Roof Slate Standards Protect Customers


1. ASTM C406-06 – Standard Specification for Roofing Slate

The American Society for Testing & Materials (ASTM - see was formed in 1898 but these days ASTM standards are truly international. ASTM C-406 is an extremely rigorous standard for roofing slate that has 3 grades – S1 (the top grade), S2 & S3. Slates that meet the S1 standard have an estimated durability of 75 years or more. Penrhyn, Pattini, Vermont Grey/Green slates and Glendyne slates are all S1 compliant.

The grades are based on physical tests:

  • flexure testing (ASTM C120-06e1)
  • water absorption (ASTM C121-06)
  • weather resistance (ASTM C217-94)
  • compressive strength (ASTM C170-90)

They also incorporate physical requirements:

  • minimum thickness 3/16" (4.76mm)
  • free from broken corners
  • curvature less than 1/8" (3.18mm)
  • no "knots" or "knurls" that affects the smoothness of split
  • freedom from "ribbons" i.e. rippled surfaces
  • less than 1% broken or cracked slates
  • maximum allowable deviation from face dimensions is 3mm from specification
  • slates to be square & corners cut to be right angles.

2. NF P32.301 & P32.302 - French Norm or Standard for Roofing Slate

These are French standards for roofing slate widely regarded as being the toughest most stringent standards for roofing slate in the world. See

This is because they only test slates randomly sampled by independent representatives of the testing house rather than having company selected slates sent for testing. In addition, sampling may take place at any time in any place. This means the test results are genuinely statistically significant. The standards incorporate all the tests outlined above.

It would be highly unlikely to achieve continual compliance with these standards unless a company had one hellishly good quality assurance program backing its products. The world’s best slates will all be NF: P32.301& P32.302 compliant.

3. BS680 Part 2 1971 – British Standards for Roofing Slate

These are superseded British standards that were applied to samples submitted (rather than randomly sampled) for the following tests:-

  • water absorption test
  • wetting & drying test
  • acid immersion test

This standard was criticized because it did not address iron pyrite content - some types of which are reactive and cause slate to rust out causing roof failures.

4. BS EN 12326-1 2004 – European Standards for Roofing Slate

This UK/European standard came into force in May, 2006 and replaces BS680 Part 2 1971. These standards were set by member countries of the EU and are a compromise and because of this some feel they are not particularly useful to the consumer, because the bar has allegedly been set "too low". However, any standard is better than none so our suggestion is to use it. More information on this standard is available from

Its contents include:

  • Symbols and abbreviations
  • Origin and petrography
  • Thickness
  • Bending strength
  • Grain Water absorption
  • Freeze–thaw resistance (only for code A2 from Table 2)
  • Thermal cycle test
  • Carbonate content
  • Sulphur dioxide exposure test
  • Non-carbonate carbon content
  • Defects
  • Dimensions
  • Release of dangerous substances
  • Number of slates required for type tests and factory production control
  • Evaluation of conformity
  • Testing frequency
  • Type of tests
  • Factory production control
  • Procedures in case of non-conformity and/or complaint
  • Marking, labelling and packaging