Industry Standards

All natural slates are required to conform to the European Standard BS EN 12326: 2004, which replaced the British Standard BS 680 in 2004.


The standard is designed to enable the specifier or end user to compare the key properties of natural slates to ensure a product fit for purpose.

The main criteria tested are as follows:

Thermal Cycle

This is a test for reactive metallic minerals within the slate, which may cause oxidisation pitting and/or delamination and potential failure of the slate. The test involves submerging a number of slate pieces in water and drying them out repeatedly over a period of up to 3 weeks. The samples are inspected and observations are classified as follows:

  • T1: No changes in appearance. Surface oxidation of metallic minerals. Colour changes that neither affect the structure nor form runs of discolouration.
    Conformity to the Standard: Acceptable
  • T2: oxidisation or appearance changes of the metallic inclusions which may develop into runs of discolouration but without structural change.
    Conformity to the Standard: Acceptable
  • T3: oxidation or appearance changes which penetrate the slate and risk the formation of holes.
    Conformity to the Standard: Acceptable, however these slates should not be used as the primary means of weather proofing. Slates showing exfoliation, splitting or other structural changes in this test are not acceptable.

Stoneleaf Slates do not supply T3 slates.

Sulphur Dioxide Exposure

This test examines the ability of the slate to resist atmospheric pollutants. All slates with less than 20% carbonate content are placed in a sealed container and subjected to sulphur dioxide before their surface is scraped to measure any softening. Results are classified and where necessary a thickness adjustment applied as follows:

  • S1: no change
  • S2: the slate must be split 5% thicker
  • S3: the slate must be at least 8mm thick

Slates containing more than 20% carbonate content are not suitable for roofing or cladding.

All Stoneleaf Slates are classified as S1.

Water Absorption

The slate is immersed in water to test it's vulnerability to frost damage. If the slates absorbs 0.6% of it's mass or less in water it is classified as A1 which is the highest grade and requires no further testing. If the test obtains a result over 0.6% a separate freeze-thaw test is required.

All Stoneleaf Slates are classified as A1.

For information call the technical department 01277 841 555