We have finished putting up the Fermacell panels up on the ceiling of the Garden Room. The system of routing a tongue and groove seems to be working but the final analysis and possible successful outcome will wait until we have painted the ceiling to the same degree as the main front garage room. The Fermacell is only 10mm thick so cutting a groove and a tongue is tricky work and rather fragile but after being glued and pushed together, it seems to be as strong as the straight forward butt joints. We will wait and see.
Garden Room ceiling
Garden Room ceiling