Sewer smells from loft conversion sink

[Diyvictorian]

Hi all, diy homeowner here, we have a loft conversion (installed pre our occupation) there is an ensuite toilet and sink, the sink however seems to let through sewer gases and in not a small amount either! The soil stack vent terminates at the original roof height of the house so quite some distance below the toilet and sink in the loft. Is this the cause of our issues? Currently fitted with a standard plastic bottle trap on the sink.

 

[Knappers]

Hi,
There are a couple of reasons this might be happening.
Have you had anyone round to have a look?

 

[Deleted member 120897]

How far below is the vent to an openable window?

 

[Diyvictorian]

How far below is the vent to an openable window?

Vent Significantly lower being at floor height of the loft conversion which then has velux window in the roof, no window in this toilet around 30ft away from the openable window and the sink in question, smells are definitely coming up the sink

 

[Knappers]

Water in trap can evaporate if not used for long periods. Most common is trap being sucked out when using toilet/shower etc.
If sink tends to 'gurgle' after use or when using shower etc then a trap with an air admittance valve can be fitted and should sort out issue.
More severe issues may require a full sized durgo valve be fitted on stack.

 

[Ben-gee]

Extend the external vent pipe so that it is the highest part of your sewerage system.

 

[tamzbackforalook]

2 reasons for a basin trap emptying. Self or induced syphonage.
The 1st, self syphonage is a thing common on wash basins because of their shape.
If you fill the basin then pull the plug the water drains and as it empties, the speed of the flow causes some water in the trap to be dragged out letting sewer gasses past. Can also be caused by pipe gradients.
The 2nd, induced syphonage, is caused by insufficient venting or overly long pipe lengths where useing that appliance causes the trap to partly drain or using other appliances (flushing the WC) will make the trap empty by suction where it is looking for air.
Unfortunately many of the basics of plumbing are lost on the younger generation and especially builders.
Get them back to fix it foc.