Grate Designs

Oi Uncle, new BPC note out on balconies — SD 02. Everyone’s shouting. But here’s what got me: they’ve drawn a typical detail. An actual picture, from the regulator. Figure 1(b). Am I allowed to just build that? /Shonky

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My dear Shonky,

Sit down. Pour something. You’ve just asked the best question of your career, and you’re not going to enjoy the answer.

Start with the word that does the work. The regulator has published a note on balconies. Very tidy, mobile-friendly. Count the musts. The plumber must obtain a copy of the approved design before commencing. Any variation must be assessed by the surveyor and the permit amended before work proceeds. Above-ground stormwater piping from balconies must be classified as roofing stormwater work. It must be installed by an accredited person under a licensed plumber. The overflow invert must sit not less than 20 mm below the adjacent internal floor substrate. The substrate must have a fall of 1:80.

Firm as a plumb bob. Now here’s the sentence carrying the whole load.

Typically responsible for design

“The architect, designer or engineer are typically responsible for the preparation of balcony designs.”

Typically.

Not must. Not shall. Not “is responsible”. Typically — the word you use for weather, not duty. It’s the one place in the document where somebody had to be named as the author, and that’s exactly where the voice goes soft and sits down. See the pattern and you can’t unsee it: must turns up wherever the obligation already exists in the Act, the Regulations or the Code, and the note simply repeats it and takes credit for the firmness. Typically turns up at the single point where the law says nothing, so there’s nothing to repeat.

The note says the architect, designer or engineer typically prepares the design. Under the Regulations, design work is a licensed plumber’s function, and this is roofing (stormwater) work. The one man whose licence includes designing it is the one told to fetch a copy of it.

Go figure the figure

And that’s what makes your question so beautiful, son, they’ve drawn you a picture, talk about leadership by example! If you lodged it, it’d bounce. Since they published it, we frame it.

Behold it. Figure 1(b). Typical Overflow Detail. Straight from the regulator. Not a manufacturer’s brochure, not some consultant’s PDF — the Commission’s own drawing, on the Commission’s own website.

If that’s the detail in the approved design, you build it. Exactly. Tile for tile, fillet for fillet. If a TikTok inspector squints at your balcony, you point at the webpage and say: that’s the detail, and it isn’t mine. Don’t add a skirting tile they didn’t draw. Don’t add a reglet they didn’t draw. Don’t dimension anything they left undimensioned. You didn’t design it, you built it, and any variation would have needed the surveyor’s blessing and a permit amendment anyway — the note says so in the imperative.

That’s your shield, Shonky. Now let me show you the hinges on it.

Look at what’s drawn. The waterproofing turns up and terminates beneath the flange of the overflow, out in the open, no cover flashing, no protection, nothing over it. Who, in the whole history of this trade, leaves a membrane exposed in the weather without skirting tiles and where a bloke with a broom comes past twice a year?

Then get out your scale rule and try to build it. The Code wants the overflow invert not less than 20 mm below the internal floor substrate. It wants a minimum 70 mm step down from inside to out. It wants a 70 mm monolithic hob at the perimeter. It notes the membrane termination height under AS 4654.2 may be more than 70 mm, and may be extended above the concrete set down. And the practice note adds that the overflow must sit above the primary outlet’s maximum hydraulic head, with “sufficient” freeboard beneath the termination.

Five constraints, one vertical dimension, and Figure 1(b) dimensions not a single one of them.

And here’s the one that’ll keep you awake. Have a hard look at what that membrane is sitting on. Go on — try. There’s a tile, there’s a couple of grey bands, and not a label between them. The Code is now very particular about this: the membrane goes directly onto the structural substrate, concrete to AS 3600, and a tile bed or a screed doesn’t count. So which is it in the picture? Nobody can tell you. It’s a typical detail that declines to identify the one layer the Code cares most about.

Then look at what the membrane’s doing. They’ve labelled “Fillet” twice on that drawing. One’s where a fillet belongs, at the foot of the wall, then folded the membrane through a dead 90° at the junction. The other’s at the top of the upstand, exactly where the waterproofing standard’s own detail puts a sealant bead under a mechanically fixed flashing. So either they’ve called the sealant a fillet, or they’ve terminated the membrane with a fillet and no sealant.

Even the picture can’t follow its own advice. The membrane runs up the wall, wrapping right round the inside of the hole and out the far side, lining the whole aperture like wallpaper. Every other trade on earth stops the membrane at a flange and lets the flange do the talking — a made thing, clamped, testable, replaceable. Not here. Here the watertightness of a plumbing component hangs on how well a bloke with a brush dressed a membrane into a hole at the wettest, tightest, most awkward spot on the balcony.

The best part? The same note lists “incorrect use of leak control flanges” as one of the reasons balconies fail. Names the villain on page one, then leaves him out of the picture entirely.

The caption is my favourite part. “Pipe or slot Overflow designed as per AS 3500.3 clause 3.8, 1% AEP.” Go and read clause 3.8. It gives you the storm event for the design, and not one word on the overflow itself: how high, how wide, how high the freeboard sits so the spill meets the hydraulic capacity for the 1% AEP. For that you’re told to use the roof provisions, and when you turn to them there’s a note waiting: vertical piped overflow outlets and covered overflow devices are not covered by this document.

The certificate isn’t yours

And where drainage doesn’t comply with the deemed-to-satisfy clause, the Code says you need a performance solution — which, per the very same note, must be attached to the plumber’s compliance certificate. Now, you’ll be feeling clever about one thing: the compliance certificate isn’t yours. The plumber signs that, and it’s backed by his insurance, and if the pipe’s wrong you’ve got someone to chase. Enjoy the feeling. It lasts about as long as the first phone call.

Because the owner doesn’t sue the plumber, son. The owner sues you. Every domestic contract carries a warranty that the work complies with all laws and legal requirements, including the Building Act and its regulations. You can’t write that out of a contract, it follows the building to the next owner, and — here’s the sting — a claim pleaded that way isn’t apportionable. No carving it four ways with the architect, the surveyor and the plumber. You wear the whole thing, and then you go hunting for contribution from a bloke with fifty grand of cover and a company he can dissolve before Christmas.

And when they come for you, the questions are already written. The SD 02 note itself lists what decides the water-carrying capacity of a pipe: diameter, length, internal roughness, hydraulic head, gradient, and the number and type of bends and fittings. No method with them, no worked example, no acceptance criteria. That’s not a standard, son. It’s a cross-examination script — six numbered items, each a separate question you either answered or didn’t:

Factor	Who actually controls it	Where it’s decided
Pipe diameter	Hydraulic designer / plumber	Design, if anyone designs it
Pipe length	Architect’s riser strategy	Permit drawings
Internal roughness	Plumber’s material choice	Procurement
Hydraulic head	Architect and RBS — set by overflow and floor levels	Permit drawings
Pipe gradient	Plumber, constrained by slab depth	On site
Number and type of bends	Builder’s structure and services coordination	On site, after the pour

And what will they say failed? The fall in the slab. The seventy-mill step. The hob. The joint that should have been on the high point. The substrate the membrane was supposed to sit on. Read that list again. Not one of them is plumbing. Every single one is yours, poured on a Thursday, by your concreter, off a drawing nobody dimensioned.

Six factors, four parties, one certificate — and none of it saves you. The plumber signs the paper. You just build the puddle.

And where did the maths behind all this come from? I went looking. The note says a handbook contains an informative note allowing you to use roof drainage principles on balconies, and it’s true — I found it. It’s a NOTE at the end of the scope clause of an installation code for metal roof and wall cladding. It says roof principles “may also be utilised” for drains from balconies, patios “and the like”.

That’s the foundation, son. One “may”, in the scope of a roof sheeting manual.

The note may be wrong; to ignore it is Wrongs

Two things for the back pocket, and know exactly what each is worth.

Go and read RP 01 — Roof Plumbing, Regulatory Framework, version 2.0, published 4 May 2023, still on the same website today. Under what is not roofing stormwater plumbing work, in black and white: “the surface drainage of a balcony.” The regulator’s own current note says your balcony isn’t roof plumbing. Its newest note says it is. Neither mentions the other. And SD 02 arrives with no badges at all — no plumbing header, no audience boxes, no version, no date, no author. Its own predecessor SD 01 said “Building Practice Note” across the front. So how was a man to know? SD. Storm drainage, I assumed. A building note, in a building series.

Enjoy that, then put it down gently. It’s worth something at sentencing and nothing on liability. If the Regulations make it roofing stormwater work, it was roofing stormwater work before anybody built a webpage, and not having read the webpage never made a single job compliant. Play it at your own peril.

Don’t go thinking the missing badges save you. The Wrongs Act 1958 never asks whether a document had a version number, an author, or a header saying what it was. It asks what you knew, or ought to have known, and whether a sensible operator would have done something about it. A webpage published by the regulator answers that question just fine — no letterhead required. They can spell grate as gate, print the same warning twice and forget to sign it, and it will still walk into a hearing room and tell everyone what a competent bloke in your position was supposed to be aware of in 2026. Unsigned guidance can’t be appealed, can’t be prosecuted, and can’t be argued with. It can only be quoted back at you.

So feed that URL to the Wayback Machine tonight. Not out of civic duty — because that detail will be quietly redrawn the moment someone with a scale rule looks hard at it, and the page will carry on as though it always looked that way. And when they ask in 2031 why you built it wrong, you’ll be the only man in the room who can prove what they were showing you at the time.

And I’ll give them this, because your uncle is fair when it suits him. The NCC 2025 Water Management provisions did real work this edition — falls in the pour, hobs, step downs, joints on the high point, membranes on concrete, self-draining finishes. That’s genuine. The one thing it left soft was how to size the pipe, and the office that could have settled it with a binding determination — the real instrument, the one with legal effect — wrote typically on an unsigned webpage and drew you a picture instead.

Build the picture, son. Just don’t stand under it.

Your loving uncle,

Dodgy Bilda

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Disclaimer: This article is a work of fiction and satire, it is not legal advice — seek your own professional counsel before acting on anything you read here. The author used AI tools for assistance but takes full responsibility for the content. See Preface for full disclaimer.