I'm a DIY planning to build a pergola over the rear patio. The overall dimensions will be approximately 14' by 16'. I'm planning to use 2X6" rafters approx. 12" on center. The rafters will be tied toogether on top with 1"X2" strips. The spacing between the 6X6 posts will need to be 14' because the pergola is directly over top of a 14X14 concrete patio pad. I don't want to tear up the concrete pad or try to drill a hole through it for an additional post. So, what size beams do I need to avoid sagging? I'm thinking of useing 2 2"X10"s, either on each side of the post, or sandwiched together and set on top of the notched posts? Is this size beam adequate? I will plan to strengthen the corners with decorative wooden corner braces. I want a strong pergola that will not sag and will look good for years to come.
Also, the main purpose is to provide shade from the sun. How do I determine the best size and spacing of the rafters and strips across the tops of the rafters to achieve this?