Sometimes roof pieces get placed so they don't quite cover the outer wall. The rough end of thatch needs to go over the sides of buildings by about a foot. I often build in the dark and then it rains and I check the walls. Occasionally a couple took rain damage, so had to reposition the thatch pieces. I often just add a 2nd on top of the 1st one when this happens and just move the new down a foot.

The sweet spot for placement can be finnicky in the dark.