I would be very leery right now for many reasons...
1- Never, ever mix business with friendship, it will end badly 95% of the time. This is really not a good idea.
2- Bad climate. Residential roofing will be put off except in the most dire circumstances. Nobody is spending for big ticket items right now unless they absolutely must. This WILL NOT change in 2009. In fact, 2009 WILL get much, much worse than 2008 ever got.
3- Life situation. No offense Gordio, but your life isn't set up for this right now. You have a 4 year old and one on the way (I have a 3 year old and one due in March). If you were single, maybe I could see it. Right now you have to be the provider, not the risk-taker. You work for a bank, good job etc. Even if you were laid off, you would most likely get severance and unemployment. If you quit, you get neither. Think about that.
4- Let the bank job ride. Play it out. If you get laid off, get your severance and unemployment and take a shot...well, not the best idea, but at least you'd have a bit of a cushion. Quitting and going for nothing....really bad idea. Think you got stress now?
5- Don't stress what you can't control. Worry about your specific job, reduce your debts and expenses and put yourself in the best position should something happen. Other than that, don't worry about what you can't control.