Blocked drain upstairs


Discussing drains today with a friend and he told me that in a very heavy rainstorm a few years ago that rainwater started filling his bath because of a blockage somewhere.Now his bathroom is upstairs and it completely puzzles me as to why this could happen when he does have sinks etc downstairs.His house would have been built in the late 1940s.Can anyone please enlighten me on this
allan
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allan wrote:

How about a pipe from bath on first floor goes through wall into a hopper also taking rainwater from guttering further up?
With a particular combination of hopper size and position, rainfall, restricted pipework*, etc - the rainwater could pressurise the bath drain whilst overflowing out of the hopper..just enough to get water to rise into the bath.
*eg the pipework lower than the hopper being blocked or constricted in some way.
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