From the chaotic regions of the Cryptosphere, The Natural Philosopher
wrote on Fri, 01 Aug 2003 09:36:23 +0100:

Hugo Nebula wrote:

Rest of the world 1, Natural Philosopher 0.

"A minimum size 100mm branch pipe may be used for a maximum of 8 WC's or
a length not exceeding 15m, gradient should be between 9mm and 90mm per
meter run" Building regs 2000 edition.

The Building Regulations don't give guidance, the Approved Documents
do. (2-0) Perhaps you should download the 2002 edition of AD 'H'
(Drainage). (3-0)

Table 2 Common branch discharge pipes (unventilated) gives the
gradient limits as:


Appliance |Max no. to |Gradient limits
|be connected|mm fall per metre)
__________________________________________________ __
WC outlet 80mm |8 |18(2) to 90
WC outlet 80mm |1 |18 to 90

The footnote (2) to this table reads "May be reduced to 9mm on long
drain runs where space is restricted, but only if more than 1 WC is
connected". So for the OP's question relating to a single WC, then
the minimum fall is 1:55 (4-0)

So min fall is 1:100 roughly.

Very roughly. (Poor defending nearly let another in)

The steeper gradients you quote are for washbasins and urinals and baths
on smaller diameter pipes.

The lower end of the gradient range is uniform at 18mm/metre(1) across
the whole range of appliances. (5-0)

External falls depend on anticipated flow rate - i.e. number of bogs
connected, and may be in large diamter pipe as low as 1:180. though
1:40- 1:80 is optimum. 1:60 is a good working figure.

The OP was about sanitary (above-ground) drainage. It sounds like
you're getting this confused with the drains below ground. I don't
know of any unpumped drainage serving normal domestic foul drainage
that can be installed at 1:180! (6-0)

(1) In the UK we use the "metre" spelling for the measure of distance.
7-0 in injury time)
--
Hugo Nebula
"You know, I'd rather see this on TV,
Tones it down" - Laurie Anderson