A structural engineer’s job is to make sure your building holds itself up. That means today, in a nasty summer storm, and in thirty years when the kids have moved out and the clay under the slab has gone through a few hundred wet and dry seasons.
So they look at the ground first, then the footings. After that they check the beams, posts and walls that carry the weight, the roof and floors, and the wind. They also check how all the bits are joined together. Then they make sure the lot meets Australian codes before anything gets built.
That’s it in a nutshell. The details are where it gets interesting.
We’re Leroy Palmer, and we get asked about this a fair bit. Usually it’s someone halfway through planning a house, an extension or a big shed. Their certifier or designer has said “you’ll need an engineer,” and they’re not sure what they’re actually paying for. If that’s you, this should clear it up. You’ll come out knowing what gets checked and why. You’ll also know when to get an engineer involved, and what you’ll have in your hands at the end. Mostly, you’ll be able to ask better questions. That alone can save you a redesign.
It starts with the dirt
Most people think about the house. Engineers think about what the house is sitting on.
In Australia, residential sites get a classification under AS 2870. Class A is stable stuff like sand or rock. Then it goes S, M, H1, H2 and E, and each step means more reactive clay. Reactive just means the clay swells when it’s wet and shrinks when it dries. There’s also Class P, which covers problem sites like soft ground or dodgy old fill.
A soil tester or geotechnical engineer usually comes out, drills a few holes and writes up the report. The structural engineer doesn’t normally do that bit. What they do is take the report and design footings and a slab that’ll cope with whatever’s down there.
Around Toowoomba, this matters more than people expect. There’s a lot of reactive clay about. After a wet summer it puffs up. Come a dry winter it pulls back. A slab that isn’t designed for that movement will tell you about it eventually. Usually it’s cracks above doorways first, then cornices pulling away, then a door that won’t close properly.
Fictional example: Say a young couple buys a block in one of the newer estates. Their soil report comes back H1. The engineer gives them deeper edge beams and more steel in the slab, and the concrete quote goes up a bit. They grumble about it. Five years later their walls are fine. Their neighbour’s aren’t, and that neighbour is now pricing underpinning.
The bits that carry the weight
Beams, columns and load-bearing walls. The skeleton, basically.
The big thing an engineer checks here is something called the load path. That’s the route the weight takes from the roof all the way down to the ground. It has to be continuous. Think of a line of people passing a heavy box down a staircase. If one person wanders off, the box ends up on the floor.
This is why knocking out a wall isn’t as simple as it looks on a renovation show. Plenty of people want the kitchen and lounge opened up into one big space, and fair enough, it’s nice. But if that wall was holding up the ceiling or the floor above, something has to take over. Normally that’s a beam across the gap with posts at each end. Those posts need something solid underneath them too. The engineer sizes all of it.
Get it wrong and the ceiling sags. Or worse.
Floors and roofs
Here’s something that surprises people. A floor can be completely safe and still feel awful.
Strength is one thing. Stiffness is another. If the joists are a bit undersized or spaced too far apart, the floor bounces, and you’ll hear the glasses in the cabinet rattle when someone walks past. Nobody’s in danger, but you’ll notice it every single day. So the engineer checks joist sizes, spans and spacing for both.
Roofs are mostly about what’s on them. Concrete tiles are heavy. Colorbond is light. Add solar panels, an air-con unit or a tank on a stand and the numbers change again. A lot of standard timber framing follows AS 1684, which has tables builders and designers can use. Once you go past those tables, with long spans, steel, odd roof shapes or a big raked ceiling, the engineer works it out properly.
Loads and wind (the part with the most maths)
Engineers split loads into a few types. Dead load is the weight of the building itself. Live load is everything that moves in and out, like people, furniture, cars and pallets of stock. Then there’s wind, plus things like soil pushing against a retaining wall. All of this gets worked through using the AS/NZS 1170 series. For houses, AS 4055 sets your wind class.
Wind is the one people underestimate.
Everyone pictures it pushing a house sideways. It does. But it also gets under the roof and tries to lift it, a bit like an umbrella turning inside out. That’s why tie-downs matter so much. A roof that weighs a lot still needs to be strapped down properly.
And Toowoomba isn’t a gentle spot for wind. Spring and summer storms roll through hard. A block on a ridge, or anywhere near the edge of the Range, can be a lot more exposed than a house tucked into a sheltered street in Rangeville. The view’s better up there. So’s the wind.
Hypothetical: Picture someone building a two-storey place on a hilltop block with views across the valley. They assume it’s a standard wind class because the house next door was. It isn’t. Their site’s more exposed, so the engineer specifies heavier tie-downs and more bracing. A few hundred dollars of extra hardware, and the roof stays on.
Materials, connections and bracing
You can pick the perfect beam and still have a problem if it’s bolted on with the wrong thing. Joints fail before members do, more often than not.
So the engineer specifies the concrete strength and reinforcement, the steel sections, and the timber grades. Concrete has its own standard (AS 3600), so does steel (AS 4100), and timber has a few. Then they work out the connections, meaning the bolts, welds, brackets, straps and tie-downs.
And bracing. Bracing gets forgotten.
Grab a cardboard box and cut the top and bottom off. Give one corner a push. It folds flat straight away. That’s a frame with no bracing. Walls with ply sheeting, steel straps across the studs, braced panels — that’s what stops a house doing the same thing in a strong gust.
Drawings, codes and the legal side
None of that helps if the builder can’t follow it. So it all ends up on drawings, and they need to be clear enough to read on a muddy site at 7am.
The engineer makes sure the design meets the National Construction Code and the relevant Australian Standards. In Queensland, anyone providing professional engineering services has to be a Registered Professional Engineer of Queensland (RPEQ). Your certifier will usually want the engineering before they’ll approve the build. If bits are missing or vague, they’ll send questions back. Every round of questions costs you time.
Engineers and designers aren’t the same job
People mix these up a lot.
The building designer works out what the building is. That covers how the rooms flow, where the light comes in, how it sits on the block and what it looks like from the street. The structural engineer works out how to make that stand up without falling down, cracking or bouncing.
On a good project, it’s a conversation. The designer draws a big opening onto the back deck. The engineer comes back with something like: “Yep, can do. You’ll need a steel beam about this deep though. Does that still work with your ceiling height?” Sometimes it’s no drama. Sometimes there’s a bit of give and take.
That conversation is cheap on paper. It’s very expensive once the frame’s standing. Honestly, it’s the main reason we at Leroy Palmer push for the designer and engineer to talk early. It’s not about selling anything. It’s just that we’ve watched late changes chew through budgets.
Fictional example: A designer draws a lovely cantilevered balcony off the main bedroom. Looks great in the render. The engineer runs it and the floor framing can’t carry it as drawn. So they run the steel further back into the house to balance it out. The balcony stays and the drawings get tweaked, not redone. Everyone’s happy because it got caught early.
When do you actually need one?
Not for everything. A basic garden shed or a minor internal fit-out often doesn’t need custom engineering.
But for most new homes you will. The footings have to suit your soil, and anything outside the standard framing tables needs working out. Extensions and renos too, especially if walls are coming out, a second storey’s going on, or new work is being joined to an old house. Commercial buildings, warehouses and big sheds definitely, because long spans and wide roller doors put a lot of strain on the frame and the wind design. Townhouses and unit developments, yes. And retaining walls past a certain height.
Sloping blocks are everywhere around here, and they tend to bring extra stuff along. That might be retaining, suspended slabs or stepped footings. One more reason to get someone looking at it early.
As for when: during concept or early design, before plans go to the certifier. And get the soil test done early, because so much depends on it.
What you’ll end up with
It varies by project, but usually you get structural drawings. Those cover footings, slab, framing, beams and posts, connections and bracing. You’ll often get the calculations too, since some certifiers want to see the working. There’ll be specifications for materials and fixings. In Queensland you might also get a Form 15, which is a design compliance certificate signed by an RPEQ, if your certifier asks for one.
Some jobs also need the engineer to inspect things during construction, like footing holes before they’re filled or steel before a pour. That can mean a Form 16 inspection certificate. Whether it’s part of your job depends on your approval conditions and what you agree with the engineer at the start. Sort that out up front so nobody’s guessing later.
Keep all of it, too. You’ll want it if you sell, renovate, or ever have to deal with an insurance claim.
A few quick questions we get
Does every project need a structural engineer?
No. Small, simple jobs often don’t. Your certifier will tell you if yours does.
Read more about What Does an Engineering Project Manager Do here >>
Do they test the soil themselves?
Usually not. A soil tester or geotech does that. The structural engineer designs around their report.
Do they come out during the build?
Sometimes. It depends on the agreed scope and what your approval needs. Ask at the start.
How early is too early?
It’s pretty hard to be too early. Too late is the common problem.
So, where does that leave you?
You don’t need to understand the maths. Nobody expects that. It’s enough to know what an engineer is looking at and roughly why. Then get them in before the plans are locked.
If you’re at the start of a project, grab your concept plans and your soil report if you’ve got one. Jot down anything a bit unusual: a big opening, a second storey, a steep block, a windy site. That’s a good list to start a conversation with.
And if you’d like someone to go through it with you, the Leroy Palmer team’s always up for a chat. No pressure either way.
This is general information only. It isn’t engineering, legal or building advice for your particular project. What applies to you depends on your site, soil, design and approval conditions, so check with a qualified structural engineer and your building certifier. The examples in this article are fictional and are only there to explain the ideas.