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??