Sheds and carports look like the simplest thing you can put on a block, which is exactly why they're the structures most often built without the engineering they needed.
The Load That Catches People Out Is Wind, Not Weight
A shed weighs very little. That's the problem. A large, light, enclosed box with a big roof area is an extremely efficient sail, and the governing design case for most sheds and carports isn't the weight pressing down. It's uplift pulling the whole thing up and sideways.
Which is why the critical details are the ones nobody looks at: the tie-down from roof sheeting to purlin, purlin to rafter, rafter to column, and column to footing. A shed fails at whichever of those connections was weakest, and the footing is very often it. Slab thickness alone doesn't hold a shed down.
Wind Classification
Every site has a wind classification determined by region, terrain category, shielding from surrounding buildings, and topography. Two sites in the same suburb can classify differently. An exposed corner block with nothing upwind of it is not the same as a block surrounded by established housing, and a site partway up a slope attracts higher wind speeds than flat ground.
Kit sheds are supplied for an assumed wind classification. If your site classifies higher than the kit was designed for, the kit is not adequate as supplied, regardless of what the brochure says.
When You'll Generally Need Engineering
- the structure is attached to the house, so its loads transfer into an existing building
- the site's wind classification is above what the supplier's standard details cover
- the structure is unusually large, tall, or has long unsupported spans
- it sits on fill, a slope, or a reactive site where standard footings don't apply
- it's close to a boundary or a retaining wall, so its footings interact with something else
- you're modifying a kit. A wider bay, a taller opening, a removed column
- the council or certifier asks for structural certification, which is common
Attached Is a Different Problem from Freestanding
A freestanding carport carries its own loads to its own footings. An attached one transfers load into the house, usually into a wall or roof structure never designed to take it, and uplift on a carport roof tries to lift the house structure it's fixed to.
Attaching to an existing building always warrants a look at what the existing building can take. This is the most common shortcut we see, and the one with the most consequences.
“It Came with Engineering”
Kit suppliers do provide engineering, and it's usually sound, for the assumptions it was written under. Those assumptions are a wind classification, a soil bearing capacity, level ground, standard footings and an unmodified kit.
Site-specific certification checks those assumptions against your actual site. Where they hold, that's a short piece of work. Where they don't, better to know before the slab is poured.
Approvals
Sheds and carports are development in South Australia and are assessed under the Planning, Development and Infrastructure Act 2016. Whether approval is required depends on size, height, siting relative to boundaries, and your zone under the Planning and Design Code, so it varies by council and by property. Check with your council before ordering, because approval conditions can affect which structure you're allowed to buy.
What Shed Engineering Typically Costs
For a standard shed, the engineering in Adelaide typically costs somewhere between $400 and $1,200. A freestanding kit shed on a sheltered suburban block is at the lower end; a large shed with big roller doors on an exposed rural site, or one attached to the house, is at the upper end, because the wind classification and the connections are what take the design time. At ANS we aim to give you a more careful check than the average for a fee below it, fixed in writing before the work starts.
What We'd Need
The address, the supplier's drawings and specification if it's a kit, the proposed position on the block, and whether it's freestanding or attached. From that we can tell you whether the standard details cover your site or whether it needs designing.
This article is general information about structural engineering practice in South Australia. It isn't advice for a specific site, and it doesn't replace confirming requirements with your local council or a registered building certifier. Requirements vary between council areas and change over time.