I have had occasional problems with rain damage to beams which I thought I had covered with a roof. It appears to me that it involves snapping the roof piece to the interior upper edge of the beam rather than the exterior upper edge.

I say that because it seems to happen when I place roof pieces from inside the building rather than outside. I have no problems snapping roofs to wood walls because walls are thinner. Wood beams are thicker than walls and log beams are thicker yet.

Sometimes I will place the first roof piece from outside the building and the rest from inside. In those cases, I snap the second roof piece to the edge of the first, the third to the edge of the second, and so on until that tier of roof pieces is complete.