reducing down for soil vent

[Mgrant]

With an internal stack that needs venting through the roof, it is usual to reduce down after the last 4inch branch in the pipe that goes in an upstairs toilet. What size do you usually reduce down to to take the soil pipe out through the roof? Think I have seen 3inch before do the readily sell this at plumbers merchants?

 

[Meady]

Easy to get hold of, that's the smallest you can go though.

 

[Mgrant]

Thanks. I will go to the merchants in the morning. Need a couple of lengths and two 45's to get around one of the roof trusses

 

[Ch4 plumbing]

Normally carry it on in 4" TBH but have used 3" like described.

If it's on a new build check with the Building inspector to make sure they are happy with it, most are but some have needed persuading.

 

[Mgrant]

Do some require you to extend up in 4 inch?

 

[Ch4 plumbing]

Have a look below.


http://www.planningportal.gov.uk/uploads/br/BR_PDF_AD_H_2010.pdf Page 13 paragraph 1.32

Can be reduced in buildings upto 2 stories.

 

[Mgrant]

Any idea why its limited to 2 storie buildings?

 

[Mgrant]

Just the the other new houses on this site well were build 7 years ago. Were finishing the site off now. Are all reduced to 3 inch to vent through the roof.

 

[Mgrant]

These are 3 story houses

 

[Ch4 plumbing]

Honest answer is I couldn't tell you, maybe 3" can't draw sufficient air for the stack?

 

[Mgrant]

Ok, all the 3 storie houses on the site that were build 7 years ago before the recession are vented in 3 inch. Thou it looks like I may not be able to do this. Shame as 3 inch just fit though the trusses. 4 inch won't

 

[SimonG]

I just use 4", probably 3" is more expensive.

 

[Ch4 plumbing]

Ok, all the 3 storie houses on the site that were build 7 years ago before the recession are vented in 3 inch. Thou it looks like I may not be able to do this. Shame as 3 inch just fit though the trusses. 4 inch won't

Contact the building inspector, if they are happy then you can, get it in writing though then there's no comeback.

 

[plumben]

If you can't get 4" through the roof spars, id go all the way to the top in 4" then reduce down at the last possible point

 

[AWheating]

As normal when a builder makes it impossible to get the 110mm pipe to vent it's the plumbers problem and not a design issue.

If it can't be done to regs then tell the builder to have a rethink.

 

[solutions]

Honest answer is I couldn't tell you, maybe 3" can't draw sufficient air for the stack?

That is my understanding, the speed at which a flushed volume falls through the stack once it exceeds 1 storey (only falls 1 storey in a 2 storey house as toilet not on ceiling) must be enough to need a fill 4 inch vent to get the right movement of air not to slow the flush.