Measuring Slate Performance Objectively
The top slate manufacturers in the world routinely test slate as part of their quality assurance programs – the ones that don't usually have something to hide.
There are a number of relevant tests for roofing slate that collectively indicate how well they will perform. There are many types of roofing slate available – the best slates have test results that validate their performance.
Each of the following tests has been designed to replicate the most common causes of failure:
Water Absorption Test – a slate should not increase in weight after repeated cycles of wetting (distilled water), boiling and drying. Levels of weight increase under 0.25% are optimal and under 0.3% acceptable. Slate is effectively impermeable to water but water trapped under the lapped zone can create failures on the under-surface of the slate.
Wetting & Drying Test – slates should not split, flake or delaminate after 15 cycles of soaking, cooling and drying in a ventilated oven. The best slates dry quickly. Water that adheres to the slate surface will dry slower and trap dirt and airborne biological matter which then holds moisture on the slate for longer predisposing the slate to premature degradation.
Acid Immersion Test – slates immersed in sulphuric acid baths for 240 hours should so no signs of swelling, softening, flaking or delamination along the edges and no gas formation or release.
Modulus of Rupture Test – this attempts to measure the flexural strength of slate to simulate foot traffic and hail on slate roofs. The test originated in the US and is based on the ability to withstand a 9000lb load on a 3/16" slate.
Flexure Test - These test methods cover determination of the breaking load, modulus of rupture and modulus of elasticity of slate by means of flexure tests
Weather Resistance Test – includes a depth of softening test and a chemical weathering test that measures slate disintegration that correlates with the durability of slate in actual weathering.
Data from these tests are used to estimate the possible lifespan of slate fixed to roofs eg ASTM (www.astm.org ), NF & BS-EN standards.