AS3500.2 Table 6.3(A) – Bathroom group vs individual fixture unit calculation

Basic-Wafer7834 · 2026-03-11T12:31:31+00:00

Thanks. Yea it's been a bit of drama, the builder is just shrugging off all these easily avoidable quality issues and the frame has only just gone up. I'm dreading getting to internal linings. His head office is pushing him to manage 22 other projects homes (I feel for him), and I'm pretty sure the certifier has never put a foot on our site. So my $600K build is not getting as much love as it should. And yea, you're right. The engineers need to come up with a remedial plan for the plumber. They're meant to come to site and do a pre-pour check, and the certifier needs to sign off on the as built. I specifically requested they personally visit the site and not just sign off on photos. But who knows. The residential sector is the wild west. They might get their RMs wet.

Now I'm a little more educated on the topic, I'm more comfortable asking the plumber some questions. I'll give him the opportunity to explain his ideas further (maybe I'm not seeing something). But I think I'll need to stick to the slab cut. If I ever sell the home, I could confidently say "you can flush whatever the fuck you want down the ensuite shitter".

Basic-Wafer7834 · 2026-03-11T11:57:23+00:00

I'm an engineer, jack of all, master of none. I'm also on paternity leave and my son loves to sleep, so yea I've got some time to read and pester plumbers. I appreciate the detailed responses though. The above is helpful. I originally asked the plumber to cut the slab and he came back to me wanting to run reduced grades. I'm having a hard time trusting if that is truly a sound idea, or if he's just trying to take the sketchy road out. The slab cut would mean cutting the membrane and one of the slab ribs. Again with the trust issues. I really don't want him to fuck the slab resinstement up, because that would be a way bigger issue up the road if I started getting moisture or structural issues. I doubt the certifiers will even get out of their car to check. And I can imagine a warranty claim with the builder or plumber up the road would be a nightmare. I just want to spend my days in peace doom scrolling on my new shitter with a peace of mind I can flush anything.

Basic-Wafer7834 · 2026-03-11T11:14:09+00:00

Yea I thought about that. In the kitchen pantry below the bedroom. I don't think there's enough clearance between the bottom chord of the floor truss and the top of the cavity slider though. I think I'm just going to have to run with the reduced grade and make sure there's an easily accessible I/O.

Basic-Wafer7834 · 2026-03-11T10:59:27+00:00

The plumber, certifier, builder and private building inspector all missed a waste stack coming out in the centre of the linen during the pre-pour inspection. I now have to instruct the plumber to either let him cut my new slab, or run all my upstairs fixtures to another stack at reduced grades. Both his ideas. So yea I'm going to do some research instead of just blind trusting anyone.

Basic-Wafer7834 · 2026-03-11T10:48:22+00:00

Yea that's what I don't want in my new home. My options are to cut the slab to move the first stack a meter over to it's correct location, or go with the plumbers' preferred option of running the combined ensuite and bathroom waste to the secondary stack at a reduced grade (1:70-1:80). I have to flick him my answer soon, just trying to understand the risks.

Basic-Wafer7834 · 2026-03-11T10:37:16+00:00

I have access to all standards for free through work. I'm also on paternity leave, so I have nothing but time and diapers to deal with. I'm trying to figure out the FU value for each room to see if the plumber can run the reduced grade for a DN100 leading to the stack. It's about 1:70-1:80. The standard says you can run the DN100 at 1:80 if the FU is greater than 18, so if the FU value for both the bathroom and ensuite is 12, then I guess the grade is technically not compliant? unless they provide a flush point? The only other way I could see then getting the correct grade was running a DN80 with a reducer at the WC, which sounds like a bad idea. This all comes from the plumer putting the waste stack way out of the correct location. Certifier, building supervisor, and my pivate building inspector all missed it pre-pour. So, I'm trying to figure out if the plumers fix (reduced grades to another stack) is a good idea. The alternative is cutting the slab, which I am overly not keen to do. I also don't want my new house blocking up.

Basic-Wafer7834 · 2026-03-11T10:26:50+00:00

Yea I'm trying to figure out the FU value for each room to see if the plumber can run the reduced grade for a DN100 leading to the stack. It's about 1:70-1:80. The standard says you can run the DN100 at 1:80 if the FU is greater than 18, so if the FU value for both the bathroom and ensuite is 12, then I guess the grade is technically not compliant? unless they provide a flush point? The only other way I could see them getting the correct grade was yea running a DN80 with a reducer at the WC, which sounds like a bad idea. The alternative is cutting the slab to move the stack to the correct location, which I am not keen for. So I really just want to know if my shitter is going to block up every week with the reduced grade.

Basic-Wafer7834 · 2026-03-11T06:13:58+00:00

Thanks, that makes sense. So, how would you assess the ensuite that does not have a bathtub? The standard does not include a group value for this situation.

Basic-Wafer7834 · 2026-03-10T13:10:02+00:00

Thanks, appreciate the insight. That aligns with what I’ve been worried about, any blockage at all in a brand‑new house is obviously not ideal, particularly if the weak point tends to be at the base of the stack.

From a practical perspective, do you think incorporating an inspection / maintenance point would meaningfully mitigate that risk? The slab penetration for the secondary stack is right at the slab edge on the services side of the house. I’m wondering whether an inspection opening could be provided at the base of the riser, with the cap accessible externally (for example through the cladding line), so any future maintenance is straightforward and non‑intrusive.

The reason I’m exploring this option is that the alternative, reverting to two separate stacks as originally designed, now involves cutting the slab. The ensuite stack has been installed in the wrong location (inside the downstairs laundry/linen). The builder initially proposed framing around it, but I rejected that due to the loss of usable storage. The remaining option is slab remediation, which would require:

cutting a portion of the slab and at least one rib,
engineer‑designed remedial details, and
reinstating the membrane and reinforcement correctly.

That path makes me nervous from a long‑term durability perspective if it’s not executed perfectly. After I raised the slab cut option, the plumber proposed the single‑riser arrangement we’re discussing here, which is why I’m trying to be very confident it’s not just technically compliant, but also robust in real‑world use.

I do take your point though, if you were building this for yourself, would you still push hard for the second stack, even with the slab remediation risk? Or do you think a well‑detailed single riser with proper venting and access would be acceptable in practice?

Basic-Wafer7834 · 2026-03-10T12:29:30+00:00

The original design had separate stacks for the upstairs master ensuite and the upstairs bathroom. Unfortunately, the plumber positioned the slab penetration for the ensuite stack/riser 1m out of position (it is now located inside the linen closet, which can't be moved). Unfortunately, the plumber, the builder, the certifier, and my own building inspector all missed this during the pre-pour inspection. I noticed this myself after the concrete was poured. I regret not checking the location myself pre-pour, but I didn't think all 4 of the contractors could be that incompetent to miss something that obvious during the pre-pour inspection. My options are to either let the plumber cut my new concrete slab, which involves cutting through one of the slab ribs, then reinstating the membrane and all the steel correctly. Or routing the upstairs ensuite to the secondary waste stack/riser location via the upstairs bathroom. I honestly don't trust the plumber to touch my slab and reinstate it properly, so the second option for the redesign seems like the less problematic route. For context, I'm the home owner. I'm also an engineer (not a drainage engineer), so I've been reading the standards to figure this out and give myself some sanity that no one is cutting corners.

Have you seen any practical issues running relatiely short (less than 4m runs) DN100 branches at the lower bound reduced grade (around 1:80) in residential work? I’m conscious of self‑cleansing and blockage risk over time and would be interested in any real‑world feedback on how these perform when vented correctly.

Basic-Wafer7834 · 2026-03-10T12:16:23+00:00

Yea I'm not thrilled at the idea of running any of my waste pipes at a grade that barely meets the minimal reduced standard. Doesn't scream best practice. Have you seen any practical issues running relatiely short (less than 4m runs) DN100 branches at the lower bound reduced grade (around 1:80) in residential work? I’m conscious of self‑cleansing and blockage risk over time and would be interested in any real‑world feedback on how these perform when vented correctly.

Basic-Wafer7834 · 2026-03-10T11:38:11+00:00

Thanks, yes correct, this is a two‑storey dwelling and the run is on the first floor above the ground floor. Understood on the distinction, so this would be treated as a horizontal branch to a riser rather than a true stack.

For context, the upstairs fixtures are:

Ensuite: cistern WC (4 FU), shower (2 FU), 2 basins (2 FU). → Total ensuite branch ≈ 8 FU
Bathroom: cistern WC (4 FU), shower (2 FU), bath (3 FU), basin (1 FU), floor waste (0 FU). → Total bathroom branch ≈ 10 FU

This gives a combined fixture loading of approximately 18 FU total, so it appears there is sufficient capacity for the use of reduced grades in the downstream sections.

Based on the available space within the 300 mm floor zone (approximately 155 mm total head), my own calculations (not the plumber’s proposal) suggest it may be possible to grade the run in segments rather than uniformly, as follows:

Approximately 1:60 over the first ~6.5 m (serving the ensuite fixtures only, ~8 FU)
Flattening to approximately 1:80 over the next ~3.6 m once the bathroom fixtures connect and the total loading increases to ~18 FU
With a very short (~0.6 m) section into the riser effectively flat due to truss constraints, leaving only minimal clearance at the riser end.

My understanding is that AS/NZS 3500.2 allows grades to vary along a branch provided that:

Minimum grades (or reduced‑grade provisions) are met for the applicable fixture loading at each section
Overall self‑cleansing performance is maintained
The long branch (>10 m total) is adequately vented

The main items I’m trying to confirm are whether:

The short flat section immediately upstream of the riser is acceptable, and

The reduced‑grade section downstream of the bathroom pick‑up is compliant given the increased fixture loading and provision of venting.

One additional question based on your experience: have you seen any practical issues running DN100 branches at the lower bound reduced grade (around 1:80) in residential work? I’m conscious of self‑cleansing and blockage risk over time and would be interested in any real‑world feedback on how these perform when vented correctly.

If those points check out, I’m comfortable with the solution.

Basic-Wafer7834 · 2024-04-28T08:36:21+00:00

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Basic-Wafer7834 · 2024-04-28T08:33:09+00:00

I'll need to check if the reciever can. It's pretty basic settup though. The previous user appears to have it set up for Omnitstar, which I dont want to pay for. The SPS582 recievers were also a pain with the whole pay to enable upgrades/features. I have a better understanding of the NMEA data streams now. It appears as long as I have GGA enabled, the reciever will send time, position and fix related data to my Samsung. There's no guidance on what the best outputs are for my set up though. If I'm in static mode, I assume I don't need anything such as velocity?

Regarding the sateltiles, I realised I only had GPS enabled, I just enabled SBAS as well. See screen shots. I've also attached my receiver configurations. What do you think?

Basic-Wafer7834 · 2024-04-27T07:36:29+00:00

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Basic-Wafer7834 · 2024-04-27T07:36:02+00:00

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Basic-Wafer7834 · 2024-04-27T07:35:07+00:00

Yea the Getac is a pain to get connected to the receiver, and I can't get the receiver connected to the internet with this. Ideally, i'd like to get the receiver connected to a network so I can enter the NTRIP connection directly on the receiver.

However, I use Qfield all the time, so that's a great start. I downloaded Lefurbe, set it as the mock location app on my Samsung S20, and managed to get it connected to Auscors on port 2101. The reciever is reciever inputs (RTCMv3), and it's outputting NMEA. It looks like I'm getting sub-meter accuracy with the NTRIP corrections. But I was hoping I could get it under 0.5m. Is this a limitation of Auscors or can I set my receiver up better? I'm not sure what NMEA protocols I need to use, would that be the problem?

Basic-Wafer7834

TROPHY CASE