Your hens will nest where they want <g>!! My ladies have several boxed, roofed in nest boxes, most of which they ignore. They have preferred building their own nest in the corner of the coopy behind the feed bin (no roof), and since they free range, they are very fond of building nests in our hay bales (also no roof).
I'd say build them without a roof, and you can always add one later if they decide they will NOT lay in them.