Cooking a roast in a crock pot?

I work all day, but I found a great recipe to make a roast. It requires either 8 hours on low or 4 hours on high. So I am going to start it at 3:00 to have it ready around 7:00. My question is should I put the potatoes, garlic, onions, and carrots in at 3:00 and let it cook along side the roast or put them in later? Will they get too soggy if I cook them on high for 4 hours?