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Testing stone slates

Testing metamorphic slates

Currently (July 2004) there is no formal system like a British Standard for testing stone slates. A set of tests are under development by the Stone Roofing Association, English Heritage and Sheffield Hallam University but this is at the stage of gathering data on the temperature and humidity conditions which stone slates experience on the roof. Surprisngly there isn't any information readily available on this.
In the absence of 'formally approved' tests any new stone should be assessed by a geologist experienced in building stones and their durability. By new stone we mean one which is from a location with no history of use. Obviously this is a bit difficult to define because a stone could come from quite close to a known stone in a particular quarry but have completely different properties. If the geologist's assessment is favourable then properties such as strength and frost resistance will be high on the list for investigation.
It is also wise to allow any stone which is from a brand new source to weather outside for at least one Winter when any problems will have a chance of coming to light before they get on a roof.
Of course the best 'test' of a stone slate is experience in use and any well established products shouldn't need to be formally tested if the have not failed in the past. Just one cautionary point though. Stone slates were traditionally hand made (and should still be) which implies careful handling. The use of modern or mechanical techniques might damage or weaken a piece of stone.
One thing that shouldn't be done with stone slates, that is non-metamorphic slates, is to test them to BS680 Roofing Slates or BSEN12326-2 Slate and stone products for discontinuous roofing and cladding - test methods. These standards are specifically designed to test the modes of failure of metamorphic slates and are not relevant to sedimentary sandstones and limestones. The scope of the European Standard BSEN12326-1 makes this very clear -
'This European Standard does not apply to products for roofing or external cladding made from the following:
a) stone other than those defined in 3.1, 3.2 and 3.3;
b) concrete;
c) polymeric materials;
d) fibre reinforced cement;
e) metal;
f) clay.

3.1 slate (commercial definition)
rock which is easily split into thin sheets along a plane of cleavage resulting from a schistosity flux caused by
very low or low grade metamorphism due to tectonic compression

NOTE 1 It is distinguished from sedimentary (stone) slate, which invariably splits along a bedding or sedimentation

NOTE 2 Slate originates from clayey sedimentary rocks and belongs petrographically to a range which begins at the
boundary between sedimentary and metamorphic formations and ends at the epizonal-metamorphic phyllite formations.

3.2 roofing slate
rock used for roofing and external cladding, in which phyllosilicates are the predominant and most important
components and exhibiting a prominent slatey cleavage

3.3 roofing carbonate slate
rock used for roofing and external cladding, containing phyllosilicates and a minimum carbonate content of
20% and exhibiting a prominent slatey cleavage'

Sandstone and limestone slates do not have a slatey cleavage.
Anyone selling stone slates on the basis of testing to either of these standards does not understand their products and has not demonstrated their durability.