3/28/2023 0 Comments Slate roofsMinor minerals can exert a major effect on the durability of slate. (The presence of calcite may also be responsible.) In contrast, some Aberfoyle slate is known to soften after 20 to 30 years, and this may be because some seams have low quartz levels. The high levels of quartz in Ballachulish slate may account for its long life. High levels of quartz and a high ratio of white mica to chlorite correlate with long service. The mineralogy of the slate is the main factor that gives slate its properties and relates loosely to its durability. Main factors affecting slate’s durability: Research involving the experimental weathering of slates has found that two main factors affect slate’s durability. But slate from some areas is much more durable than slate from other locales. Scottish slate has a well-earned reputation as a long-lasting material. Larger slates were laid at the base of the roof and smaller ones at the top, to make best use of all of the slate quarried. The variable size of Scottish slates led to the distinctive roofing technique known as ‘diminishing courses’. More wastage would result if larger slates were produced, while a smaller slate size made the most of the available material. The spacing of the fractures formed during this process, and the angle at which they crossed the cleavage plane, set the typical slate size. The size of slates that a quarry could supply was also affected by the folding of the rock. But as long as slaty cleavage is present, the slate will make a good roofing material.
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