Decks, Patios, Porches and Docks - Railing on balcony

I have a 10x25 balcony off of my second story bedroom and it is over my finished garage and my living room. I am installing a vinyl railing and need to know how to install the posts. The plates I bought screw to the bottom of the posts and you are to install a second plate below the mounting surface and bolt them together. Since I cannot get below the surface can I use stainless steel lag bolts and screw them to the wood below? It has a roll roofing on the surface. How long of a lag bolt should I use?

A picuture is worth a thousand words. First of all, you want to make sure you're securing your post into solid framing. Otherwise, it doesn't matter how long your lags are. Second, it sounds like this is on a flat roof. I personally would never penetrate any type of flat roof without knowing before hand exactly what I was getting into. But like I said, if there is any way you can post a picture of your situation, you may be more apt to get the help your looking for here.