[Cob] Attaching concrete roofing tiles to cob walls

[Hugh Withycombe]

Hey there to cob world
I have a question about getting concrete roofing tiles to stay attached to the top of cob walls.  My family and I have built a garden wall, with bench seat, windows and arched gate as an introduction to more complex cob projects.  Now that we are starting to get regular morning frosts (Canberra, Australia) the curved concrete roofing tiles with which I have capped off the walls have detached.  I bedded them on with a slightly clay rich, straw poor mortar mix which adhered well during the warmer months but differential thermal expansion seems to have broken them free.  They were stuck with about a 20 degree pitch and even loose the weight still seems to hold them in place.
So, should I worry at all or can someone suggest a better fixture?
Hugh Withycombe