"I did some research on "edge beams" and decided on a double 2x6 beam for my 42" deck width. Since a 2x4 is actually 1.5x3.5 and you want your beam to be the same width as a 2x4, you have to sandwich a piece of 1/2" plywood between the 2x6's to get a width of 3.5. I used liquid nails between them all and then two 3" deck screws every six inches."