Slates - no more grey areas

From Standards Extra 43

For many years slate roofs have been acknowledged as the premier roof covering for pitched roofs but in recent times there have been concerns about the durability and appearance of certain slates.

The change from the British Standard, BS 680 to European Standard BS EN 12326-1 and -2 has resulted in a need for the user or specifier to specify what level of performance and durability they want. Whereas BS 680 gave a simple pass or fail for several tests and two quality 'grades' BS EN 12326 does not. Because of the wide range of qualities of slate which are acceptable throughout Europe, each with different durability, it provides a set of levels of performance against a series of tests. The user can and should choose what level of performance they want for each test.

To address this issue NHBC has been working with the industry to establish what should be the minimum specification to meet with NHBC Standards to ensure the product has acceptable durability.

Under BS EN 12326 consignments of slates should now have an 'accompanying commercial document (ACD)' from the producer/supplier indicating the slate's performance. An example of the full ACD, containing an explanation on the information it should provide, can be view at

Below is an example of the first page of the ACD together with the level of test results that NHBC now looks for to meet our Standards.

Accompanying commercial document - Information requirements

Producer - Sufficient details are required to identify the producer/supplier.

Number of this commercial document - This can be a number issued by the test house or the producer/supplier.

Date of issue - The ACD should be relevant for the batch of slates being supplied. Retesting and updating of ACD's should be carried out at least once a year and more frequently where large volumes of slates are being extracted and for every new source or vein of rock extracted.

Commercial document issued by - this can be the test house or the producer/supplier.

Location of mine or quarry - required for traceability of slate. 

Date of sampling & date of testing - If more than one date is applicable to sampling and testing they should be indicated against the individual test results. The sampling and testing dates should be as close as possible to the 'Date of issue' of the ACD. The 'Date of issue' should be used to establish when further sampling and retesting is required at the frequencies set out in NHBC Standards.

Product description & commercial name - Name or description given to slate by the producer/supplier.

Dimensional tolerances - These are to be within those set down by the Standard.

Note: Slates, which fall within the tolerances, still require proper selection and fixing on site to achieve an acceptable finished roof covering.

Nominal thickness - Thickness of the roofing slates, as declared by manufacturer and the amount by which individual thickness varies as a percentage of the nominal thickness.

Characteristic modulus of rupture (MoR) - a measure of the slate's mechanical strength both along and across the slate. Slates from weaker rocks may need to be supplied thicker than those from stronger rocks to achieve a satisfactory performance in use. The Standard provides equations, which are used to establish the minimum slate thickness. The minimum thickness is calculated using the CMoR and a National (i.e. UK) factor, which takes into account the weather conditions of the country of use.

It is the producer's / supplier's responsibility to ensure all the slates in a consignment are at least as thick as the minimum value obtained using the UK factor.

Water absorption - For compliance with NHBC Standards the water absorption should be no greater than 0.6% i.e. A1 code.

Freeze thaw - This test is not required when slates have a 'Water absorption' no greater than 0.6%

Thermal cycle - This is a test for reactive metallic minerals within the slate, which may cause staining, pitting and/or delamination and potential failure of the slate. For compliance with NHBC Standards a slate should achieve a T1 code.

Carbonate content - High carbonate content can cause colour changes in the slate and potential for accelerated erosion and break down. For compliance with NHBC Standards a slate should achieve a S1 code.

Note: It is acknowledged that some slates, that do not achieve a S1 code, have a history of satisfactory performance for at least 60 years in the UK. If such a slate is being considered please discuss its acceptance with NHBC Technical in the first instance.

Non-carbonate carbon content - to meet the standard this should be less than 2%.

Exposure to fire, reaction to fire & release of dangerous substances - Roofing slates meet these requirements.

An abbreviated version of the ACD, as shown in BS EN 12326 should also be attached to each crate of slates.

Slate:Accompanying commercial document