What are engineered drawings, and when do you need them on the Gold Coast?

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What are engineered drawings?

Engineered drawings, or structural drawings, are a structural engineer's design of a building's footings, slab and framing. The City of Gold Coast's building plans search page says structural drawings "are prepared by a structural engineer and show the design of the footings, slab and framing of the building". Its plan types show how they differ from architectural drawings:

Drawing set	Plans in the set	What the plans show
Architectural	Site plan, floor plan, elevations	Where the building sits on the property, the layout and dimensions of each room, and the outside of the building from the front, sides and rear
Structural (engineered)	Footing, slab, frame and bracing	The design of the footings, slab and framing, prepared by a structural engineer

In Queensland, anyone providing professional engineering services must be a Registered Professional Engineer of Queensland (RPEQ), work under an RPEQ's direct supervision, or only work to a prescriptive standard, according to the Board of Professional Engineers of Queensland (BPEQ). Its definition of a professional engineering service covers applying engineering principles and data to a design.

What do structural drawings show?

Structural drawings show the design of the structure and, for footings on reactive sites, the site classification. For new footings and slabs, the Queensland Building and Construction Commission (QBCC) guide Defective residential construction work causing subsidence says drawings for reactive sites shall include the site classification, citing Australian Standard AS 2870. It says the engineer's design drawings should include:

• the selected footing systems and any special site works;
• the means of diverting surface water away from the slab;
• the location of control joints in brick and masonry construction;
• the location of retaining walls;
• requirements for articulation (flexible joints) in stormwater and sanitary drainage.

The design also follows the structural standards the Building Code of Australia calls up. For retaining walls, the City of Gold Coast's retaining wall guidelines say the Building Code of Australia (BCA) calls up structural codes for each material, such as AS 1720 Timber structures, AS 3600 Concrete structures, AS 3700 Masonry structures and AS 4100 Steel structures.

When does a Gold Coast project need engineered drawings?

Engineered drawings come into a Gold Coast project when the work needs building approval and the building certifier relies on an engineer's design as evidence that the structure complies. An engineer's design isn't the only accepted evidence: the City says building plans can be prepared by a builder, engineer, designer or architect, and Form 15's notes say a manufacturer or supplier that designed its product can certify that design. The Queensland Government says a building development approval is needed before construction can start on most types of domestic building work. On the Gold Coast, approvals are now usually privately certified, and the City says the first step is to have drawings prepared.

Under National Construction Code (NCC) 2022 Volume Two, clause A5G3, evidence that a design meets the BCA can be required by an appropriate authority or a person certifying compliance. One accepted form is a certificate or report from a professional engineer that sets out its basis and the standards relied on.

City of Gold Coast pages set out when these projects need building approval:

Project	When building approval is needed
Deck	An elevated deck needs approval unless it's smaller than 10 m², less than 1.0 m high and meets all building requirements (building works).
Retaining wall	Unless it meets all the City's criteria, such as being less than 1.0 m high, more than 1.5 m from a building or another retaining wall, supporting nothing above it and not over a sewer or stormwater service (retaining walls).
Repairing or replacing a retaining wall that isn't exempt	When the work affects more than 20% of the wall's structural components or changes its height (retaining wall guidelines, citing the Building Regulation 2021, schedule 1). A full replacement needs approval, even if built back the same (retaining wall guidelines).
Swimming pool or spa	All pools and spas need approval to build or demolish, and must be registered with the QBCC (pool approvals).
Renovation or repair	Minor repairs or maintenance that don't affect a building's structural or fire safety may not need approval, but a change of use needs approval even with no building work (building works). A building certifier can advise whether your work does.

Two more local rules apply. A deck over 1.0 m high may also need the City's approval for building within setbacks, which the private certifier arranges. And a private building certifier can't issue an approval until other approvals, such as a town planning approval from Council, have been given.

How do Form 15 and Form 16 relate to engineered drawings?

A Form 15 certifies the engineer's design to the building certifier, while a Form 16 certifies that a built stage complies with the building development approval. Both are approved building forms in Queensland.

Form 15, Compliance certificate for building design or specification, states that an aspect of building work will comply with the building assessment provisions if carried out as stated. On it, the engineer describes the aspect, identifies the reference documents, such as numbered structural engineering plans, and states the standards and codes relied on. Its notes say it informs the certifier's decisions, including amending the approval for updated information such as truss specifications or revised excavation drawings.

During construction, an engineer the certifier has appointed can inspect an aspect, such as the frame, and give the certifier a Form 12, Aspect inspection certificate (BPEQ).

Certificate	What it certifies	Who signs it	Given to
Form 15	A design or specification will comply with the building assessment provisions if carried out as stated	An individual the building certifier has appointed as a competent person (design-specification), such as a Registered Professional Engineer	The building certifier
Form 12	An inspected aspect is complete and complies with the approval	An appointed competent person (inspection)	The building certifier
Form 16	All relevant aspects of an inspected stage are complete and comply with the approval	The building certifier, another building certifier or an appointed competent person (inspection), with the limits below	The builder

The limits are in the notes to Form 16, Inspection certificate. For a single detached class 1a building and class 10 buildings and structures, only a building certifier can sign the excavation, footing and slab stages (each before the concrete is poured) and the final stage. For a pool, the final stage is at completion of the pool and its fencing and, if no temporary fence was built, before it's filled to 300 mm or more. The certifier may rely on a competent person's Form 12, such as an engineer's, for an aspect of those stages, but still signs the Form 16.

Under NCC 2022 Part A6, class 1a includes a detached house, and class 10 includes private garages, carports, sheds, retaining walls and swimming pools. For class 10 work other than pools, the only mandatory stage is the final one, though the approval may list more (stages of inspection).

What does the engineer need from you?

The engineer needs plans of what you want to build, information about the site, and any records of what's already there.

1. Plans and approval details. The architectural site plan, floor plans and elevations, if a designer, architect or builder has prepared them, and the certifier's reference number and, if available, the building development application number.
2. The site. For new footings and slabs, the QBCC's subsidence guide lists what the builder must give the engineer, including the site's location, contours, trees on and next to the site, overland flow, platform levels, cut and fill, and searches such as flooding and easements.
3. Soil information. The same guide expects a site investigation by an engineer or a licensed soil tester, with at least two exploration positions or bore holes and laboratory testing to determine the site classification.
4. Council records. Through its building plans search, Council may have copies of approved plans, including structural drawings, a soil or engineer's report (for buildings constructed after 1989), and earlier Form 15, Form 16 and Form 12 certificates. Its officers can't identify specific documents such as footing details.
5. Earlier approvals. The City's retaining wall guidelines note that a subdivision (reconfiguration of a lot) approval may limit the heights and locations of future retaining walls.

When to call a structural engineer

Call a structural engineer before you apply for building approval on structural work, and whenever your building certifier asks for a Form 15. Triggers include:

• a deck of 10 m² or more, or 1.0 m or more above the ground;
• a retaining wall 1.0 m or higher, within 1.5 m of a building or another wall, supporting anything above it, or over a sewer or stormwater service;
• major repairs to, or a full replacement of, a retaining wall that isn't exempt;
• a new swimming pool or spa, or an extension that changes the structure.

Our structural design page covers certifier-ready structural drawings for new homes, renovations and repairs. For walls and pools, see retaining wall design and swimming pool engineering. Send your plans, site photos and suburb with your enquiry.

Example from our projects

Our pool and deck structural design in Ormeau shows a set: eight sheets issued for approval, covering a 200 mm reinforced concrete pool shell and a timber deck on 450 mm concrete piers with adjustable steel stumps.