One of the things I love about the area we work in is how much the job changes the moment you head out toward the water. A driveway in Southside is a different proposition from a shed slab in Goomboorian, and a coastal slab at Tin Can Bay throws up its own set of challenges that we’ve come to know intimately. We finished a substantial shed slab job out at Tin Can Bay last week — 12m × 9m of concrete, designed for a serious machinery shed — and it was the kind of job where you remember why the planning matters more than the pour. Here’s how it came together and what we learned doing it.
The Tin Can Bay brief
The clients had bought a five-acre lifestyle block on the outskirts of Tin Can Bay about two years earlier. The house was already there. What they were missing was a proper machinery shed for the tractor, a small Bobcat, the boat trailer, the camper, the ride-on, and a workshop bay for the owner who’s a serious hobbyist welder. The shed itself was a kit they’d already ordered from a reputable supplier — 12m × 9m, single-sided lean-to extension, 3m eaves height. What they needed was the slab to go under it. They wanted us to handle the slab, the drainage, and the council compliance side because the build itself wouldn’t trigger approval but the slab specifications are part of the package the certifier wants to see.
The site walk
The block sloped gently away from the planned shed location toward a treeline at the back. The soil was sandy-loam over what we found, when we dug, was a layer of more reactive clay starting at about 600mm depth. Tin Can Bay generally is wetter underfoot than Gympie itself — the water table’s higher, the soil holds moisture longer, and you get genuine standing water across blocks during summer storms. None of that is a deal-breaker, but it changed how we designed the slab. We marked out the footprint, did three test holes to check the soil profile, and figured out where the drainage falls would need to go to keep storm runoff away from the back of the shed and toward a planned soakaway in the lower corner of the block.
The engineering decisions
This was a serious slab. We specified 150mm thick concrete with SL92 mesh in the main shed area and SL102 reinforcement under the planned tractor and Bobcat traffic zones. We added a thickened edge beam (300mm deep at the perimeter) to handle the steel shed columns, and additional bar reinforcement at the door openings where vehicles would constantly traffic over the same line. The lean-to side, which would mostly carry foot traffic and the welder’s bench, came down to 125mm with SL82 mesh. None of this is overkill — it’s what a 12m × 9m shed taking real machinery loads needs to perform reliably for the next thirty-plus years on a Tin Can Bay block. For a fuller picture of how we approach shed slabs across the region, see our piece on concrete shed slabs in Gympie.
The sub-base prep
Two days went into the sub-base before any concrete arrived. We stripped the topsoil down to firm subgrade, dug the perimeter beam to the right depth, brought in 150mm of compacted DGB20 road base in two layers (each compacted with a vibrating plate), and laid heavy-duty plastic vapour barrier across the lot to stop ground moisture wicking up through the slab. On a coastal block where the water table is closer to the surface than most clients realise, the vapour barrier isn’t optional — skip it and you get persistent dampness at the slab edges that ruins anything stored on the floor over time. We taped every overlap and folded the barrier carefully up the perimeter beam edges to seal the slab as a complete moisture envelope.
The drainage design
Drainage matters enormously on a Tin Can Bay block. We designed a 1:100 fall across the slab from back to front (toward the open shed door), a small upstand at the rear wall to keep storm runoff from sheeting under the back wall, and a 100mm ag drain wrapped in geotextile filter fabric running along the rear and side perimeters, leading to a soakaway in the back corner of the block. The shed gutters and downpipes are tied into the same drainage system. We had a conversation with the clients early about why they shouldn’t think of the slab and the drainage as two separate jobs — they have to be designed and installed together or the slab loses to the drainage every wet season.
Council compliance and the certifier
The shed itself sat under the size threshold for triggering full Gympie Regional Council building approval, but the certifier still wanted documentation of the slab specifications, the soil testing, and the engineering details. We provided the structural certification, the moisture testing records, the reinforcement schedule, the pour records, and photos of the rebar and vapour barrier in place before the pour. This is increasingly standard practice for any rural block above a certain size — the council and the certifiers want a paper trail, and clients who skip this step often find the lack of paperwork shows up as a problem when they go to refinance or sell the property.
The pour day
108 cubic metres of concrete is not a small pour. We had a line pump on site, two trucks staggered through the morning, six crew on the slab, and a tight weather window between two storm fronts that the radar was tracking. The pour itself took the better part of the day. Screeded off, levelled with bull floats, then a power trowel finish across the main shed area to give a smooth-but-not-slippery surface for the floor. The lean-to area got a broomed finish for added grip. Saw-cut control joints went in the next morning before random cracking could start — a 4m × 4m grid for the main shed, with proper expansion joints where the slab meets the perimeter beam. By the end of day three the slab was firm enough for the shed builders to come in and start setting the steel columns.
The finish coat for the workshop bay
The clients wanted something a bit more refined for the welding workshop bay — easier to clean, less dust, more pleasant to spend time in. We came back two weeks after the slab cured and applied a penetrating concrete sealer with a slight gloss enhancement over the workshop bay only. The rest of the shed slab stayed unsealed, which is fine for a machinery shed where the floor is going to take dropped tools and dragged equipment regardless. For other clients thinking about decorative or sealed concrete options, our piece on decorative concrete finishes for Gympie homes covers the broader options.
What this kind of job teaches
Three things stand out from the Tin Can Bay slab. First, water management on a coastal block is half the job. Second, the engineering decisions made before the pour determine the next thirty years more than anything we do on the day. Third, paperwork matters more than it used to — even on rural lifestyle blocks where things used to be a handshake and a pour. If you’re planning a substantial shed slab anywhere across Tin Can Bay, Cooloola Cove, Goomboorian or out toward Rainbow Beach, the first conversation worth having is with the concreter, before the shed kit is locked in. The slab design follows the use, the soil and the drainage — and the cheapest way to do it well is to plan from the start.
If you’ve got a similar block
For owners thinking through driveways, pool surrounds and other concrete decisions on coastal lifestyle blocks, our pieces on exposed aggregate vs plain concrete driveways and concrete pool surrounds in Gympie are useful starting points. Every block is its own story — but the underlying principles of sub-base prep, reinforcement, drainage and proper jointing apply universally. If you’re starting to plan, we’d love to come and walk through the block with you before any concrete is booked.