I am not exactly certain — this is not a topic I’ve ever deigned to do much research about… I’ve always thought it was pipes in older properties, because in upscale hotels and restaurants, this does not exist.
But I asked my waste-removal consultant (my husband) and he says the problem is at the local (entry-level) site, for two reasons:
1)
Older houses have clay drainpipes and TP sticks to them as if they were velcro, hence blocking the pipe.
and 2)
If it is modern construction and they use PVC, they often uses a small-diameter pipe to save money. And these can easily block up.
Suzan