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What is the maximum gradient for a foul drain? - Green Building Forum


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What is the maximum gradient for a foul drain?

I'm looking to install a new 63m long 110mm foul drain from my new cabin in the woods to the existing septic tank.

The ground slopes and the gradient is around 1:21 over the first 40m and 1:100 over the rest.

What is the maximum gradient I should be looking at to meet building regs ? Also are there any rules on manhole size as I want to keep them a small as possible as they will be in lawned areas. Do I have to use any special bedding materials when installing the pipe?


  • Its all in approved doc H in Building Regs. Gravel to lay pipe in; manhole / inspection chambers dictated by depth below ground level which are in turn dictated by your fall. More experienced people than me on here I am sure, but we did our own for the SB and touch wood they're fine / were passed by inspector. Planning is the main thing to minimise inspection chambers, etc whilst still allowing all to be rodded if necessary... Do a few drawings!
  • I would lay at a consistent gradient minimum 1:60. No maximum, but you may find that if it speeds up / slows down you could get solids build up. Bed inspection chamber bases in concrete. Line bottom of trench with pea shingle. Lay pipe. Shingle to crown of pipe. Call building inspector, then 100mm shingle over pipe and backfill to fill trench.
  • Extract from government website:-

    "If a gradient is too steep i.e. steeper than 1 in 40, the liquid may run faster than the solids in the sloping foul water pipe thus leaving the solids stranded, which could then block the pipe.
    If the gradient is not steep enough, i.e. less than 1 in 110, then the pipe could still block if the solids slow down and become stranded."
  • Maguire's rule (for intermittent flow drainage)

    pipe diameter divides by 2.5

    so 100mm/2.5=40 so 1:40 gradient ( self cleaning gradient for intermittent or low flow rates)

    see part H as mentioned for exact info.
  • Pea Gravel only needed for plastic pipe of course.

    For the 1:21 you may need a backdrop drain (google it).
  • Any idea how much pea gravel for a 63m run of pipe?

    Goggles backdrop - yep, can do that.
  • Posted By: TriassicAny idea how much pea gravel for a 63m run of pipe?
    Depends how much you use!! Seriously, few people put 100mm on top, depends what your backfill is like.

    Oh and do not gravel to top of pipe and leave exposed if likely to rain. If the trench fills with water the pipe floats to the top and all the gravel along the sides ends up under the pipes. Had to re-lay 30M of pipe on my first build Grrrrr.
  • I understand that the theory that too steep a drain was likely to cause problems is proven not to be the case. No need for a backdrop, just ensure the gradient is > 1:60 throughout.
  • At least put good roding access at both ends if a drain is too steep, problem can come down to as little as if the wc is used after the shower rather then before.
  • edited October 2014
    This came in handy for converting a gradient of 1:60 to a figure I can use on site.
  • Paving expert is a great site with loads of practical info. Well done Tony McCormack!! Really professional.
  • edited October 2014
    Right, I've dug three trenches, the first two will be able to accommodate the falls but the last part of the run is a problem, it's 10m long and I have rock close to the surface and the ground sloped around 15 degrees. So can I put a single run in at 15 degrees or will I have to put three backdrop manholes to achieve the correct fall?
  • 15 degrees is fine.
  • Just a quick update... drains passed BC inspection, steep gradient and all. Thanks everyone.
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