levelblue

Hi all, I will shortly be re roofing as our roof is loosing slates at an alarming rate, got new battens 25x50 and underlay, hoping to re use most of the old slates as they appear in good condition and get some reclaimed ones to make up whats required. I'm wondering what's the best for nails /hooks ? Any one got a preference personally I think I would prefer to use the hooks with clouts around the verges etc. Is 30mmx 2.65mm the right size ? That's what was in, but from the nhbc I thought they had to be at least 38mm....30mm would give 15mm penetration into the batten though... thought I had also read somewhere that needed to be 3.35mm thick.. would that help if reusing good holes in the slates ? Any input great fully received LevelBlue