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Grading Slates


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We've used Spanish Prime slates on the garage and outbuildings. The first roofer we used didn't grade the slates for the garage roof and it's  slightly uneaten finish still annoys me now (but it may be my OCD as another roofer who I know well said that a slightly rustic finish is very normal around these parts!).


Anyway on the house I want a "church job" (another phrase of my roofer!!) so want to ensure the slates are sorted and graded. 


I fully understand the process and subsequent use (I.e. Thicker slates towards eaves) but......


Whats the best technique for sorting thicknesses. I'm sure the pro's can do it by eye but there must be a technique? Is sorting to three thicknesses enough?


I have a vague memory that @Construction Channel mentioned this as being one of his "trade secrets" which his boss would not be happy if he shared? 



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3 hours ago, dogman said:

i made a sort of jig with different sized slots in it and slid all 3000 slates through it to sort into 4 grades

i used roof batten with plastic wedges to get the gap right



I like your thinking. How did you decide on the 4 jig sizes? By selecting 4 from the 3000 as your "template"?

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I had two slots I think they were 6mm and 7mm.  So ended with four sizes. less than 6mm and thicker than 7mm. 

I bought most on eBay so had a lot of different thicknesses 


It's what you do with those  that just have a small area that's thicker 


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