Concrete Perimeter Help and advice

Adeez · May 22, 2023 11:04 pm

I am trying to work out what correct concrete perimeter to have done on my home.

There is conflicting advice and recommendation everywhere. Is anyone able to help or provide some guidance please?

I’ll put all concerns and thoughts below which are affecting this decision:

I require a concrete permitter done around for a house in West Melbourne. It is on highly reactive clay soil. The easiest solution for this would be to go by the NCC. Which recommends 50mm over the first 1m
Low intensity rainfall areas state 25mm over the first 1m is acceptable. The NCC states Melbourne in general is 132 mm/hr - is this outdated too?
Concreters I’ve spoken to have recommended 25-30mm. Some laughed when I said I think I require 50mm. But they also said we’ll do whatever you want us to do based on my specifications...
I have asked my builder what they recommend. As I have heard stories of warranty be affected or void by incorrect concrete placement. The builder has referred to the CSIRO’s slab and maintenance document, which recommends 1:60 (17mm over the first one metre). From my research this recommendation is outdated!
There are further concerns regarding a 50mm slope. I believe the water would pool against the fence to the Neighbours property. For a 50mm slope I cannot see this being viable unless there is a spoon drain going all away from the house. Am I right?
Wouldn’t it be most effective to have the concrete done in manner which merely channels the waters correctly into the drains? I have plenty all around the house…But having it structurly designed in such a manner to cater to the specific house, would go against NCC recommendation.
I have walked around my area extensively. Many homes are non compliant or do not have correct slope. Are they voiding their warranty?

Little bit lost here!

Any advice would be great. Some people are saying I'm looking into this much...But I'm trying to get this right

Thank you!

SaveH2O · May 23, 2023 7:37 am

Adeez

I require a concrete permitter done around for a house in West Melbourne. It is on highly reactive clay soil. The easiest solution for this would be to go by the NCC. Which recommends 50mm over the first 1m.

The NCC is the primary document and correct.

Quote:

Low intensity rainfall areas state 25mm over the first 1m is acceptable. The NCC states Melbourne in general is 132 mm/hr - is this outdated too?

Melbourne's 1:20 year rain intensity use to be referred to as an Average Recurrence Interval (ARI) which was 130 mm/hr but then the boffins started using other terms such as an Intensity Frequency Duration (IFD) etc. I think that the 132 mm/hr is an IFD reference. Regardless, Melbourne is not in a low rainfall intensity area.

There is little doubt that Melbourne's various rainfall intensity figures will be higher in the coming years as the South Polar - Equator Pressure Gradient continues to weaken.

Quote:

Concreters I’ve spoken to have recommended 25-30mm. Some laughed when I said I think I require 50mm. But they also said we’ll do whatever you want us to do based on my specifications.

Concreters are generally a slap happy bunch with little regulatory knowledge.

If you have slab heave, a builder's get out of jail free card will be a non compliant concrete perimeter slope. The biggest get out of jail free cards are having gardens against the house wall or there being no concrete perimeter.

Quote:

I have asked my builder what they recommend. As I have heard stories of warranty be affected or void by incorrect concrete placement. The builder has referred to the CSIRO’s slab and maintenance document, which recommends 1:60 (17mm over the first one metre). From my research this recommendation is outdated!

It is staggering that a builder would refer to this non existent regulation given the high incidence of slab heaves in Melbourne's western and northern suburbs a decade ago. Back then, builders were giving homeowners the CSIRO booklet without advising of the incorrect slope advice but then using the non regulatory slope as their get out of jail free card.

What else does the builder not know? No point asking him!

Quote:

There are further concerns regarding a 50mm slope. I believe the water would pool against the fence to the Neighbours property. For a 50mm slope I cannot see this being viable unless there is a spoon drain going all away from the house. Am I right?

Yes, you are right. Not having drainage at the edge of the path also allows the pooled water to seep under the path which could/will/ cause the reactive soil to expand and heave, causing the path to reverse slope.

When doing drainage calculations, you also need to be aware of the affect of additional wind driven rain draining down the wall during heavy rain..

Quote:

Wouldn’t it be most effective to have the concrete done in manner which merely channels the waters correctly into the drains? I have plenty all around the house…But having it structurly designed in such a manner to cater to the specific house, would go against NCC recommendation.

The NCC is a performance based document that states Performance Requirements. This allows the use of a Performance Solution that has been certified by an industry expert as meeting the Performance Requirements within the NCC.

Quote:

I have walked around my area extensively. Many homes are non compliant or do not have correct slope. Are they voiding their warranty?

Yep!

Adeez · May 23, 2023 8:44 pm

Thank you. Ridiculously helpful. Although can’t believe how much the ‘professionals’ I talk to don’t know or don’t care about these things.

SaveH2O · May 23, 2023 9:16 pm

groundzero is the forum's super helpful guru on this subject. You can access his posts and discussions by visiting his profile.

https://www.homeone.com.au/member/groundzero

ponzutwo · May 24, 2023 7:48 am

I noted the same discrepancies 10 years ago, so I reached out to Steve Lawler who wrote the csiro guide, and his advice was the NCC I the prevailing document. He had advised that a lot has been learned since the csiro guide was written and I was written at a time where there was ambiguity in the ncc and industry.

I raised this with my builder that their doxume T's were outdated. They didn't care. And 10 years later the outdated csiro guide is still being issued.

ponzutwo · May 24, 2023 7:57 am

SaveH2O while having a drainage solution at the ends of a path is the gold standard, the heaving of the path reversing the slope is a big maybe despite what the potential movement of a soil will be. Ive only seen this extreme situation when the path was Installed flat to begin with and then, the movement only ever needed to be ever so slight.

Not saying it shouldn't be a consideration, but for the most part, if the fall is compliant or more, and your join between the path and the house is sealed properly, the path will perform as expected.

SaveH2O · May 24, 2023 9:19 am

Hi ponzutwo. In the past, I have replied to and communicated with some Homeone members who have had this happen, If they still visit Homeone and read this thread, maybe they will post of their experience. The bottom line is that houses are expensive and having adequate drainage to stop water pooling on highly reactive clay is cheap insurance.

ponzutwo · May 24, 2023 9:52 am

SaveH2O

Hi ponzutwo. In the past, I have replied to and communicated with some Homeone members who have had this happen, If they still visit Homeone and read this thread, maybe they will post of their experience. The bottom line is that houses are expensive and having adequate drainage to stop water pooling on highly reactive clay is cheap insurance.

no doubt, though it would be good to know whether their paths were compliant to begin with. The guides or the NCC (IIRC) don't note the collection of that run off as a going concern. Not saying it shouldn't be, but if the path is installed correctly, not having a spoon drain to collect wont be a get out of jail free card for the builder.

"cheap insurance" objectively sure. The quotes for concrete around my house without any additional spoon drains or required plumbing to connect to storm water came in at between 16k -22k. The full perimeter run off coverage of strip/spoon drains isnt an insignificant cost on top of this. The unfortunate reality is that people are building to the ends of their budgets without much consideration for the concrete let alone the drainage and have zero left n the budget when they learn about this. So the proposition is unrealistic for most.

Again, not saying that it shouldn't be done, or not a good idea to do. But if you could only get the concrete and not be able to afford the strip or spoon drains, the risk of foundation damage is still reduced, rather significantly. For a highly reactive clay to swell to its maximum and have a material effect on the path the prevailing conditions would need to be rather persistent, The clay expands, but it shrinks back too. So under "normal" conditions, for most its not an issue.

WE know about installing concrete perimeters because of what was happening to thousands of houses without these protections. While edge cases exists on the lifting of paths without Propper collection of run off, were not hearing about the same problems with perimeter paths as we did houses. rather the opposite. If the outcomes were as poor (which they should be if theyre going to direct water towards the house from heave without run off collection) then we'd be hearing a lot more about it today is my guess.

SaveH2O · May 24, 2023 2:12 pm

ponzutwo

The guides or the NCC (IIRC) don't note the collection of that run off as a going concern.

This is a very good point.

Abutting walls are factored in roof drainage calculations within AS/NZS 3500.3 yet a wall that has a far greater surface area than a one metre wide concrete path it abuts will receive more water during wind driven, heavy rain when facing the weather.

.

Adeez · May 29, 2023 12:41 am

Great discussion here folks...hopefully will be helpful to others also doing same work also.

I do have a follow up question. The distance of weep holes to concrete.

Is the minimum 75mm?

Should it be more if can be?

And given that the DPC is typically below the weephole, the minimum with concrete perimeter installed would continue to be 75mm? Otherwise 150 without...

Am i understanding this right?

SaveH2O · May 29, 2023 9:07 am

Yes, unless you are in a low intensity rainfall area or the area is protected from rain.

You would have to have a bad drainage issue if the 75mm gap plus the path slope was not enough but if you have a rainwater harvesting system that diverts to a water tank via a standard (sub standard) charged or wet system, just be aware that the stormwater pipes that would normally also drain surface water are invariably annexed for the wet system.

Do you have access to NCC Part 2 or can you download it? I can email you a copy if not.

A forum search should also bring up the regs as i know that I have posted them previously.

Found one: viewtopic.php?p=1756378#p1756378

The above link gives information that is still current.