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### Why is the slope on a pipe so important?

Posted: **April 30th, 2017, 1:29 pm**

by **RPM**

Another question that comes up from time to time...............

I`m changing things around and want to put a new toilet in upstairs, what is the slope/ angle/ fall on the pipe meant to be and is there a maximum distance this pipe can run?

### Re: Why is the slope on a pipe so important?

Posted: **April 30th, 2017, 4:39 pm**

by **DEFENDER**

the slope should not be too steep, this is to ensure all solids slide along the pipe smoothly

### Re: Why is the slope on a pipe so important?

Posted: **April 30th, 2017, 8:06 pm**

by **RPM**

DEFENDER wrote: ↑April 30th, 2017, 4:39 pm

the slope should not be too steep, this is to ensure all solids slide along the pipe smoothly

Thanks for that and from that I guess there is a minimum and maximum angle to take into account when thinking of the run of the pipe then?

### Re: Why is the slope on a pipe so important?

Posted: **April 30th, 2017, 9:01 pm**

by **Masood**

Justpumps+ wrote: ↑April 30th, 2017, 8:06 pm

DEFENDER wrote: ↑April 30th, 2017, 4:39 pm

the slope should not be too steep, this is to ensure all solids slide along the pipe smoothly

Thanks for that and from that I guess there is a minimum and maximum angle to take into account when thinking of the run of the pipe then?

There is - for soil, there is a fixed fall of 18mm/m with a maximum length of 6m per branch.

32mm basin wastes need a fall of 18-20mm/m with a maximum length of 1.7m, 40mm bath and shower wastes need 19-90mm/m with a maximum of 3m.

This is for a primary ventilated stack. May be different on other systems but I can't remember them off the top of my head!

### Re: Why is the slope on a pipe so important?

Posted: **April 30th, 2017, 9:01 pm**

by **Masood**

Justpumps+ wrote: ↑April 30th, 2017, 8:06 pm

DEFENDER wrote: ↑April 30th, 2017, 4:39 pm

the slope should not be too steep, this is to ensure all solids slide along the pipe smoothly

Thanks for that and from that I guess there is a minimum and maximum angle to take into account when thinking of the run of the pipe then?

There is - for soil, there is a fixed fall of 18mm/m with a maximum length of 6m per branch.

32mm basin wastes need a fall of 18-20mm/m with a maximum length of 1.7m, 40mm bath and shower wastes need 19-90mm/m with a maximum of 3m.

This is for a primary ventilated stack. May be different on other systems but I can't remember them off the top of my head!

### Re: Why is the slope on a pipe so important?

Posted: **April 30th, 2017, 9:09 pm**

by **Masood**

PS - if you want more details I'll happily dig them out. As Defender pointed out above, the fall must not be too steep, otherwise the liquid waste will runaway, leaving the solids sitting on the bottom of the pipe. It's called a self-cleaning gradient...

Another possible problem is self-siphonage - if the gradient is too steep, the slug of water released when a plug is pulled can gather enough velocity to pull the water out of traps by creating a vacuum behind it.

### Re: Why is the slope on a pipe so important?

Posted: **May 1st, 2017, 9:53 am**

by **jonnyswamp**

I've always gone with 1 in 40 for soil and what ever looks right for waste pipes

1" in 40" was what I was taught in college

### Re: Why is the slope on a pipe so important?

Posted: **May 2nd, 2017, 5:49 pm**

by **Rocketmanbkk**

Good info that Mas.

Thx

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### Re: Why is the slope on a pipe so important?

Posted: **May 2nd, 2017, 8:03 pm**

by **Mr KJ**

Mcguires law innit?

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### Re: Why is the slope on a pipe so important?

Posted: **May 2nd, 2017, 9:08 pm**

by **REDSAW**

who is mcquier?

### Re: Why is the slope on a pipe so important?

Posted: **May 3rd, 2017, 10:28 am**

by **Masood**

jonnyswamp wrote: ↑May 1st, 2017, 9:53 am

I've always gone with 1 in 40 for soil and what ever looks right for waste pipes

1" in 40" was what I was taught in college

It's an interesting one Jonny. The City & Guilds / NVQ books all state 18mm/m which I'd around 1:55, but Building Regs Doc H states 1:40 so I guess a fall in that range is going to be OK.

### Re: Why is the slope on a pipe so important?

Posted: **May 3rd, 2017, 5:19 pm**

by **jonnyswamp**

It stuck in my mind (one of the few things that did) but in reality, I've seen all sorts of gradients working fine

### Re: Why is the slope on a pipe so important?

Posted: **May 3rd, 2017, 5:27 pm**

by **REDSAW**

i thought it was 18mm per meter !!

then came the sun and sagged the pipes and buggered all calculations up

### Re: Why is the slope on a pipe so important?

Posted: **May 4th, 2017, 5:40 pm**

by **REDSAW**

Go on, explain please

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### Re: Why is the slope on a pipe so important?

Posted: **November 1st, 2017, 8:29 pm**

by **kozak1968**

Different materials of pipework will have different frictional resistance aswell which will require a different gradient.

### Re: Why is the slope on a pipe so important?

Posted: **November 1st, 2017, 10:20 pm**

by **Last Plumber**

jonnyswamp wrote: ↑May 1st, 2017, 9:53 am

I've always gone with 1 in 40 for soil and what ever looks right for waste pipes

1" in 40" was what I was taught in college

I was taught same.

1:40 for 4"

Gives self cleansing velocity

### Re: Why is the slope on a pipe so important?

Posted: **November 2nd, 2017, 9:42 am**

by **oz-plumber**

For us..

Waste pipes - 40mm & 50mm: 1:40

Soil Pipes - 65mm, 80mm & 100mm: 1:60

These are minimum falls.

### Re: Why is the slope on a pipe so important?

Posted: **November 21st, 2017, 12:26 am**

by **REDSAW**

not sure on the pipe but, i know why the slope was on the bridge!

(no insult intended and i still dont really get it

)

### Re: Why is the slope on a pipe so important?

Posted: **November 23rd, 2017, 12:42 pm**

by **Davey**

Ha ha. Reminds me of the boiler training workshop at Inverness college where you get your OFTEC quals.

The main toilet is directly above the workshop and all the soil pipes drop down vertically through the workshop.

Think it was originally meant to be a store room or something.

Some interesting noises........