i absolutely swear by Mercury trainers , this is for ladderwork only. The fabulous grip on slate/pantile or whatever is due to the soft marshmallowey sole (usually mainly a white colour) ,the sole on Mercurys turns up a bit at the toe end too (good feature)
the uppers are not waterproof and they are not very long lasting (about 2 or3 months before the sole cracks across the centre, but they are usually £9-99 so best to buy a few pairs while in the shop. Most cheap shoe shops sell Mercury brand ,impossible to buy a Mercury online ,they have excellent grip on ladder rungs and never need any breaking in like some shoes do