# Forum Home Renovation Plumbing  Stormwater - minimum fall?

## gsouth

All, 
After searching the net for about an hour I thought I'd post the question here.... 
I am looking to install a water tank under the deck and connect to the 2 downpipes at either end of the deck. 
Currently the pipes run thru the deck and into the ground.
My plan is to cut them off and run them across under the bearers to the top of the tank (about 9m away).
What is the minimum fall I need to have on the pipes to ensure a good flow? 
Cheers
Geoff

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## echnidna

As long as there is slope you will have flow.

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## Wood Butcher

When I did the shed at my last house I was advised to have at least 5mm of fall for every metre of pipe/gutter.

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## Termite

Irrigation ditches in cotton fields have a fall of 1 foot to the mile. :Biggrin:

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## Iain

Currently having a sh!te fite with our council as our land is 3 metres lower than the drain, they still seem to think that water runs uphill, saga continues...

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## ozwinner

Love the new avatar.
Horse stops mid air, must be the council horse? 
Al  :Biggrin:

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## Iain

Few years ago, it did, I didn't, 8 cracked ribs, 1 broken wrist.
Horse suffered sore bum when I got home from hospital, re admitted, 1 broken foot.
(last one was a lie)

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## ozwinner

Is that you on the horse?
I would have stopped too if I was the horse.  :Tongue:   :Biggrin:   
Al  :Biggrin:

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## echnidna

Is that why yer ride donkeys now. :Biggrin:   :Biggrin:   :Biggrin:

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## elphingirl

A fall of 1 in 100 is adequate in a closed pipe situation, in a larger pipe (250mm diameter) you could go to 1 in 200. In an open gutter, where litter such as leaves and dirt could get trapped it would be safe to allow more like 3 in 100. 
Cheers
Justine

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## maglite

Have you given any thought to disposal if/when the tank fills up?

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## gsouth

maglite,
The overflow is directed back into the stormwater channel... 
Geoff

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## TassieKiwi

People get carried away with this.  If this section of pipe is below downpipes etc the 'head' of this water will push it through this section, even with reverse fall.  To illustrate this better our last house had a syphon system: The kerb on the roadside was only about 1.5m below the guttering, but the pipe had to be about 1m below the kerb outlet as it was buried under the driveway.  I sealed all joints, had an overflow on the downpipe, and it worked a treat.  Note that the system is permanently full when not running. 
That said, most inspectors like to see 'normal' pipes sloping towards the outlet.  Put a 'turn of the bubble' on it if you can.

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## soundman

The other answer is...... as much fall as you can practicaly get.
If thats all the fall you have....  :Biggrin:

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## Iain

If it's likely to build up a retarding basin may be order of the day to save any problems.
Thanks Al, just because I'm 5'2" and 55kg you have to make fun of me :Frown:

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## Christopha

In a closed system you don't need *any* fall at all as long as the outlet, whether it is to the street or into a tank is lower than the inlet..... at my beach shack the rainwater discharges into a large fibreglass tank which is approximately 45 metres from the front of house downpipe.... sealed with a 30mm difference in height... Ever seen/used a water level??????

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## seanr

You dont need any fall!!!! as long as the gutter is higher than the tank which it is!!! i think by what you are saying . The water will always flow , even the gutter can be dead level . If you fail to clean the gutters doesnt matter what fall you have you will have trouble . So once a month spend 15 mins cleaning them .

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## Skew ChiDAMN!!

The main reason for fall is to scour out any detritus.  You don't _need_ fall, but it doesn't take much in the way of sediment to create problems, especially if gal is used in the system somewhere.  And PVC weathers badly, so I don't really want puddles lying around in it. 
Anywhere there are horizontal drains or gutters running over "internal" areas, I want some decent fall.    
Any horizontal runs without easy access, whether above ground or below, I want some decent fall. 
Both to reduce blockages (more shed time) and better get the water away from where it can do damage if something, god forbid, breaks.

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## Ivan in Oz

G'Day Ppl,
When we finally do get rain:confused: 
it is usually ALL or nothing :Rolleyes:  
With the Downpoors we get,
I'd like to have some decent fall,
else there is too much 'back-up' and the water will overflow out of the pipework where it SHOULD be going into, to flow away. 
Result :Redface: 
Flooded ceilings and Decks:mad:

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## TassieKiwi

> G'Day Ppl,
> When we finally do get rain:confused: 
> it is usually ALL or nothing 
> With the Downpoors we get,
> I'd like to have some decent fall,
> else there is too much 'back-up' and the water will overflow out of the pipework where it SHOULD be going into, to flow away. 
> Result
> Flooded ceilings and Decks:mad:

  Sounds like you need an overflow in the system somewhere to cater for this.

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## Christopha

In a "tropical" downpour which we get *all* over Australia no amount of fall will stop your gutters overflowing.... proper design of the building elements will however ensure that wet ceilings or eaves is not a problem. Gutters should have a fall just so they aren't continually wet which promotes rust and growth of all sorts. Fitting IPs' so that the sealed system can be accessed easily will keep things easily cleaned, in reactive soils fitting a Storm Plastics telescopic joint will sort out that problem and painting with a good quality exterior acrylic paint will stop any weathering of the PVC. My sealed system which is now over 20 years old has never failed, every so often, usually during a bit of heavy rain I pop an IP and let the system flush itself.

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## mudguts

i agree that any fall is good enough....worked a treat for me

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## journeyman Mick

> In a "tropical" downpour which we get *all* over Australia .............

  Christopha,
agree with all your other points but I doubt that all of Australia would get downpours of, say, 250mm in 2hrs like you get in a real tropical downpour. 
Mick

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## seanr

Doesn't matter if there is no fall , 1:1 or what ever , the water can only escape at the same rate . If your gutter overflows its because the exit point is too small , nothing to do with fall . I thought you guys were Aussies not Kiwis!!!!

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