Pitched roofs are traditionally covered with slates or tiles. Slate rock is formed when successive layers of mud are metamorphosed under heat and pressure to form solid rock, although the layers result in a laminar structure which enables blocks of stone to be cleft into the sheets which are more familiar as roofing slates. Most of the roofing slates in use in the British Isles were quarried in North Wales, although slates of less uniform but perhaps more attractive colour and texture are also available in the Lake District and in Cornwall, where they are used locally. Most roof slates for new construction and repair are obtained from less expensive sources in Spain; most of the Spanish slate is similar in appearance to Welsh slate but it is usually less durable. The long-term durability of slate depends mainly on the nature of the bonding between adjacent laminations, some slate delaminating quite rapidly in polluted urban atmospheres; durability is usually assessed using an acid resistance test. It is a matter of great concern that, when individual slates commence to slip from a roof due to nail corrosion, it is often recommended that the entire roof covering should be replaced, whereas the correct solution to the problem is to carefully remove and refix the original slates which have proved their virtually infinite durability, a replacement covering of any other material being probably less durable, even if it is new slate. Recommendations for complete replacement are usually prompted by the greater labour cost that is involved in carefully removing slates for refixing, although some roofing firms will obviously make more profit from supplying new covering, and some may be particularly attracted by the possibility of both supplying new covering and making extra profit by selling the removed slates which are now very valuable!