Is your loft going to be a mostly enclosed structure with walls and a floor or are you talking like a bare ground and wire aviary?
If you’re building something like this:
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Then I strongly recommend some good thick plywood for the floor. Easy to scrape the poop when it’s wet without making a mess, and it’s even easier to scrape dry. If you’re going to have the pigeons actually touching the ground I would say try to go for bare earth or something you can easily take out. Maybe chopped dry leaves, mulch, something like that? They don’t scratch around like chickens so the ground quickly gets loaded up with droppings. I know
@WVduckchick has her pigeons in a bare ground setup that works well for her. The key to any clean pigeon housing is to keep it
dry.