Waste sink outlet blockage

[Wings]

Block of flats, blockage in first floor flat's sink, washing machine waste pipe. The waste pipe consists of four right angle elbows, three 32mm compression elbows and one 32mm push fit elbow, with a horizontal nine foot straight 32mm run.

The waste outlet runs under floorboards to the outside of the building. I believe the blockage is caused through washing powder solidifying, and whilst the waste water empties from the sink tap, when the washing machine is used, water cascades into the flat, and the flats below.

I do not believe drain unblocking chemicals are going to have any effect, I have tried rodding from outside to inside, but may be pushing blockage further in towards elbow. Tomorrow will attempt using wet blower vacuum cleaner, if that fails any suggestions please.

[Pheo]

I would have thought caustic soda would have worked quite well on washing powder...

[Fastpedeller]

remove wash m/c flexi waste from stand pipe. Fit a stop end to pipe (this is to stop air flow either way). Similarly block overflow on sink. Use a very good plunger (one that will also shift water) and plunge like mad! Avoid (until you've tried everything else) undoing joint or poking things in pipe - this only makes things worse IME.

[Wings]

Managed to unblock by reducing vacuum cleaner hose, by attaching 22mm tubing to wet and dry vacuum cleaner.