Every club run I organise starts the same way: my own bike, the spare I keep for the member who’s always borrowing a wet-weather loaner, and the box trailer that hauls the marquee, the first-aid kit and forty litres of fuel for the sag wagon. All three used to live in a single-bay shed in Mandurah that I built for one bike and optimistically assumed would stretch. It didn’t. The trailer tongue lived in the doorway for two years before I finally poured a slab sized for what I actually own rather than what I owned when I built the thing.
Two flat-six cruisers parked side by side with enough clearance to walk between them and swing a leg over without clipping a mirror need roughly 900 mm of gap, which on top of each bike’s own footprint puts a genuine two-bike bay at around 3.6 to 4 metres wide. Add a single-axle box trailer at typical dimensions of roughly 2.4 by 1.5 metres, plus the turning space to back it in rather than manhandle it sideways, and a shed that comfortably holds two bikes and a trailer is closer to 6 by 6 metres than the 4.5 by 4.5 metre single-bay kit most people start with. It’s cheaper by a wide margin to size the slab correctly the first time than to extend a shed footprint later — an extension means a new footing, a step or seam in the slab, and matching a roofline that was never designed to be added to.
Sandy coastal soil drains fast, which sounds like an advantage until a shed slab is going down without proper compaction underneath — sand that isn’t compacted to a consistent density settles unevenly over a year or two, and a shed slab that starts flat can end up with a lean that shows up as a door that no longer closes square or a visible dip where the trailer wheel always sits. A well-compacted sand base with a slab at least 100 mm thick, reinforced with SL72 mesh for a domestic shed floor carrying vehicle loads, handles the combined weight of two bikes and a loaded trailer without the cracking that undersized reinforcement eventually shows.
Salt air off the water also plays into shed slab decisions more than people expect. A slab with a light broom finish sheds tracked-in sand and moisture better than a hard steel-trowelled surface, which matters when the trailer’s been used for a beach-adjacent club event and comes back with sand packed into the tyre tread. That sand ends up on the slab either way — the question is whether it gets swept out easily or works its way into a smooth surface’s pores.
A shed sized correctly inside is still a bad shed if the trailer can’t actually get in without three-point turns on soft sand. Working out the trailer’s real turning radius before pouring the apron in front of the shed — not just the shed slab itself — avoids a common Mandurah problem where the internal floor is generous but the approach outside is too tight for anything but a perfectly straight reverse. A hardstand apron extending at least the trailer’s own length in front of the door gives enough room to straighten up before backing in, rather than fighting the angle in the last two metres.
A slab this size, with two vehicle loads and a trailer’s point loads from its jockey wheel and stands, is not a job to undersize on reinforcement to save on the quote. Getting the base compaction, mesh spec and finish right for a shed that’s actually going to carry what it’s built for is worth pricing properly rather than matching the cheapest kit-shed slab quote going around, and Mandurah Concrete is who most people in this club end up talking to once they’ve worked out what they actually need to fit.
If I were building this shed again from scratch, I’d add another half a metre of width for the day someone’s borrowed loaner overstays its welcome, and I’d never again let door width dictate trailer width — measure the trailer with its jockey wheel extended and its safety chains hanging, because that’s the real footprint, not the folded travelling one. A 28-day cure before anything heavy rolls across the new slab is non-negotiable regardless of how tempting it is to load the trailer back in early.
Is SL72 mesh actually necessary for a home shed floor? For a slab only ever carrying foot traffic, lighter mesh is fine, but once a bike, a spare bike and a loaded trailer are all sitting on it regularly, SL72 gives enough tensile strength to resist the cracking that vehicle point loads cause over time.
How much bigger should I build than I think I need? As a rule of thumb, add at least 15–20% to whatever dimensions feel comfortable on paper — most people undersize because they’re picturing the bikes they own today, not the trailer, tools and second bike that tend to accumulate within a few years.
Does sandy soil in Mandurah need special base preparation compared to other areas? Yes — sand compacts differently to clay and needs to be brought to proper density in layers rather than assumed stable on its own, which is the step most commonly skipped on budget shed slab jobs in this area.