I'm adding a garage addition out the back of my existing garage. Should I, and if so how much, slope the floor? The addition will be 21' deep and I'll be storing my boat in it so I won't be parking a car in there that could have ice and snow melting off of it but it would be nice to get good drainage when I hose it down. If I do slope it then I guess I'd build the walls with slightly shorter studs as I move toward the rear of the garage (the higher concrete elevation) to keep my top plate level?