Testing to the extremes
Ken Clark, Technical Development Centre Manager at Marley Eternit, explains the different product standards that affect European roof tiles and discusses whether minimum standards are really enough to meet the UK’s challenging weather conditions
From CE marking to BS EN 490, 491, 492 and 539, as well as BS5534 and BS8000-6 - all of these standards affect different aspects of roof tile manufacture and installation and all play a vital role in controlling the quality of both products and workmanship.
The implementation of the Construction Products Regulations (CPR) and mandatory CE marking last summer has gone a long way towards ensuring consistency of quality and product performance within the EU. Essentially the CPR means that any product manufactured to a European standard must be CE marked and accompanied by a declaration of performance about the product’s physical characteristics.
There are European standards for concrete, clay roof tiles and fibre cement slates, which came into effect in the mid 1990s, replacing the old British Standards. BS EN 490 and BS EN 491 cover the performance of concrete tiles and fittings in key areas such as strength, impermeability to rainwater and freeze thaw resistance. BS EN 539 determines the frost resistance of clay roof tiles and fittings, with products tested to agreed levels for each country. While BS EN 492 covers fibre cement slates and fittings including product specification and test methods to determine qualities such as strength.
In addition, there are British standards affecting the installation of roof tiles, including BS8000-6, covering workmanship on building sites and BS5534, the code of practice for slating and tiling, which is due to be revised this summer to take account of more extreme UK weather conditions.
For those contractors using products imported from outside of the EU, it is important to be aware that each country may have its own individual product standards for clay and concrete roof tiles. However, only standards prefixed by the BS and EN reference have been drafted to meet the UK requirements. There are no ISO or other internationally accepted standards that cover roof tiles or other roofing products.
The European standards ensure that roofing products meet a minimum quality, strength and performance standard that has been agreed at a level for the UK but does this go far enough?
At Marley Eternit, we don’t believe that meeting just the minimum requirements is testing our products to an acceptable level. For example, our ISO 9001 quality system has strength requirements which far exceed BS EN 490. In some circumstances, we believe these higher strength tolerances are required to ensure customers do not experience excessive breakage or handling problems that could come from just mere compliance with minimum strength levels. In fact, in the majority of cases for our concrete tiles, we seek strengths which are 25% higher than the BS EN 490 standard.
When it comes to product testing, one of our main areas of focus is the resistance to UK weather conditions and long term product performance. Rather than simply testing our products for ordinary levels of rainfall or wind, our roof tiles are subjected to wind tunnel testing for driving rain and deluge conditions equivalent to a once in 50 year extreme weather event, following the test methods stipulated in CEN/TR 15601: 2012. Whilst most roof manufacturers test their product to the same extreme weather conditions, they have different pass or fail levels. At Marley Eternit, we are particularly conservative and have set the level high, so it is much harder for our roof tiles to pass.
In our experience the tests required in standards, such as BS EN 491, are not sufficient for freeze-thaw resistance in UK conditions. The UK doesn’t have particularly cold winters in terms of the typical minimum temperature but does have a lot of night time frosts followed by warmer spells during the day, with conditions frequently cycling between freezing and thawing. This means that water trapped in pores near the surface of a roof tile expands as it freezes and the cycles of this place stress on the material, leading to fractures and loss of the surface layer if it is not frost durable.
As a result, not only do we test tiles to meet the relevant product standards but also to more onerous test conditions in accordance with recommendations from RILEM’s technical committee (the international union of laboratories and experts on construction materials). These tests maximise the severity of freeze thaw conditions, for instance by using salt solutions, more extreme temperature conditions (+/- 30°C) and longer test sequences. Performance is also monitored on natural weathering exposure sites to verify correlation between accelerated tests and real life performance.
Frost resistance of clay tiles is one area that needs particular care. The challenging weather conditions in the UK mean that clay has to be very well fired to form a strong and frost resistance matrix. Until recently, a UK and North West Europe specific test method, Method D of BS EN 539 was used. However, this has now been replaced by a pan-European Method E but with the UK in the highest performance category, meaning that the tiles must pass 150 cycles of the test to demonstrate compliance.
This change was carefully checked by the Roof Tile Alliance (RTA)and provides confidence that RTA members’ clay tile durability is fully assured. However, contractors need to take care with imported products that have not been subject to such rigorous assessment.
When it comes to deciding on a product’s quality, performance and suitability for the UK climate, as a minimum roofing contractors should always look for products that have CE marking or BS EN standards. However, as the UK looks set to encounter more extreme weather and variances in temperature over the next 25 years, our experience as a manufacturer and our understanding of the stresses placed on roof tiles, means we believe that just meeting minimum testing standards is not good enough. We will continue to push our products to the extreme with the tolerances we expect in strength, impermeability to rain water and freeze-thaw resistance.