Sorry, I understand I didn't give enough info before. The two side walls are 36" long by 48" in the front. I want the roof to slope back at a 20 degree angle. (I read online that a 20 degree angle on the roof would be good for this.) I just need to know how high the back of the side walls need to be. (Which would also be the height of the board for the back wall.) Hope that is enough info for an answer. Thanks!!!