The longevity of a roof covering is generally governed by the quality of the source materials used, the age of the slate/tile and the degree of exposure.
Old tiles and slates can absorb water which then freezes and expands. White powdery salts under the tiles can indicate erosion and the projecting nibs at the top of each tile can break.
The need for complete re-covering is rare; a well laid roof should survive for 40 – 50 years before substantial works are required. Surveyors can sometimes be guilty of erring on the side of caution, so get a second opinion before commissioning a complete re-roof. Most issues can usually be overcome with the aid of localised repairs rather than a complete re-roof however there will always come a time, unfortunately, when a series of patch repairs is no longer the correct way forward.
It’s worth noting that some roofers may be tempted to utilise inferior, cheaper materials when recladding older roofs.
Things you should be aware of: