Flexible soil stack

[mildcheese]

The 4" vertical soil pipe in my ensuite terminates with an air admittance valve below the toilet cistern. I'd like to extend the soil pipe into the loft space and either finish in the loft with an air admittance valve or possibly vent externally.
Can I extend it using a flexible pipe or does it has to be PVC? I need to navigate round the cistern slightly and other obsticles.
Thanks

 

[kirkgas]

if it is only in the dry section of the pipe above the wc then it will be fine to do in flexi

 

[mildcheese]

It is only the dry section, many thanks for your response.

 

[swbjackson]

If you vent through the roof make sure the flexible section is well supported and can't sag. You will get condensation form inside the pipe and if the pipe sags it will collect and eventually distort the pipe enough to create a water seal and all the wastes will start gurgling. I had this happen on a new build estate.