ISS categories’ s its slates into the three following criteria, “Ultra”, “Prime” and “Commercial/Standard”.
Its worth noting that all three classifications will have been tested and will confirm and surpass all the top UK Tests for water absorption, thermal cycle resistance and Sulphur dioxide exposure. Only slates advertised, classified and sold as”T2” will not conform to the UK tests and can potentially have a higher chance of oxidisation.
It is the physical selection process at the quarry, and the individual characteristics of the rock that determines which category individual slates fall. Due to lack of consistency & transparency across the market and with no set industry definition, ISS wanted to offer a more detailed description of what to expect when selecting and working with a slate, from each category.
Everyone is searching for the holy grail of slates, an inexpensive, flat, thin, clean and consistent slate unfortunately does not exist.
Ultra Slates, a range of premium slates in terms of cost that offer the finest selection from the best quarries. A generally very clean and often thin, uniformed slate requiring minimal sorting and grading. The most stable rock with few inconsistencies. Due to the above, the ultra slates are offered with a 75 year guarantee.
Often purchased by the sharper tradesman/contractors, the additional upfront cost of the slates should be offset through increased efficiency and reduced labor required in sorting and grading, loading, laying and wastage. The uniformed nature of this range, allows a simplified installation process, and therefore, can be used by all roofers. Expected wastage 5%.
Prime Slates, a range of mid-priced slate. The prime grade, offers excellent quality but will require more sorting and grading and will have slightly higher inconsistencies than ultra grades. They will cover a broader range of textures and thickness. Often more characterful slates, they will require more working and time invested on site by roofers to lay professionally, however they will often result in a more aesthetically pleasing look on completion. Prime slates are sold with a 50 year guarantee.
Generally purchased by the slightly more experienced roofer who has greater knowledge, time to spend on sorting & grading, and experience to work with a slate with more inconsistencies. Will have slightly higher wastage then ultra grades. Expected wastage 7.5%.
Commercial/Standard Slates, A range of affordable slates, often opted by budget conscious building projects. Although the slates have been put through the same rigorous tests to achieve UKCA marking as the Ultras and the Primes, the selection will show more inconsistencies and have a greater level of variance than their prime counterparts. The Commercial/Standard grades are sold with a 30 year guarantee.
The commercial/standard grade should only be purchased by experienced roofers who are able to work with the slate, potentially allocating more time sorting & grading, thus allowing the roofer to maximize the coverage and minimizing slate wastage.
Historically the demand was high for the commercial/standard grade slates, however with increased labor costs, the shrewd roofers are looking to purchase higher grade slates offering increased efficiency. Expected wastage for Commercial/Standard slates 10%.