Is the moist spot only underneath the roosting area, or is it on the ground near their chicken door from bringing it inside? Have you stuck a sprinkler on the roof and gone inside to look for drips? Maybe one roof fastener is letting in water when it rains. Pellets will quickly turn into...
^that. Condensation drips was my guess too, since it's a metal roof and you can see markings on the wood rafters/purlins where they've gotten moisture in the past