Shed slab preparation is the part of the project where amateurs save money and then pay for it for the next decade. I have helped people fix enough bad slabs that the patterns are clear. The five mistakes below account for most of the problems I see.

Mistake one: skimping on the base preparation

The cheap shed slab pour goes onto whatever ground happens to be under the shed location. The good shed slab pour goes onto a compacted base of crushed rock, properly graded to drain, with a moisture barrier underneath.

The difference shows up in three to five years. The cheap slab cracks because the underlying ground moves seasonally. The good slab does not.

The cost to add proper base preparation at the build stage is a few hundred dollars in materials and a day of work. The cost to fix a cracked slab later is starting from scratch.

Mistake two: wrong concrete thickness

Standard shed slab thickness is 100mm. For sheds storing heavy equipment, 125mm or 150mm is appropriate. For oversized sheds, additional thickness in the high-traffic areas is worth the marginal cost.

I see slabs poured at 80mm or less to save concrete. These slabs will crack under predictable loads. The saving at the pour is trivial relative to the consequence.

Mistake three: missing the reinforcement

Reinforcement mesh is not optional. F62 or F72 mesh, properly chaired to mid-slab height, distributes the loads and limits crack propagation when cracks do appear.

The mesh laid on the ground, then poured over, does not do its job. It needs to sit at the right height in the slab. Chair stands cost a few dollars and are the single most important detail in the reinforcement.

For sheds in expansive soil areas, additional reinforcement around the perimeter and at penetrations is worth the discussion with the engineer.

Mistake four: poor edge detail

The slab edge is where moisture enters and where the structural connections happen. A clean edge with proper formwork, adequate thickness at the perimeter, and proper integration with the shed wall plate produces a slab that lasts.

A sloppy edge with informal formwork and minimal perimeter detail produces a slab that develops perimeter cracks and lets in moisture.

For sheds with wall plates anchored to the slab, the bolt placement matters. Bolts pre-cast into the slab in the correct positions are much stronger than bolts retrofit afterward with concrete anchors.

Mistake five: not letting it cure properly

Concrete needs time and moisture to cure properly. Pouring a slab and walking on it the next day, putting the shed up two days later, and storing equipment within a week produces concrete that has not reached design strength.

The standard guidance is seven days minimum before loading and twenty-eight days for full design strength. In practice, three weeks before the shed goes up and four weeks before serious equipment loading is the sensible timeline.

Wet curing with damp hessian over the slab for the first week produces stronger concrete than letting it dry naturally. The water is the catalyst for the cement hydration that gives the concrete its strength.

What the right way costs

A properly prepared shed slab — base prep, reinforcement, correct thickness, proper edges, proper cure — runs maybe 25% more than the corner-cutting version. The 25% premium buys decades of additional life.

I have seen plenty of sheds where the slab outlasts the structure. I have also seen plenty of sheds where the slab needs replacing while the structure is still good. The difference is mostly the slab prep.