My chickens want to roost on the edge of the nest boxes
making a terrible mess inside. Needless to say this causes extra cleaning and some dirty eggs. Can anyone tell me how to stop this ?

The best way is to cover the nest boxes after they finish laying for the day and uncover it after everyone is on the roost for the night. This is not very convenient for most people, however. You can also go in just after roosting time and make them get off the boxes. Once it's dark, they wont be able to see where to get up on the nests. If your nesting area is very dark anyway, at roost time they should not be able to see how to get up there. Can you isolate your nest area with some cardboard or paneling so as to make it atractive for laying, but difficult for roosting? I have two hens who have insisted on doing this for more than two years. Every night I put them on the roost, and next evening they are back on the nests again. Everyone else eventually got the idea. Good luck.

My friend has a hen who roosts on the nest box. If you covered the box with a peaked roof, wouldn't that discourage the hens from roosting on it because it is too steep?

I made my nest boxes with an empty space in between each box. I may be just lucky, but they roost on the sides of the boxes, with their behinds hanging over the edge, and the droppings are dropped neatly into a litter box that is set below the nest boxes!!. I just fill the litter box with cedar shavings, leaves, hay, etc., and dump it every day or two. If you study the way your roosts are, you should be able to manipulate where they are sitting at night.

Thanks, I'll try covering. They can't get on top, they sit on the edge on the entance.

Once in a while I'll have a hen that didn't make it to the roost before dark so she'll go to the nest box instead. Similar to Srbucks above, I have a perch right in front of the nest boxes, and they tend to roost there. It works out okay because there's a space between the perch and the box and their droppings just fall to the floor.