ASTM C406

ASTM C406, Standard Specification for Roofing Slate, sets material and physical requirements for natural slate used as roof shingles in North America. The active version listed by ASTM is C406/C406M-22.

What it covers

The specification deals with the characteristics, physical requirements and sampling procedures for selecting slate for roofing. It excludes slates containing soft carbonaceous "ribbons" (bands of different composition), because their properties vary widely and their service life is uncertain.

Grades

Roofing slate is classified into three grades:

• Grade S1 – the highest grade, intended for the longest service.
• Grade S2 – intermediate.
• Grade S3 – the lowest grade.

Each grade must meet requirements for:

• Breaking load (modulus of rupture) – how much force the slate can carry before snapping.
• Absorption – how much water the slate takes up. Lower absorption generally means better resistance to freeze-thaw damage.
• Depth of softening – a weathering test in which slate is exposed to acid to see how deeply its surface softens, simulating attack by polluted rain.

The specific numerical limits are in the standard itself. ASTM C406 itself does not state a service life for each grade.

Use

Architects specify a grade in project documents, and quarries test their product and state the grade on data sheets. Slate from the same quarry can vary between beds, so testing is done on representative samples.

Comparison with Europe

The European equivalent is EN 12326, which uses a different set of tests and declared codes rather than three grades. Imported European slate sold in North America is sometimes tested to both standards. The two systems do not convert directly.

Installation

ASTM C406 covers only the material. Slate roof installation in North America follows manufacturer and trade guidance, such as that of national roofing contractor and slate associations.