Jenny, I'm working on the assumption that French farmhouse roofs are much like German ones, i.e., layered (terracotta) tiles, with insulation tacked underneath. After having a whole colony in our garage, and the occasional "stray" bat in the attic, my husband tacked up thick plastic foil over (actually under) the insulation, and since then, we haven't had any bat visitors, other than those Jamie finds on the balcony.