Much of the slate quarried in Brazil comes from ‘sedimentary mudstone’*. At the time of writing (Spring 2013), there is some debate over the future of this type of Brazilian roofing slate.
In Summer 2012, the National House Building Council (NHBC) published a technical bulletin reminding members that any roofing slate (including that sourced from Brazil) which does not comply with BS EN 12326, will not be warrantied or insured by themselves. In February of 2013, the National Federation of Roofing Contractors (NFRC) published a warning to its members advising that Brazilian grey slate was found to be delaminating on several properties. This bulletin went on to say that Green slate from Brazil appeared not to be affected.
The current European Standard for roofing slate (BS EN 12326) applies only to slate that has been manufactured from metamorphic rock (rock that has been formed through the application of heat and pressure). It does not allow for the testing of slates manufactured from sedimentary rocks (rock formed through the application of pressure alone).
The result of these statements has ignited furious debate, the results which are confusing to all. There are clearly very good and very bad slates manufactured in every country.
There are those who point towards the high-quality Brazilian roofing slate that has been used successfully and have withstood the test of time. Others point to those suppliers who have chosen to sell cheap, low-quality Brazilian roof slates which delaminate and are not fit for purpose.
Whilst the debate for Brazilian roofing slate is likely to continue, the responsibility for solving this issue lies with the quarries and testing authorities. Between them, they must agree to the implementation of a stringent quality test that will confirm which Brazilian slate roof tiles are suitable for use.
If they fail to grab this issue, Brazilian roof slates will continue to gain a reputation for concern.
*Mudstone: A sedimentary rock that has been created through intense geological pressure being applied to sedimentary deposits. **Metamorphic Slate: A hard mudstone that has been subjected to metamorphism (intense geological pressure and geothermal heating of between 200-250degrees centigrade).