer are needed and the roof goes down faster. The result is a flatter, bolder surface with strong lines. Slate, pantiles and other coverings are different again, and the range is mapped out in our roof tile types guide.

Pitch is the biggest practical difference. Marley's published minimum pitches show it clearly. Its interlocking concrete tiles range from 12.5 degrees for the Mendip 12.5, through 15 degrees for the Wessex and 22.5 degrees for the Double Roman, up to 25 degrees for the Anglia. Its concrete plain tile needs 35 degrees, and its clay plain tiles need between 30 and 40 degrees depending on the range, with the Canterbury at 40.

So a shallow roof such as a porch, a bay or an extension can often take an interlocking tile but not a plain one. Figures change from one product to the next, so the data sheet for the tile in question has the final say. Our answer on the minimum roof tile pitch goes through how the angle is measured and what follows when a roof is lower than a tile allows.

An interlocking tile is bigger and heavier than a plain one, but plain tiles are laid in two layers, so the weight over a square metre of roof depends on the product. Do not guess. Compare the weights on the data sheets, because a heavy covering puts more load on the rafters, and the difference matters when you swap one type for another.

The fixing rules differ as well. Wienerberger insists that every tile with a single lap carries at least one mechanical fixing, in practice a clip or a nail, and the maker's data sets the pattern. Plain tiles are double-lapped, and the fixing pattern in BS 5534 and the tile maker's guidance is set out for them separately. Either way, fixings should follow the data for your tile and your location. Our guide to fixing roof tiles to BS 5534 covers the detail.

Plain tiles suit older houses, because the small scale looks right on Victorian and Edwardian roofs and on cottages. In Bognor's Steyne and Waterloo Square area, Arun's conservation document wants slate or plain clay reinstated wherever a modern material has taken over. Interlocking tiles read as plainer and more modern, and for Aldwick Road the same document records slate being swapped for concrete interlocking tiles, which it counts as a loss to the street.

Repair differs too. A single broken plain tile is small and easy to swap, but a patch shows if the colour does not match. An interlocking tile leaves a bigger gap, and because of its locking edge the tiles beside it must be eased out of the way. On lifespan, Marley expects concrete tiles to serve for around 60 years but says that is no guarantee, while for clay it passes on a Building Research Establishment minimum of 60 years. Treat both as typical figures, with the battens and felt beneath often giving up first.

Swapping plain tiles for interlocking, or the reverse, is a change of covering, which is a bigger step than a repair. A new tile can change the weight on the roof, the pitch it needs, the look from the street and the fixing pattern. According to the Planning Portal, a new covering that increases the load by 15 per cent or more brings the job under Building Regulations. It also advises talking to a structural engineer or surveyor if the new tile weighs a great deal more or less than the old.

In a conservation area, or on a listed building, check with Arun District Council's planning team before ordering, because a change of material can alter how the building looks. Our answer on heavier roof tiles and the regulations explains the weight test. Our survey looks at pitch, battens and fixings before any tile is recommended. Our pitched roofing page lists the tiled roof work we take on.