Slow draining basin

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Hi

Have a hand basin in the bathroom. It is frustratingly slow to empty when the plug is pulled and in fact when running the hot tap waiting for hot water, it will back up as the water exits too slowly.
I have tried the stuff you squirt down the plug hole, no difference. Tried a plunger a few times. some black smelly gunk appeared but eventually it stopped. Still very slow.
I have now unscrewed the water trap thing from under the basin and managed to scrape out a load more gunk from the downpipe under the plughole. The trap itself was fairly clean.
The water exit is still too slow.

Any ideas?

Thanks

[4getmenot]

Call your building insurance and get a CCTV survey done on the drainage. Check water levels in manhole if this is high you have a problem. Ask neighbours if they are having problems, if they are could be a blockage in the main sewer.

[zzxxee]

yep put a big handfull of soda crystals and a whole bottle of white vinegar down there leave over night then boil kettle and pour down sink run taps and should be ok worked a treat on mine xx

[4getmenot]

yer try that first. Just dont try the chemicals they are not good for your drains

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cheers for that.
I dont thnk its the main sewer or anything as I am on top floor of a block of flats.

Trouble I'll have will be finding the bottle of white vinegar!

[4getmenot]

I;d suggest cheking the manhole level then. if it is full no point even trying soda crystals. You need to get a company out to unblock, if it still happens then get a camera survey to see what the cause is

[The Builder]

If you're on the top floor, and you're sure nobody else is having problems, then it could be the 32mm waste pipe from the basin to the soil stack. Disconnect the trap from the pipe, and rod the pipe with something like stiff fencing wire .......... rather like a straightened out wire coat hanger.
Give it a go and come back if it's still bunged.

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Thanks Builder will attempt it at the weekend.