We have been slowly building our coop out of them. One thing we kept in mind is that we want the pallets to be turned on their sides. Everyone stacks them upright, but that discourages rain run off.. not what we want when we move to the valley of the downpour (near Eugene, Oregon)...
We are using plywood on the floor. Not because we have to, but because it's easier
. The majority of our pallets are 4 feet long and 3'4" wide, so two of those will be cut down to 2' to make a nice even 12' by 8' floor.. A nice number to work with, since our wall pallets will be stacked on side all the way around, so each one will be 4'.. so two walls will have two pallets, and two walls will have three pallets (4x2 = 8', 4x3=12').
We are stripping some of the boards off of some of the pallets and nailing those boards onto the others to cover up the gaps. This makes it predator proof, and looks much much nicer, we think. We will cap the ends so that little gap made by overlapping the pieces doesn't allow a sneaky mouse in.
We are using 4x4s at each corner for a strong support system, and 2x4s to support the roof. The roof will be some old tin roofing we have off a falling apart ready-built shed left by the last inhabitants of the property.
We are using plywood on the floor. Not because we have to, but because it's easier

We are stripping some of the boards off of some of the pallets and nailing those boards onto the others to cover up the gaps. This makes it predator proof, and looks much much nicer, we think. We will cap the ends so that little gap made by overlapping the pieces doesn't allow a sneaky mouse in.

We are using 4x4s at each corner for a strong support system, and 2x4s to support the roof. The roof will be some old tin roofing we have off a falling apart ready-built shed left by the last inhabitants of the property.