Decks, Patios, Porches and Docks - Deck Beam specs

I am building a deck on my townhouse that will extend over my garage and driveway. Because it's over the driveway, I can only have 2 posts to support the deck (in addition to the ledger attached to the house). The distance between the posts will be 20'.

The question I have is will I need to use a steel I-beam in order to support the joists, or can I use wood (for that matter, can I even find 2X14's or 2X16's that are long enough)?

Second, how do I go about getting a design for the steel beam if I need one? Do I have to find a structural engineer? Is there a good place to find them?

Thanks

I would start at a lumber yard. I doubt you can span 20' without going to something like a 4x16, which will probably be more expensive than a steel beam. It'll also be a special order (and special price) to get one/two that long. You could also look into "micro-laminate" lumber, not sure of the correct name, but it's fabricated boards similar to plywood, much stronger than standard lumber. I would use a steel I-beam, and if the lumber yard can't order them they can tell you where to go to get one. When you buy the beam just tell them what it's for and the span and they'll tell you the size requirement. Getting a 20' steel I-beam on top of two 8 foot posts should be another story all together. I've set two foundation beams for a house at that height and it took seven guys for a 16' beam (about 600 lbs.). It's been a while, though, and although I am a mechanical engineer I have limited structures experience.