# Forum Home Renovation Concreting  Size of stormwater pipes

## Flok

Hi all,  
What diameter plastic stormwater pipes should we use to channel rainwater from some small  (future) drainage pits and maybe a long channel drain along garage? 90 or 100mm? The pipe would be in the front yard, but should never get any car traffic. The 90mm ones have much thinner walls and are almost half the price  :Eek:

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## president_ltd

i think you answered your own question....... 
i'd only ever put in 100mm.

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## Oldsaltoz

I would avoid any thin walled pipe, you just know it's going to fail and have to be dug up and repaired again and again. 
Good luck.

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## an3_bolt

100 DWV - more impact resistant. Go cheap (90mm) and you will find out why most don't use it.
Stay happy - use 100 DWV. :2thumbsup:

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## jonesyGT

Im in the middle of repairing mine @ the moment
Its 175mm OD, most have never heard of that size pipe
The reason Im repairing it is because when our place was built, they laid the pipe about 30mm underground, then I dig to put some plants in the ground & "_CRACK_".
Has now taken me 3 days to sort out a repair for this 
Id suggest use something that is readily available like 150mm & make sure its deep enough that you cant hit it.

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## Flok

Thanks for all your replies!  
Jonesy, what is OD? Diameter? How terrible it was only 3cm deep! Was there not enough fall so they couldn't burry it any deeper??  
Strangely too, ALL our numerous concretors and landscapers have ONLY ever quoted and suggested a 90mm pipe.  :Mad:  Even after we asked them which size they would suggest.

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## Danny

100 mm PVC Outside Diameter (OD) is actually 110 mm but 90 mm PVC is measured as (and is) 90 mm OD. The 100 mm PVC pipe has +40% more volume than 90 mm PVC. 
I regularly come across rainwater tank 'wet system' plumbing that has used 90 mm PVC buried between the house and tank. This, I assume, is only used to cut costs on packaged quotes. The customers are none the wiser but an ethical plumber who has pride in his work would not bury 90 mm.  
If you are considering burying the pipe as per your intended use, I would not be using 90 mm.   
BTW, this thread is more applicable to the Plumbing forum.  :Bump:

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## jiggy

90mm for stormwater and 100 mm for sewerage is the standards, makes it easier to identify which pipe is which !!

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## intertd6

I  think you will find plumbing pipes are measured & specified by ID inside diameter, the ID has to be known to work out flow & capacity for calculations.
regards inter

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## Danny

So what is the ID of: 
90 mm PVC?
100 mm PVC?
PVC Class 12 20 mm?
PVC Class 12 25 mm?
PVC Class 12 32 mm? 
PVC Class 12 40 mm?

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## Compleat Amateu

Only 3 things to be said:
1.  Do the rainfall/runoff calculations for your area and that'll tell you the minimum volume of water you have to be able to shift ... no doubt some people in Brisbane and Melbourne now now the truth of that ....
2.  Look at the standards for depth of burial and pray like crazy that you can meet your drainage requirements at that depth
3.  Don't, don't, don't bury 90 mm PVC.  Even if it meets #1.

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## Flok

> Only 3 things to be said:
> 1. Do the rainfall/runoff calculations for your area and that'll tell you the minimum volume of water you have to be able to shift ... no doubt some people in Brisbane and Melbourne now now the truth of that ....
> 2. Look at the standards for depth of burial and pray like crazy that you can meet your drainage requirements at that depth
> 3. Don't, don't, don't bury 90 mm PVC. Even if it meets #1.

  Thanks, but not sure I quite understand what you mean. Why not 90mm - I agree not to use 90mm, but I am not basing that decision on much information other than it looks tougher to me - can you please tell me what are the bad but non-obvious things about it?  
Also, can you point me in the direction of standards for depth of burial ?  :Frown:  
And by rainfall/runoff calculations for your area - how do I go about that? The rainfall info on the BOM web site?? How do I marry the rainfall with roof runoff and what is the formula?  :Cry: 
Thanks

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## Bloss

> 90mm for stormwater and 100 mm for sewerage is the standards, makes it easier to identify which pipe is which !!

  Once I've broken the pipe (cos that's what we always do!  :Redface:   ) always been able to tell - sniff test works every time . . .  :Wink:

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## Danny

> Why not 90mm - I agree not to use 90mm, but I am not basing that decision on much information other than it looks tougher to me - can you please tell me what are the bad but non-obvious things about it?  
> Also, can you point me in the direction of standards for depth of burial ?  
> And by rainfall/runoff calculations for your area - how do I go about that? The rainfall info on the BOM web site?? How do I marry the rainfall with roof runoff and what is the formula? 
> Thanks

  90 mm is flimsy. If there is ground movement due to changing climatic  conditions as could be reasonably expected since the recent heavy rain and subsequent  high subsoil moisture levels, 90 mm pipes could crack. 
You need to check your local regulations. You also need a slope to allow  for an adequate sediment flushing velocity. This should be a minimum  1:100 but again, check your local regulations. 
Your details show you are in Sydney. Unless it has been recently updated (and many areas have), your 1:20 year Average Recurrence Interval (ARI) is 214 mm rainfall in an hour or a rate of approximately 3.5 mm per minute. Calculations are 1mm of rain on 1 sq m = 1 litre. The garage roof area is easy to work out. The area feeding your future drainage pits is obviously not as there are no details posted as to where this water will be coming from. Also, where will the pipe feed to?

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## Flok

Thank you very much Danny! I suppose these new pipes and drainage are just to capture the heavy rainfalls which quickly create pools of stagnating water next to the house slab and this water doesn't actually come from roofs.  (Our roof (house/garage) run-offs are captured by gutters and they all go into a water tank.) So this is for those heavy downpours and considering that we have a bit of a problem with those cut-in sections at the front of the house, we thought if we should help drain the water away by installing an inground system of pipes, drain grates and similar  :Confused: . I probably explained this very poorly to start with  :Blush7: . 
Hope this helps clarify our situation  :Blush7:

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## Danny

I have misunderstood your question. 
Unless I am still wrong, it now seems that you are referring to AG pipe draining wet areas to then drain to the stormwater system. For this, you need AG pipe, geotextile material and gravel in the trench.  
It is always handy to post some photos showing the problem areas 'in flood'.  
I would also ask the Mods to transfer this topic to the Plumbing section. Woodwork is fortunate to have the contributions of a Sydney plumber who uses the very appropiate handle of Wonderplumb. He may not see this post in this section. 
Plumbing also looks simple to most people but that is only because a good plumber can make it look simple. Even running AG pipes can have complications. 
It is essential that water not be allowed to pool next to a house's exterior although the building code does specify that water must not pool within 1 metre of the house. You are wise to address the problem.

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## Pulpo

I have used 90mm to fill my rain water tanks. 
Its a flooded system so the water stays in the pipe until rains then pours out the end into the tank. 
Do have a flush system of sorts, so can be flushed anytime. 
I have since been told this is below standards it should be 100mm. 
But am not going to dig 30 metres, 500mm to replace. 
I did not know any better but will only use 100mm for now on. 
What price difference are we talking $100? 
The standards for stormwater is 90mm.

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## intertd6

> So what is the ID of: 
> 90 mm PVC?
> 100 mm PVC?
> PVC Class 12 20 mm?
> PVC Class 12 25 mm?
> PVC Class 12 32 mm? 
> PVC Class 12 40 mm?

  That darn plumbing lecturer has got me caught out on the 90mm pvc SW & a couple of others.
regards inter

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## stevoh741

totally agree with earlier post, 90mm stormwater, 100 for turds etc

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## Danny

I got caught out in the past by cross referencing stated PVC pressure pipe sizes and then referring to friction loss/flow rate charts.  
The one I was initially caught out on was 32 mm class 12 PVC pipe, all my test results were unbelievably good. I now know that  32 mm class 12 PVC has an internal diameter of 37.5 mm.  
20 mm ID is 23.7mm, 25 mm ID is 29.8 mm, 40 mm ID is 42.8 mm and 50 mm ID is 53.7mm.  
Great when you are doing comparative theoretical V's field tested flow rates for a new product! 
For 90 & 100 mm PVC, the respective ID's are 86 mm & 102 mm.

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