Pergola Help

gway84

Thanks in advance for your input. I have built a large deck 16X30. On one half we are going to add a pergola and I have some questions about materials. The pergola will be apprx. 12X15 with (4) 6X6 posts on each corner that are already in concrete and integrated into the deck. My inital thought is to put (2) 2X10X16 on each side of the post to go the 15 foot span. I then need to put something on top to provide the 'shade" factor...what do i use here? I dont want something too small that is going to warp all over the place but i also dont want something soo heavy that it will cause the 2X10x16s to sag. Any help and input is greatly appreciated.