ITYWF the original material was most likely thatch.

Only wood shingles are as light as thatch is.

Even pantiles will sag a roof designed for thatch, and they are about
the second lightest I can think of.

Nah - often happens in the middle of a terrace of slate roofs.

Former thatched buildings round here can usually be identified by the
steeper roof pitch - around 50-55 degrees.

Usually these are done over with sagging red clay pantiles (though
that may be due to roughly hewn roof timbers on buildings of that
age).

On these pre-Victorian properties, usually with fairly random masonry,
sagging roofs on pantiles look like character - though I'm not the
person inside having to live inside with the roof leaks.

Sagging concrete tiles otoh, just look terrible - inappropriate to the
building, glaring disjoint to neighbouring properties etc.