Peter Howell
Required fall on foul water waste pipes is 1:40 So the problem you have could be one of many things A blockage of some kind ie the tiler pouring water with grout residue in it down the waste pipe this then goes off solid in the pipe Incorrect waste pipe size Long run if pipe work not supported correctly and then sagging Incorrect fall on the waste pipe You could try fitting an anti vac trap on the hand basin this would help putting air behind the water
