Modern interlocking concrete tiles

Modern interlocking concrete tiles spoil the appearance of a small lodge (top), while the ‘turnerised’ coating of hessian and bitumen (centre) not only looks awful but traps moisture and invites frost damage. A recent development, spray-on foam (above), may secure the tiles, but this too traps moisture and is extremely difficult to remove, making it unlikely that any of the original tiles on this roof will ever be reused.

Problems that can affect the tile surfaces, apart from those rare occasions when moss or lichen cause damage, are usually brought about by matters such as pollution, the premature failure of poor quality tiles, saturation from leaking pipes or drips from overhanging details such as TV aerials.

Frost damage can occur where moisture is retained on the surface and this sometimes happens at the laps. Sometimes localised frost damage can cause a tile to break at the head lap. Machine-made tiles are particularly prone to frost damage as the surfaces are more even and regular, enabling moisture to be trapped on the underside. Handmade tiles on the other hand have a natural variation which is both less moisture-retentive and more pleasing to the eye.

Other problems can arise due to poor laying in the first instance. Such problems include inappropriate detailing at verges and hips, poor setting and laying of the ridge and poor detailing of abutments such as chimneys and walls. Abutments need particular care. Local vernacular may dictate the use of tile creasings, or else mortar fillets, or lead. Lead flashings are usually the more reliable and mortar fillets the least. Whichever detail is used, lead soakers should always be incorporated between each tile to resist the passage of rainwater horizontally.

Another common failure with clay-tiled roofs is brought about by the failure of the fixings or battens due to rot or rusting. The battens often use sapwood, which is much more vulnerable to decay than heartwood. Pressure-treated battens should always be used for repair and replacement. If care is taken, many of the tiles themselves can usually be salvaged and reused. A word of warning, however: because peg tiles tend not to be pegged every course and therefore rely on friction and/or the torching, there is a risk of mass failure and slippage if a careless roofer steps onto the roof. Before attempting to repair a clay tile roof it is important to check the fixings below in case the attempt at repair itself causes more damage.

When repairing a tiled roof it is important to obtain as close a match as possible to the original in terms of texture and colouring. Non-ferrous fixings should be used to reduce corrosion risks. Any lime torching should be continued across new areas of work, and with the existing torching properly reinstated.

Most roofs can be satisfactorily patch-repaired rather than having to be completely stripped and re-covered. However, if complete re-covering is to take place, every attempt should be made to salvage the tiles and as a rule of thumb one would hope to salvage approximately 70 per cent.