Why Does a Concrete Slab Crack? Causes, Prevention and When to Worry

Introduction: Every Slab Cracks Eventually

If you've noticed a crack in your shed slab, garage floor, or concrete footpath, you're not alone — and in most cases, you're not in trouble either. Cracking is one of the most normal, predictable things concrete does. What actually matters is understanding why it cracked, what kind of crack you're looking at, and whether you need to fix it or leave it alone.

This guide walks through the real reasons concrete cracks, which causes are especially common in Brisbane and South East Queensland, how to prevent the worst cracking at the pour stage, and how to tell a cosmetic hairline from a crack that needs attention. We won't sugar-coat it — we'll just explain the physics, the soil conditions, and the practical steps that keep a small slab performing for decades.

The Hard Truth: All Concrete Cracks

There's an old saying in the concreting trade: "There are two types of concrete — concrete that's cracked, and concrete that's about to crack." It sounds cynical, but it's essentially true. Concrete is a rigid, brittle material made of cement, sand, aggregate and water. As it cures and then ages, internal stresses build up. At some point, something gives way.

The good news is that the industry has been designing around this reality for over a century. Modern slabs use mesh reinforcement, control joints, proper subgrade prep and correct mix designs specifically to manage cracking — not eliminate it entirely. A well-built shed slab will still crack; it will just crack in predictable, controlled, cosmetic ways rather than structural ones.

So the question isn't "will it crack?" — it's "will the cracks matter?"

The Main Causes of Concrete Cracking

a) Shrinkage Cracks (The Most Common)

When concrete cures, water evaporates out of the mix. As that water leaves, the concrete physically shrinks — typically by around 0.5mm per metre. A 6m shed slab can shrink by 3mm during curing. That movement has to go somewhere. If the slab can't shrink evenly (and it never can, because the base restrains it), internal tension builds until the concrete cracks to relieve it. This is why shrinkage cracks are basically inevitable. Control joints and mesh are how we manage them.

b) Settlement Cracks

If the ground below the slab moves, compresses, or washes out, the slab loses support and cracks from the stress of unsupported spans. This is usually the result of poor subgrade preparation — uncompacted fill, soft spots, or organic material left under the slab. Settlement cracks are often diagonal and can be wide.

c) Thermal Cracks

Concrete expands when it heats up and contracts when it cools down. A slab sitting in full Brisbane summer sun can swing through a 30°C temperature range in a single day. Over a long slab, that movement adds up. Without expansion joints, thermal movement has nowhere to go and the concrete cracks.

d) Overload Cracks

Every slab is designed for a particular load — foot traffic, a light vehicle, a shed with stored gear, or heavier plant. If you park a 3-tonne ute on a 75mm footpath slab with SL62 mesh, it will eventually crack. Overload cracks usually appear under the point of stress and may be accompanied by surface spalling.

e) Reactive Soil Movement

This one is a Brisbane and SEQ special. Reactive clay soils swell when wet and shrink when dry. The slab above them moves up and down with the seasons, and if it's not thick enough or reinforced enough, it cracks. We'll cover this in more detail below because it's genuinely common in SEQ.

f) Poor Curing (Dried Too Fast)

Concrete needs moisture to cure properly. If the surface dries out during the first few days — particularly in 35°C Brisbane heat with a dry breeze — the surface shrinks faster than the interior, creating tension that shows up as fine crazing or plastic shrinkage cracks. Proper wet curing prevents this. (See our guide to concrete curing times for the full picture.)

g) Insufficient Reinforcement

Mesh doesn't stop concrete from cracking — it holds the cracks tightly closed so they stay hairline rather than widening into a structural problem. If the slab has no mesh, the wrong size mesh, or the mesh sits on the ground instead of at mid-depth, cracks open wider and behave worse. A shed slab with SL62 mesh where SL82 was needed is a classic cause of wider cracking.

Brisbane-Specific Crack Causes

Concrete cracks for the same reasons everywhere, but SEQ throws a few extra ingredients into the mix:

Reactive Clay Soils

Large stretches of Brisbane, Ipswich and Logan sit on highly reactive black and grey clay soils. These soils can swell up to 10% when saturated and shrink equivalently when they dry out. A slab poured directly on untreated reactive clay essentially rides a slow-motion seesaw between summer wet season and winter dry. Without adequate thickness and mesh, long diagonal cracks are the usual result.

Tropical Heat and Rapid Curing

A midday pour on a hot January day in Brisbane will start to skin over within minutes. If the concreter doesn't control curing properly, plastic shrinkage cracks appear before the slab has even fully set. This is why good concreters pour early morning in summer and start wet curing almost immediately.

Heavy Rainfall Saturating the Subgrade

SEQ storms can dump 100mm of rain in an hour. If the subgrade under a slab wasn't properly compacted or if drainage around the slab is poor, water saturates the base. Soft spots form, the slab loses support, and settlement cracks follow.

Tree Root Invasion

Large eucalypts, poincianas and jacarandas are everywhere in Brisbane backyards. Their roots chase moisture and can lift or undermine concrete slabs over time. Footpaths running close to established trees are particularly prone to this.

Types of Cracks and What They Mean

Not all cracks are created equal. Here's how to read what you're looking at:

Crack Type	Width / Appearance	Severity	What It Means
Hairline crack	Under 2mm wide, thin line	Cosmetic	Normal shrinkage — no action needed
Map / crazing cracks	Fine web of shallow surface lines	Cosmetic	Surface dried too fast during curing — appearance only
Straight-line crack at joint	Follows a saw-cut or tooled joint	Normal / expected	The control joint doing its job — this is what joints are for
Diagonal crack	2–5mm, runs across slab at an angle	Monitor	Often shrinkage or mild soil movement — watch over 2–3 months
Wide crack (5mm+)	More than 5mm wide or continues to widen	Structural concern	Get a professional assessment — subgrade or reinforcement issue
Heaving crack	One side raised above the other	Structural concern	Soil movement or root lifting — needs attention
Spalling around crack	Concrete flaking or breaking away at edges	Moderate concern	Water penetration or overload — seal and monitor

The quick rule: if you can fit a business card into the crack, it's hairline and cosmetic. If you can fit a 5c coin, it's worth looking at properly.

How to Prevent Concrete Cracks

You can't stop cracking entirely, but good practice at the pour stage prevents roughly 90% of serious problems. The fundamentals are:

a) Proper Subgrade Preparation

The ground under the slab has to be compacted, level, and free of soft spots. Organic material (grass, roots, topsoil) is removed. A compacted base of road base or crusher dust gives the slab something uniform to sit on. See our guide to base preparation for the details.

b) Adequate Thickness for the Load

Under-thick slabs crack under loads they weren't designed for. A 75mm footpath is fine for foot traffic but will crack if you park on it. A 100mm shed slab is standard for small-to-mid sheds; 125mm is better for larger sheds or workshop use. (More in our shed slab thickness guide.)

c) Correct Mesh Reinforcement

SL72 is the standard mesh for most shed slabs. SL82 is used for heavier slabs like double garages or workshop floors with plant equipment. The mesh needs to sit at mid-depth (not on the ground) using chairs or bar supports. Mesh that sits on the base does almost nothing.

d) Expansion and Control Joints

Joints give the slab a planned place to crack. Without them, the concrete decides for itself — usually badly. Spacing is typically 24 to 36 times the slab thickness (so a 100mm slab gets a joint every 2.4–3.6m).

e) Proper Curing

Wet curing for at least 7 days in Brisbane summer is essential. That means keeping the surface moist with hessian, sprinklers, plastic sheeting, or a curing compound. Letting a fresh slab bake in the sun is the fastest way to guarantee crazing cracks.

f) Right Concrete Mix

25–32 MPa is standard for residential shed and garage slabs. The slump (how wet the concrete is) matters too — too wet and you get excess shrinkage; too dry and it's hard to finish properly.

g) Skilled Finishing

Timing matters. Troweling too early seals water into the surface; troweling too late damages the hardened top. A good concreter reads the slab as it sets and works with it.

Expansion Joints Explained

Expansion joints — also called control joints or saw cuts — are probably the most misunderstood feature of a concrete slab. People see a straight line across their slab and assume it's a crack. It's not. It's the crack's designated parking space.

Concrete will crack due to shrinkage. The only question is where. A control joint is a deliberate weakness — a groove tooled into the surface or saw-cut in after the pour — that tells the concrete "crack here if you have to." When shrinkage tension builds up, the slab cracks at the weakest point, which is the joint, and the crack is hidden inside the groove where it's invisible and irrelevant.

Joint Spacing

The rule of thumb is 24 to 36 times the slab thickness:

• 75mm footpath: joints every 1.8–2.7m
• 100mm shed slab: joints every 2.4–3.6m
• 125mm heavy slab: joints every 3.0–4.5m

Joints should also divide the slab into roughly square panels where possible — long thin panels crack worse than square ones.

What to Do If Your Slab Cracks

First — don't panic. Work through the following steps:

Step 1: Document It

Take photos with a ruler or coin next to the crack for scale. Note the date, the crack's location, width at the widest point, and length. This baseline lets you tell if the crack is growing.

Step 2: Monitor Over 2–3 Months

Check the crack monthly. A crack that stays the same width over a full season is almost always cosmetic. A crack that widens, lengthens, or starts heaving is telling you something is still moving underneath.

Step 3: Classify It

Use the table above. Hairline and map cracking are cosmetic. Wide cracks, heaving, or growing cracks are a different category.

Step 4: Act Accordingly

Cosmetic cracks can be filled with a concrete crack sealer to keep water out and improve appearance. Structural cracks need a professional assessment before repair.

Repair Options

Polymer Crack Fillers

For hairline and small cracks up to about 3mm, flexible polymer fillers from a hardware store work well. They seal the crack against water, flex slightly with the concrete, and are easy to apply. Good for preventing further deterioration of cosmetic cracks.

Epoxy Injection

For structural cracks that have stopped moving, epoxy injection bonds the two sides of the crack back together and restores strength. This is a specialist repair and usually done by a concrete repair contractor rather than a general concreter.

Overlay / Resurfacing

For surface damage, crazing, or slabs with many minor cracks, a thin polymer-modified overlay (typically 5–15mm) can be applied on top to restore a clean finished surface. Works well on shed slabs and garage floors.

Slab Replacement

For badly cracked or heaving slabs — especially thin footpath sections — removal and replacement is often more economical than patching. If the subgrade failed, a repair won't fix the underlying cause.

When to Call a Professional

Most cracks don't need anyone involved. The ones that do share common features:

• Cracks wider than 5mm
• Heaving — one side visibly raised above the other
• Cracks that are actively growing week to week or month to month
• Water coming up through a crack (indicates subgrade or drainage failure)
• Cracks under a load-bearing structure like a shed wall or post
• Multiple cracks radiating from a single point
• Crumbling or spalling concrete around the crack edges

If you're seeing any of the above, get an experienced concreter to look at it in person. Photos help, but someone on site can tell in minutes whether it's a patch job or something more.

Final Thoughts

Concrete cracks. That's not a failure of the material or the tradesperson — it's physics. What separates a well-built slab from a poorly built one isn't whether it cracks but how it cracks. Thin hairline cracks along a control joint are a slab doing exactly what it was designed to do. Wide diagonal cracks across the middle of a slab mean something went wrong at the subgrade, reinforcement, or thickness stage.

The best defence is good preparation: a compacted base, correct thickness, proper SL72 or SL82 mesh held at mid-depth, control joints at the right spacing, and proper wet curing through the first week. Get those right and your slab will be boring for decades — which is exactly what you want a slab to be.

If you're planning a new shed, garage, or concrete footpath in Brisbane or SEQ and want it done right the first time, get in touch for a quote. You can also check the pricing guide for a starting-from idea of costs, or read about common mistakes with small concrete jobs to avoid the pitfalls.