I have found quite a few posts about smells from bathroom drainage but all appear to have the smell in the bathroom.
My problem is different in that the smell appears outside in the drain.
I know it is not the drain itself - I have checked that by running water into it from a hosepipe. We also do not get the smell when any of the kitchen waste is run into it.
The smell (like a really foul drain) only happens when the washbasin appears to be used . I have not noticed it when the bath or the shower empties.
The plumbing has been in place about 15 years but this smell has only started happening recently.
I thought at first that it was the outside drain but, as I said, this has now been ruled out.
There is no smell appearing in the bathroom itself.
I have checked the U-bend which is clear.
If your getting smells from the drains, then there's a problem with the seal/trap. The seal is made by an amount of water being held in the trap, if the seal is lost - the smells can come back up from the drains.
If all the drains are clear, it could be the fall on the waste pipe - too great and you'll get a suction on the trap and water will be lost - or there's a fracture in the gully/trap - quite common when people try rodding drains themselves, not knowing what they're doing - if the bottom of the trap has been damaged/fractured then the water will be lost gradually.
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