More Than Just Size: Why Garage and Shed Slabs Differ
At first glance, a garage slab and a shed slab are both just "concrete slabs." But they're actually quite different – designed for different purposes, different loads, and different conditions. Understanding these differences is crucial for getting the right slab for your project.
This guide walks you through the key differences, helps you understand which you actually need, and explains how these differences affect cost and durability.
Key Differences at a Glance
Here's what separates a garage slab from a shed slab:
- Purpose: Garage slabs support vehicle weight; shed slabs support building weight only
- Thickness: Garage slabs are typically 100mm minimum; shed slabs are often 75-100mm
- Load type: Garage slabs handle dynamic vehicle loads and parking; shed slabs handle static structure weight
- Surface finish: Garage slabs need slip resistance; shed slabs require less grip
- Drainage: Garage slabs must shed water; shed slabs have simpler drainage needs
- Edge details: Garage slabs often have edge beams; shed slabs can be simpler
- Cost: Garage slabs typically cost 10-20% more due to specifications
Thickness Comparison: Sheds vs Garages
Shed Slabs
Typical thickness: 75mm to 100mm
Most standard sheds use 100mm concrete. Some smaller garden sheds or storage pads might be 75mm. This thickness is adequate for the static weight of a timber-frame shed structure and the contents inside.
The 75mm option is the absolute minimum and only appropriate for very small, light structures (tiny garden sheds, not full-size working sheds).
Garage Slabs
Typical thickness: 100mm to 150mm
Garage slabs are always at least 100mm, with 150mm increasingly common for modern builds. The vehicle weight (2,000–2,500 kg for typical cars) creates concentrated dynamic loads as the vehicle drives and parks. This requires a thicker, stronger slab to handle the stress.
Some engineers recommend 200mm for heavy-duty garages or areas with reactive clay, but 100mm is the code minimum in most circumstances.
Why the Difference Matters
It's about load type. A shed distributes its weight across the entire floor as one static load (the building + contents). A car is a moving, dynamic load that concentrates weight on tires. The stress patterns are completely different, requiring different slab designs.
Load Requirements: Foot Traffic vs Vehicle Weight
Shed Slab Loads
Shed slabs carry predictable, static loads:
- Weight of the shed structure (timber frame, roof, walls): typically 2–5 kg/m²
- Contents stored inside: varies, but shed slabs assume typical storage (tools, equipment, boxes)
- Foot traffic: occasional people walking across the floor
- Light equipment: lawn mowers, small tools, hand-operated equipment
There's no dynamic impact, no vehicle wheels, no repetitive heavy machinery. The load is consistent and evenly distributed.
Garage Slab Loads
Garage slabs must handle:
- Vehicle weight: 2,000–2,500 kg concentrated on four tire contact patches
- Dynamic loads: the impact and flexing as the vehicle drives, accelerates, and brakes
- Repeated loading: the same spot bears vehicle weight every time the car parks
- Heavy tools/workbenches: if used as a workshop, additional equipment weight
- Potential hydraulic fluid spills: vehicles leak, and concrete must resist these
This is why garage slabs must be thicker and stronger. The repeated dynamic loading – the vehicle driving over the same spots – creates fatigue stress that a thin shed slab couldn't handle.
Edge Beams and Footing Differences
Shed Slabs
Shed slabs typically have simple edge details:
- Finished edge on all sides, but no substantial edge beam
- Minimal reinforcement at edges
- Simple footing (usually just gravel base with compacted fill)
- Suitable for small structures where edge stress is minimal
Garage Slabs
Garage slabs often feature edge beams (raised concrete edges):
- Edge beam: a reinforced concrete edge, typically 100-150mm deep
- Purpose: prevents edge cracking from vehicle loads near the perimeter
- Footing: often includes concrete footings beneath the edge beam for added support
- Termite barrier: mandatory in Queensland, installed at the footing
The edge beam provides additional strength where vehicles might park close to the edge, and it serves as a structural frame for the entire slab.
Surface Finish Differences
Shed Slab Finishes
Shed slabs typically receive:
- Broom finish: textured surface for basic slip resistance, standard for storage sheds
- Plain finish: smooth surface without texture, perfectly acceptable for sheds
Aesthetics matter less for a shed floor. A basic broom finish is standard and adequate.
Garage Slab Finishes
Garage slabs require:
- Broom finish (standard): textured for traction, essential for vehicle safety
- Slip-resistant finish (recommended): can include special additives or pattern finishes for extra grip, especially important for wet conditions
- Appearance matters: people see the garage floor, so finish quality is visible
Safety is the key difference. A wet garage floor can be slippery for footsteps and vehicle tires. A proper textured finish is not optional.
Drainage Considerations
Shed Slab Drainage
Shed slabs have simple drainage needs:
- Slight slope (1 in 100 fall) to allow water to drain away from the structure
- Adequate base preparation to prevent waterlogging (gravel base layer)
- No specific front fall or perimeter drainage required
Garage Slab Drainage
Garage slabs require more careful drainage:
- Fall toward the front: typically 1 in 80 to 1 in 100 fall from back to front door, allowing water to exit the garage
- Edge drainage: sometimes includes a perimeter drain or step at the garage entrance
- Moisture barrier: often includes a vapour barrier beneath the slab to prevent groundwater ingress and moisture rising through the floor
- Purpose: vehicles bring water into garages (rain, snow, washing). The slab must shed this water to prevent puddles and moisture damage
Garage slabs are in contact with water more frequently and for longer periods, requiring more robust drainage design.
Typical Size Differences
Common Shed Slab Sizes
| Type | Common Size | Area |
|---|---|---|
| Small garden shed | 3m x 3m | 9m² |
| Standard shed | 6m x 6m | 36m² |
| Large storage shed | 9m x 6m | 54m² |
| Large work shed | 12m x 6m | 72m² |
Common Garage Slab Sizes
| Type | Common Size | Area |
|---|---|---|
| Single car garage | 3.5m x 6m | 21m² |
| Double car garage | 6m x 6m | 36m² |
| Large double garage | 6m x 7.5m | 45m² |
| Three car garage | 9m x 6m | 54m² |
Feature-by-Feature Comparison
| Feature | Shed Slab | Garage Slab |
|---|---|---|
| Thickness | 75-100mm | 100-150mm |
| Load Type | Static building weight | Dynamic vehicle weight |
| Reinforcement | Standard wire mesh (SL72) | Enhanced mesh or rebar |
| Edge Treatment | Standard edge finish | Often edge beam + footing |
| Surface Finish | Basic broom finish | Slip-resistant finish |
| Drainage | Simple slope away | Fall + moisture barrier |
| Moisture Control | Basic gravel base | Often includes vapour barrier |
| Lifespan | 25-30 years | 30-40 years |
Cost Comparison
Typical Costs
Garage slabs typically cost 10-20% more per square metre than shed slabs, primarily due to:
- Greater thickness (more concrete)
- Enhanced reinforcement
- Edge beams and footings
- Superior finish quality
- Drainage and moisture control features
Real Numbers
- Shed slab (6x6m, 100mm): approximately $2,400–$3,200
- Garage slab (6x6m, 100mm): approximately $2,800–$3,800
The $400–$600 difference for a standard double garage is the cost of the features that make it suitable for vehicles. It's money well spent – skimping on a garage slab can lead to cracking, surface deterioration, and expensive repairs down the line.
Which One Do You Need? Decision Guide
You Need a Shed Slab If:
- Building a storage shed, workshop, or equipment building that won't support vehicles
- Installing a garden structure (but a very large one, not tiny)
- Creating a covered work area where no vehicles will drive
- Cost is a priority and vehicles won't be parked on it
You Need a Garage Slab If:
- Building a garage, carport, or any structure where vehicles will be parked
- Creating a workshop where heavy machinery or vehicles might be driven
- Planning a multi-purpose space that might eventually be used for vehicles
- Want maximum durability and longevity (garage slabs last longer)
- Need a slab that resists moisture and chemical spills (vehicle fluids)
Consider a Garage Spec Shed If:
Sometimes the best choice is a shed slab built to garage specifications. This is ideal if you're unsure about future use or want extra durability for a premium shed. You get better longevity at minimal extra cost.
Common Questions About Garage vs Shed Slabs
No. A carport is where vehicles are parked, even if it's not a fully enclosed garage. Vehicles will be on that slab regularly. A carport slab must meet garage specifications – proper thickness, reinforcement, and drainage. Using a shed slab under a carport will result in cracking and premature deterioration. Always spec a garage slab for any vehicle parking area.
Yes. There's no reason not to. If you're unsure whether you might eventually park on the slab, or if you simply want better durability, upgrade to garage specifications. The cost difference is typically 10-20% – a relatively small investment for significantly better longevity and safety. Your concreter can easily build a shed slab to garage specs.
In Brisbane, with our clay soils and high water table in some areas, a vapour barrier is highly recommended for garage slabs. It prevents groundwater from rising through the slab, which can cause dampness, mold, and spalling (surface deterioration). It's not always mandatory, but it's worth the modest extra cost. Shed slabs in low-moisture areas might not need one, but a garage where people spend time should have one.
A well-built shed slab typically lasts 25-30 years. A garage slab, with its superior thickness and construction, typically lasts 30-40+ years. With proper maintenance (keeping surface sealed, managing water ingress), both can last even longer. A garage slab's durability advantage makes it a worthwhile investment if you think you'll be on the property long-term.
For sheds: 100mm is standard; don't go below this. For garages: 100mm is code minimum, but 150mm is increasingly recommended for durability and to handle reactive clay soils found in Brisbane. Talk to your concreter about your specific soil and intended use – they'll recommend the best thickness for your project.
Important Disclaimer
Not a licensed builder. This guide covers general slab design principles. Specific requirements for your project depend on local building codes, soil conditions, and intended use. Always consult with appropriate professionals for your specific project needs.
We specialize in small concrete jobs only – shed slabs, garage slabs, concrete footpaths, and small pads. We do not construct house slabs or provide structural engineering. For house slabs or commercial projects, consult a structural engineer.
Final Thoughts
Garage slabs and shed slabs look similar, but they're designed for fundamentally different purposes. Garage slabs are stronger, thicker, better drained, and built to handle vehicle weight. Shed slabs are simpler and more economical for building foundations.
The choice depends on your project's purpose. If there's any chance vehicles will be on it, upgrade to garage specs. The 10-20% extra cost is well worth the durability and safety you gain.
Ready to pour your slab? Contact us today to discuss which slab type is right for your project.