Advise on Single ply flat room

[Simon Man]

Hi there. Im after some advise on a flat roof. I had a quote for an extension with a flat roof. The builder had quoted to fit Sarnafil 1.2mm lead grey PVC membrane single. The area of the roof is 4.4m x 5.5m and I was wondering if I would save cost by using 2 ply and does anyone have advise on if 2 ply would be suitable and is the difference

im assuming the 'lead' is the colour not the material!!???

thanks in advance!​