Seams were sealed like any other surface, flashing and sealant as well as controlling the direction of water flow. The sheets come in a variety of widths up to 82". Your local plastic supplier should be able to source for you and the manufacturer is quite helpful. http://polygal-northamerica.com/

Thanks. I forgot to ask, is it very noisy during a downpour?

It's a hollow core product so it can make a little some noise.

how much sun/temperature reduction is gained under the polygal? I'm wanting to do something very similar, but have an infant son, and want a good place to keep him out of the sun when we're all outside