Putting the nesting boxes lower than the roosts can help keep the birds from roosting in the boxes. I think it's a protective instinct for chickens to want to be up higher for sleeping so if given the choice between sleeping on a roost and sleeping in a nesting box, they will choose the roost if it is higher. Also, if your boxes are lower and you plan on hatching chicks with a broody, the chicks will be safer in a lower nesting box.
If your chickens aren't laying yet, cover the boxes so they can't get in to make a mess. When they start to show signs of laying, you can take off the covers and put golf balls or plastic eggs in so the chickens will get an idea that is the safe place to lay their eggs.
Thanks for joining us!