Roof construction

I am after some advice on constructing the roof.
Have all the ribs and valances in place but can't work out how the roof boards go. It says in instructions to start with a board either side of the centre line of the ribs which will only use up 22 of the 23 boards supplied and will not leave me with the required overhang at the rear. If I place a board along the centre line so I use the 23 boards it leaves nothing at the sides to fix the last boards to.
If anybody who has built a roof could give me some guidance it would be much appreciated.
Thanks,
Simon

Hi Simon, when I did my roof I glued and screwed the roof boards to the ribs it was fairly straight forward until I got to the outsides. I did soak the outside roof boards in warm water, I clamped the boards to the ribs and let them dry this will take some of the stress out of the wood. I glued them and screwed them where I could. I left the outside boards full length and trimmed them after they dried. I used an expanding glue, there is a bit more cleaning up but the joints seem to be strong. sorry I have just seen your post. David.