# Help Calculating Soil Fall within Joist

#### rossyl

Hi Guys

If you'd seen my scribblings on a piece of paper you'd laugh. Please can someone with a few more braincells help!

Toilet is to be on Upper Floor of house
Soil Pipe must be run along joists, it cannot drop into ceiling below.

Joists are 7 inches deep
Soil Run is max 7 metres (likely slightly less)
Will I have enough fall?

I realise I'm over max Unvented horizontal soil pipe run, however, Soil initially runs into Eaves space (could an AAV be fitted?) then is to run along joists.

Thank you!

My rudimentary maths is...
7" joist = 177mm
Soil Pipe = 110mm
Leaving 67mm for fall.

Min fall is 18mm per meter
18mm x 7m run = 126mm

I'm therefore lacking 59mm of fall.

Is that right?

If so, is there any way to run a soil with less of a fall?

Or are my calculations completely wrong!

Thanks

If you check out building regs document H you will see that if your maximum flow requirement is below 6.3 lts/sec you can use a fall of 1 in 80 .
I think you would be very lucky to get 67mm as that doesn't account for collars on pipe. The other thing that gives you difficulty on low falls is the fact that all 110mm plastic is not strait.
You can find yourself with a fall each end and back fall in middle!
Normal good practice would be to have a AAV at highest point of drain .

Even at 1 in 80 you're not going to make it totally within the joist depth (yes your rough sums are correct). Easiest workround (presumably this is a loft conversion) may be to move the loo so the run length is down to 5 metres, if that's not possible then the top of the pipe will have to start above floor level (and be boxed in or hidden under the bath or a cupboard or whatever)

Damn. Nothing is ever simple is it!

I can't shorten the run.
I'm using the space in the Eaves to run the soil above and across the joists. As then I can drop the soil between the correct two joists, to line up with the exterior.

The AAV at the start of a 6.5m run, doesn't lessen the need for the fall I require, does it?really done want to enter macerator territory.

Thanks all. Really appreciate it.

No it doesn't and very wise to stay away from macerator. Presumably building control are involved- give your inspector a ring, see what they reckon. Remember, the Approved Docs give numbers that (if you follow them) are deemed to comply. Doesn't mean you have to follow them all the time.....

Think I might have to run it on a shorter run, in between the "wrong" joists, then do a few turns outside.

82mm soil is available, might help a bit, as will trimming the floorboards a bit for the first metre. Setting the pipe run straight is critical (a string line and plenty of timber supports).

82mm soil is available, might help a bit, as will trimming the floorboards a bit for the first metre. Setting the pipe run straight is critical (a string line and plenty of timber supports).
That combined with the (hopefully) shorter run should work.

Is there any disadvantage to using an 82mm waste.

It will have two bathrooms being fed into it. Though, I doubt both will see much use.

I believe you can only have 1 WC on 75/82, so you're back to 110, but you can reduce fall to 9mm/m if more than 1 WC.

Don't know where other contributors get 1 in 80 from as its 18mm in 1000 i.e. 1 in 55 BUT so you don't get the 1 in 55 fall, so what? Do you think water will stop flowing downhill because of a lesser gradient, no it will still go to the lowest point. The only thing you (and others?) will no doubt question is the deposition of solids along the length, well its not the gradient that moves them along its the 2 gallon flush of water that does it, a bit like a tsunami, and its a question of will it run out of power before it delivers the "goods" to the discharge end, hence the limiting 15m max lengths of pipe stated. The 1 in 55 is not a magic figure where things suddenly stop working but gives gives a reasonable expectation that "things" will be moved along depending how many Nr "ones" flushes compared with Nr "twos", I wouldn't get to hung up on official gradients unless your BC is a "jobs worth"

I believe you can only have 1 WC on 75/82, so you're back to 110, but you can reduce fall to 9mm/m if more than 1 WC.

View attachment 256394

Thank you very much for looking into that for me.

Really appreciate all the help I'm getting on here.

OK, so back to 110mm but, with a significantly shallower fall of 9mm, as its two bathrooms connected.

Strangely enough, that means I've got enough fall even on the 7m run

7" joist = 177mm
Soil Pipe = 110mm
Leaving 67mm for fall.

Min fall for 2 WCs on one soil is 9mm per meter
9mm x 7m run = 63mm

So I've 4mm spare, but that doesn't allow for collars, the fact it has to be dead straight etc!

Need to see this all in person now, and lift some floor boards, to see if I can shorten the run.

Will let you know how it goes.

110mm can be had in 6m lengths to reduce collars if you can get one in. Otherwise use the pipes with the built in socket on one end rather than collars to reduce leak points.

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