waste run length..

That depends upon the bore and flow rate. In one of my factories, building control insisted that I used 50mm waste pipe, instead of the more usual 32mm, for hand basins with a long run and, just to be sure, fit a running trap just before it entered the soil stack.

Colin Bignell

That's the theory. Approved Document H gives limits on diameters of pipes;

I have over 5m on the minimum slope - it does not empty traps - in fact, opening up a rodding point (cap on top) shows even an emptying bath to only fill the pipe 1/2 way, leaving an air break over the top.

I should add that is in 50mm. It did exceed the building regs guides, but it works in practice.

I suppose if the outlet from the basin etc. was sufficiently restricted the pipe wouldn't fill up. I noticed a few years ago that modern outlets had as low as, at a guess, 25% open area - my 60 yo outlets must be >80% and do fill the pipes. Of course, a HepvO has no such limit.