The sewer on Sydenham Road is on the opposite side of the road and at (if I remember correctly) 1.6m below the surface. As Sydenham Road slopes it would be possible to get the correct fall (gradient the pipe is laid) by having a longer run. Bit tricky to see from the online version of this drawing. The blue dashed line is the datum that everything is measured from; the red line is the gradient of the road and the green line is the sewer with the 1:80 fall. None of it is to scale. As the green and red lines are at different gradients they converge. At the point where they are 1.6m apart is where we join the main sewer.