Page 7 – British Standard
Standard specification and details
The traditional British Standard for roofing slate was set out in BS680: Part2: 1971. There never used to be any agreed international or even European standard for natural slate. This changed, however, in July 2004 when BS EN 12326-1 and -2 were published. These superseded and replaced the old British Standard.
BS680 was very straight forward and relatively simple to understand. Slates would need to be put through three different tests in order to be approved: 1 – Water Absorption. 2 – Wetting and Drying. 3 – Sulphuric Acid Immersion. Test 1 had a maximum value over which a slate would fail. Tests 2 and 3 had a simple pass or fail result. The outcome of theses tests gave us two levels of conformity. Those which had passed all three tests and a lower level of conformity for those slates which failed on the acid immersion test but had a proven, long durability based on many years of use in areas of little or no acid pollution. Unfortunately many foreign imported slates claimed compliance under the lesser conformity without the proven life expectancy in British conditions and this gave rise to the introduction of the new European Standard. The thickness of imported slate was also an issue. Welsh slate is particularly strong and durable and therefore traditional thicknesses of slate produced in Britain are relatively thin. Some imported slate made to British thicknesses were simply not strong enough and would break under wind loads and conditions experienced in the UK. There was no specific standard in relation to the strength of a slate other than the grain having to run north to south. The new Standard rectifies this by requiring slates to be a specific thickness in relation to their strength.
BS EN 12326 is far more complex and detailed in both its testing and its classification of slate. The new standard includes requirements for the following properties:
- Origin and petrography
- Bending strength and grain
- Water absorption
- Freeze-thaw resistance
- Thermal cycle test
- Carbonate content
- Sulphur dioxide exposure test
- Non-carbonate carbon content
- Dimensional variation
For the durability tests (water absorption, freeze-thaw, thermal cycle, carbonate content and sulphur dioxide exposure) slates are graded depending on how well they perform in each instance.
Key grades to look out for:
- Water Absorption – the more water a slate absorbs the sooner it is likely to fail. In the UK a slate should have a maximum water absorption of 0.6% or an A1 grade. Note: NHBC require slates to the A1 grade for use on NHBC registered sites.
- Freeze-Thaw – this test is not required for slates with a water absorption rating of less than 0.6% (A1 grade).
- Thermal Cycle – the better the thermal cycle grade the less likely a slate is to discolour due to the presence of metals and minerals within the slate itself. Always look for a slate with a T1 rating. Although T2 slates can be used in the UK it is not recommended as they will soon give an unsightly appearance and have a reduced lifespan. T3 slates are also permitted but only in conjunction with a waterproof membrane due to the possibility of holes appearing where impurities are severe. These slates should really be avoided at all cost unless solely for some cheap decorative cladding. Note: NHBC require slates to the T1 grade for use on NHBC registered sites.
- Carbonate Content/Sulphur Dioxide Exposure – the better the grade the stronger the slate. This is the general rule of thumb. Slates with a high grade in this category are both stronger, more durable and can be cut thinner. Using an S1 graded slate in this category ensures you have chosen a slate suitable for use in all exposures. S2 and S3 slates are prone to softening and weathering problems. Note: NHBC require slates to the S1 grade for use on NHBC registered sites.