I'm just going to start by saying I love my Ultimaker 2 and the guys at Ultimaker UK are all great and very helpful. But, I've had 3 build plates for my printer, first that came with the U2 was perfectly flat, but I had a little bit to much heat on the bed and the print ripped up bits off glass. Ordered a new one, arrived and put it on my surface table. Carefully measured it with my mitutoyo vernier and had a high point of almost 0.5mm This was fine for small prints not for anything over 25/30mm either side of the centre point. Ultimaker kindly sent me a replacement which had the same problem, only not as bad. Spoke to them and the said I should be using a raft to print on, but I don't won't rough prints on the bottom. As this printer can print at 20Microns, if the glass isn't flat, it's never going to work. Might try and find a glass supplier who can guarantee a minimum tolerance on warp.