How much fall,on 110mm waste pipe

[Tricky Dicky]

At present the waste from my toilet pan is about 0.5m between pan and vertical stack originally it was in the kickspace of some bathroom fitted cabinets. I cannot measure the angle but it looks almost level. I am going to move the pan a further 1m away and the new cabinets have a smaller kickspace of 140mm so there will be absolutely minimal fall. Since we never had any problems with the existing fall does it matter if the fall is further reduced as long as it is level or has a slight fall?

I note that bends and tees in 110mm have 92.5 deg angles which by my dodgy maths equates to a 43mm fall over over a distance of 1m. There is no way the present fall is anywhere near that and the new fall is even less likely to be near that.

The tee in the vertical stack is already at floor level but could be lowered below except the stack goes through the roof and I am loathe to disturb it in case it causes problems where it exits the roof plus it will make an awkward hole through the floor.

I would be grateful for any advice.

[Andy Burns]

A WC requires between 18 and 90 mm of fall per metre, see Approved Document H for chapter and verse.

[Tricky Dicky]

I think I will just about make that lower figure. Thanks Andy

I lifted a few floorboards yesterday and Plan A is the only option with a possible slight modification of it to gain a little more height by making a small platform of 18mm ply for the cupboards to sit on, a couple of cabinet door replacement companies sell gloss white edging strip to match the cupboards so I can effectively conceal the edge.

Richard

[Steve Jones]

A 110mm foul drain taking the discharge of less than 1 l/s should be laid at a 1:40 (25mm per metre) fall. A foul drain taking the discharge from a minimum of one WC can be laid at 1:80 (12.5mm per metre).

And check out

Gradients Foul water drainage systems are generally designed to run at a maximum of three quarters full bore. Pipe gradients should be established such that the velocity does not fall below 0.70 m/s to ensure adequate self-cleansing. A 110mm foul drain taking the discharge of less than 1 l/s should be laid at a 1:40 (25mm per metre) fall. A foul drain taking the discharge from a minimum of one WC can be laid at 1:80 (12.5mm per metre). Gullies incorporating in foul water or combined drainage systems must have a 50mm minimum water seal. The table below is taken from BS EN 752 and provides guidance on minimum gradients for different size drains

[David]

I think a short run directly from the WC could get away with minimal fall, because of the initial rush of water when flushed.

Quite how far this initial rush will carry any waste is an exercise for the reader. ;-)

Cheers

Dave R