Harry,Hi. I dont know how wide your patio is, so I'm going to say its X wide. You may have a drive way (flat area) draw a line on the floor about the lenght of X, on the one side draw another line at 90deg, ( thats the wall (house) side) Take the size of the header beam and draw that on the other end of the X line.( this is called full size layout) When you draw all this on your drive way you will see what I'm trying to tell you.you now take a offcut of the lumber (nice word that) that you will use as a rafter this will be a template for you birdsmouth, play with it till you think it's right, and away you go. Have fun. John 2000.