1. Roof Spread
It was common throughout the 1970s & 80s for dilapidated slate roofs to be replaced with heavy interlocking concrete tiles. Large concrete tiles weigh considerably more than slates and unless additional supports are provided the rafters will bend under the weight and in extreme cases place additional pressure of the tops of the walls in a defect referred to by surveyors as ‘roof spread’.
2. Nail Rot
Slate coverings to older roofs were secured with steel nails which are susceptible to corrosion (known as nail rot or nail sickness). Warm air rising from within a property reaches the underside of the roof covering, cools and deposits moisture on the underside of the slates saturating the nail heads.
3. Build up of Moss
Although a covering of moss appears harmless it can cause surface water to back up and eventually penetrate through the roof covering. It may also lead to the rainwater goods becoming blocked as it eventually dislodges and falls away. Moss will also retain moisture and may lead to:
4. Frost Damage
Surveyors often report ‘unevenness’ or ‘delamination’ to small plain clay roofs; the root cause of such problems is likely to be frost. Some types of clay tile are more porous than others and as a result absorb moisture easily – either in the form of condensation rising from with a property or rain falling from above. When a wet tile is exposed to sub-zero temperatures the moisture within expands causing the nibs to become brittle or forcing the surface of the tile to spall (break away). The nibs of concrete tiles tend to be more resistant to frost but they can also suffer spalling to the surface if the protective layer has been washed away.
5. Cracked Cement Fillets
Cement fillet joints are a poor alternative to lead flashings. By its nature such a joint is stiff so any slight movement will cause it to crack and allow moisture to penetrate.