Welcome ewoc(wasn't that an animal offa Star Wars?)First off you'd be doing yourself a favor if you got your sparky to show you how to test your heat mats,it's very simple,Of course you'll have already purchased a multimeter,not some clever electrical gadgetry(that already sounds like more money just for saying it)they're not expensive but invaluble if your tiling over heat mats,once you have the know how,test your mats after every row you set,it only takes a second and if your trowel has nicked a wire through a thin spot in your screed you'll be right on top of it then your sparky can throw in a splice kit,which could be done by yourself but it depends on your coding situation and how sure you are of yourself,but check them mats often.They do get nicked and you don't want to find out after you've finished your set.Secondly,and I think someone mentioned this but get a high end enhancer,it'll make the job,don't skimp on cheap sealer,Over here in the colonies we have a product called Aquaseal and I swear by it,It's expensive yeah but you make way more money than it costs by selling it to the customer before you set they're
tile,(only works on natural stone)over here I pay $200.00-$250.00 for 4litres,but it's well worth the money ,it goes a long way and I always make 10 times or more my money back,Before you start a job just put enhancer on a piece of scrap slate,the look will sell itself,then you can double your sealing rate because of the high end finish,Anyways,I've said to much,I just love natural stone and done right it's the best,good luck,Mike