external drains

sorry people for a rather simple question, but i have just had some drainage fitted to a restoration job and the ground work guys have run the wc waste pipes at 80 to 1 slope from the house to the sewage treatment plant. This sounds like not much slope to me. THey say its ok if using 3m plastic pipes. The pipes are 110mm diameter and the run is around 20m long. it aslo carries kitchen and other bathroonm waste What do you experts think??

any manholes in the 20m. for cleaning .........usual rule of thumb is 1:40 so invert @plant should be half a meter lower than invert @ top of drain run .........I personaly would need to look at job to judge, because 1:40 is an old R.O.T. from the days of 2 foot long stoneware pipes..........they have a Valid point, your G/workers, and before I condemn them, I`d want to know other factors like manhole layout and how the pipes are bedded etc. and check out the falls with a (borrowed) Cowley level I`m Sussex born+bred and I like groundworkers a bit more than itinerant roofers

thanks nige f, there is a man hole at each end of the run, the fall was set using laser level so i think it is pretty good for accuracy. the guys doing the work seemed very concentious. the pipes are set in the trench with shingle before back filling.

See document page 16 here http://www.planningportal.gov.uk/uploads/br/BR_PDF_ADH_2002.pdf

1 in 80 OK if Wc installed.

we have had success with drains laid with as little as 3" of fall over 22 feet (75mm over 6700mm), this equates to less than 1in 80.