Most foundation problems trace back to decisions made before a single truck of concrete arrives. Get the planning right and the pour itself is straightforward. Get it wrong and you're looking at cracking, settling, or a slab that's fighting the ground underneath it for years. Here's where we see it go wrong, and how we plan around it.
Site prep that skips the boring parts
A site looks ready once the vegetation and debris are cleared, but that's only the visible half of the job. The ground underneath needs to be properly graded and compacted, not just cleared and levelled by eye. Skip the compaction step and you'll get uneven settling months or years later, showing up as cracks that follow no obvious pattern because the movement underneath isn't uniform either.
We test and compact the base before anything else happens on site. It's not the part of the job anyone photographs, but it's the part that decides whether the slab lasts.
Choosing the wrong foundation type for the soil
Not every site takes the same foundation the same way. Reactive clay soils move with moisture, sandy soils drain differently to loam, and a slab-on-ground design that works on one block can be the wrong call fifty metres away if the soil profile changes. Foundation design in Victoria for residential work is governed by AS 2870, the Australian Standard for residential slabs and footings, which classifies sites by soil reactivity and sets the design requirements from there.
Skipping a proper site classification, or assuming last door's foundation design will do, is how you end up with a slab under-engineered for the ground it's sitting on. It's a false economy: the cost of getting a soil report is small next to the cost of a foundation that moves.
Drainage as an afterthought
Water sitting against or under a foundation is one of the most common causes of long-term movement. Poor grading, missing stormwater provisions, or a site that simply hasn't had drainage thought through at design stage all lead to the same outcome: moisture builds up where it shouldn't, and the ground beneath the slab stops behaving consistently. Getting the fall right and planning where the water goes before the pour is far cheaper than fixing it afterwards.
Reinforcement planning left too loose
Reinforcement isn't something to sort out on the day. The steel mesh or bar layout needs to match the load the slab will carry and the soil class it's sitting on, specified as part of the design, not adjusted on the fly. Underspecifying reinforcement to save time or material is one of the more expensive mistakes to discover later, because by the time it shows up as cracking, fixing it means addressing the whole slab.
Ignoring what's already on site
Established trees, buried services, and neighbouring structures all affect how a foundation should be planned. Tree roots move soil and draw moisture unevenly, and services that aren't properly located before excavation cause their own problems. A proper site survey before design work starts picks these up early, when they're cheap to plan around.
Getting the planning right from the start
None of this is complicated, but it does need to happen in the right order: site assessment, soil classification, drainage planning, and reinforcement design, before formwork goes in. We handle this planning stage on every job so the pour itself, covered in a separate guide on its own common mistakes, has a solid, well-designed base to work with.
If you're planning a new slab, shed base, or extension footing anywhere across South-East Melbourne, get in touch through our shed and garage slab page or head to contact us and we'll walk the site with you before anything is designed.