A carport that has been up for two summers and a wet winter tells you what is underneath it. If the roofline has dropped at one corner, or a gap has opened at the base of a post, the frame is usually fine. The ground moved.
What decides your footings is not the carport. It is the soil under it. The same flat roof on four posts can sit on modest pad footings in one street and need bored piers two streets away. A site classification settles it, and that comes from a soil test, not from what the neighbour did.
What reactive soil actually does
Reactive clay takes on water and swells, then dries out and shrinks. Over a Melbourne year that is a slow up and down cycle, worst around the edges where rain, sun and garden beds get at the soil, mildest in the middle. The soil is not settling under load. It is breathing.
Much of Melbourne sits on heavy clay, particularly through the west and north, with sandier profiles around parts of the bayside and east. It still changes block to block, and old cut and fill can leave uncontrolled fill under half your footprint.
Site classification is what your engineer designs to
Residential footings are designed against AS 2870, which classifies the site on how much the ground is expected to move with seasonal moisture change.
The classes in plain terms
- Class A: stable, mostly sand and rock, little or no movement expected.
- Class S: slightly reactive clay or silt.
- Class M: moderately reactive clay or silt.
- Class H1, H2 and E: highly through to extremely reactive clay.
- Class P: problem site, covering uncontrolled fill, soft soil, landslip and abnormal moisture.
Those letters tell the engineer how much movement the footing has to swallow without passing it up into the frame. Ask for the classification in writing. If someone quotes a footing without knowing the class, they have priced a guess.
Why a carport is a harder problem than a house
A house is heavy and holds itself down. A carport is a light roof on four to six posts, and in a big blow that roof behaves like a wing. Uplift, not downward load, usually governs the footing. Each post is a tall lever with the load up top and the overturning force landing at the base.
Then there is differential movement. If one footing lifts through a wet winter while the one diagonally opposite does not, a house slab absorbs that quietly. A four post frame does not. It shows up as racking, roof sheets working at the fixings, cracks radiating from a post base, and a roller door that binds if it is enclosed.
The footing options, and what each is for
Pad footings under each post
Concrete under every post, sized and reinforced to the engineer’s detail. Fine on lower movement sites. On reactive clay the pad has to sit below the zone that dries and swells, or it just rides the movement.
Bored piers
A drilled hole, caged and filled, taken down to more stable material. The usual answer on reactive or filled sites: it puts the structure below the seasonal moisture zone and resists uplift. Costs more, and finds rock more often than planned.
Thickened edge slab, or a slab left independent
Poured as one, the slab and its perimeter beam work as a single stiff raft with the posts fixed to it. More concrete and more steel. The alternative is piers or pads carrying the structure with the slab floating around them, so a cracked slab is far less likely to drag the carport with it. The trade off is a visible joint, and detailing that keeps a heaving slab off the post bases.
Tie down is what keeps it there
Melbourne sits in the lower wind region, but your site wind classification still turns on terrain, shielding and topography. An exposed corner block is not the job a sheltered one is.
What holds the roof down is a chain: sheet to batten, batten to beam, beam to post, post to footing. Every link has to be rated, and the last one is where people skimp. The footing resists uplift with its mass and depth, so a shallow pad on an exposed site is not a saving. It is the failure point.
Where jobs quietly go wrong
- Topsoil left in: organic material under a footing keeps breaking down and settling. Strip it.
- Water management: a downpipe discharging beside a footing keeps that soil wet while the rest dries. That is the movement you are paying to design out.
- Mature trees near the footprint: a big tree pulls moisture from clay for metres around and can hold that ground shrunk for years. Tell the engineer what is there and how close, and ask about losing it as well as keeping it, because taking a big tree out lets that clay rehydrate and swell.
- Rushing the concrete: pouring into a pier hole with water in the bottom, or standing the frame before it has cured, gives you neither the strength nor the behaviour the design assumed.
On a carport already standing, the faults read off the base: posts out of plumb, a gap at a post base, a low corner in the roofline, or a door that has started to catch.
What to line up before you get quotes
- A soil test and written site classification for your block, not the estate.
- An engineer’s footing detail drawn to that classification and your site wind class.
- Confirmation from your council or a registered building surveyor on whether a building permit is required. Victorian building and siting rules sit under the Building Act 1993 and the Building Regulations 2018, and the Victorian Building Authority is where to start reading. Never take a permit exemption on trust from a sales brochure.
- A quote naming the footing type, the number of piers or pads, and who wears the cost if rock turns up.
