One must learn from his mistakes. However, in the case of roofing, this learning is bound to cost you a lot of money and a huge waste of time & energy. Better yet, avoid making errors.
The first and foremost thing to consider is the type of slate you opt to buy. You might get a “surprising” discount at some hardware malls, which will surely surprise you with the need of a new roof in the next five years. To be on the safer side, buy from a renowned dealer, one who has a good history. It is wise to pay a bit extra than lose a lot in the coming years.
The next important decision is the size of the slate. You might think getting smaller tiles will go cheaper. Quite on the contrary, their number & the strenuous installation will cost you much more. Instead, calculate the optimum size based on the roofing area. Make sure you shuffle the cards before you lay them – gives you a much acceptable output.