Spanish Slate

Spanish slate is natural roofing slate quarried in north-west Spain. Spain is the world's largest producer and exporter of natural slate and supplies roughly 90% of the roofing slate used in Europe.

Where it comes from

Most production is in Galicia, with the larger companies concentrated in the Valdeorras area of Ourense province. Other workings are at Quiroga, Ortigueira and Mondoñedo. The deposits are Palaeozoic, more than 500 million years old.

Properties

Spanish slates are usually dark blue-black or grey. Good-quality material has a water absorption below about 0.4%, which gives strong frost resistance. Because quality varies from bed to bed and quarry to quarry, buyers rely on test results to EN 12326, the European product standard for roofing slate. Key classifications are:

• A1 water absorption (0.6% or less by mass);
• S1 in the sulphur-dioxide test, linked to low carbonate content;
• T1 in the thermal-cycle test, meaning any oxidation of metallic minerals such as pyrite causes only minor colour change without runs of staining.

Slates graded T2 or T3 may develop rust spots or staining, and UK industry commentators have warned that lower-grade imported material has caused disappointment on new roofs.

Use

Spanish slate is widely used on new housing and re-roofing in the UK, Ireland, France and other parts of Europe. On the European continent it is commonly fixed with metal hooks rather than nails; in the UK both methods are used. It is produced in many standard rectangular sizes as well as cut shapes for decorative patterns.

Comparison with other slates

Where an original local slate is no longer available, Spanish slate is often considered as a substitute. Heritage guidance generally advises matching the colour, texture and thickness of the original as closely as possible.