Domestic Roofing Guide - Common Faults with Old Roofs

The life of a roof is finite; one should, therefore, acknowledge the fact that at some point in time it is liable to fail.

The first signs of deterioration in the roof covering are usually indicated by the presence of broken and delaminated slates/ tiles or daylight showing through to the loft area. Cracks and eventual breakage are caused by the effects of weather and movement within the roof structure, which increase as the fixings decay.

Delamination occurs with slates and clay tiles, which are of a laminated structure. Water, which is absorbed into the slate or tile during the winter, can freeze and expand, causing the layers to separate and flake off.

A roof, which has an uneven surface, gives a reasonable indication that the roof timbers and battens are sagging. This may be due to decay in the timbers caused by water ingress or settlement within the building structure. The end result is a general disturbance of the roof covering, producing strain on the fixings and breaking of mortar joints.

Slates or tiles, which have slipped, indicate deterioration in the fixings, which may be nails or wooden pegs. If slipping has occurred in several places on the roof, it must be assumed that all the fixings are in an advanced stage of decay.

On closer examination, it may be found that the bedding mortar used at the ridges, verges and hips is cracked and decomposing allowing movement and water penetration to take place. Metal flashings and valleys may also require replacement as erosion and fatigue can produce hairline cracks allowing water to enter, which is mostly caused by incorrect installation.

Mortar fillet flashings are cheap and were common before the 1940ís, but they are generally less effective than metal due to their durability and inability to accommodate minor thermal and structural movement, which causes cracking. They are also susceptible to frost attack, which also results in the cracking of the mortar fillet.

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