Learn How Thick A Concrete Patio Should Be Before You Order Concrete Or Build Formwork. This Guide Explains Why 75 mm Is Only A Lower-End Reference For Light Non-Load-Bearing Slabs, Why 100 mm Is A Common Practical Planning Thickness For Many Residential Patios, And When Loads, Ground Conditions, Edges, Reinforcement Or Engineering May Require A Different Slab Design.
There Is No One Thickness That Is Correct For Every Patio. Cement Australia Notes That A Non-Load-Bearing Slab May Have A Minimum Thickness Of 75 mm, But Also Advises Consulting A Concrete Professional Because The Required Thickness Depends On The Load Placed On The Slab.
Lower-End Reference For Light Non-Load-Bearing Slabs Only; Not A Universal Patio Rule.
A Common Practical Planning Thickness For Many Pedestrian Residential Patio Slabs When Site Conditions Suit.
May Be Appropriate Where Loads, Poor Ground, Local Thickening Or Structural Requirements Demand More Capacity.
A Firm, Compacted, Well-Drained Base Is Critical To Supporting The Slab Even When Thickness Is Adequate.
Mesh Or Bars Must Be Correctly Selected, Supported And Positioned If The Design Requires Them.
Control And Isolation Joints Help Accommodate Shrinkage And Movement Instead Of Relying On Thickness Alone.
A Good Patio Relies On The Concrete Slab, Correctly Positioned Reinforcement Where Required, A Stable Compacted Base And A Subgrade That Drains And Supports The Work.
Patio thickness is often reduced to one question: “Is 75 mm enough, or should I pour 100 mm?” The useful answer is that thickness cannot be separated from the job the slab must perform. A patio used only for outdoor furniture and foot traffic does not have the same loading as a slab carrying a masonry barbecue, large spa, outdoor kitchen, heavy planter walls, posts or occasional vehicles.
The ground below the slab matters just as much. A well-compacted base over stable, well-drained soil can support concrete more uniformly than soft fill, poorly compacted ground or a site that becomes saturated. Increasing thickness can add capacity, but it does not automatically correct poor drainage or unstable ground.
Cement Australia states in its slab quantity guidance that a minimum thickness for a non-load-bearing slab would be 75 mm and advises consulting a concrete professional because slab thickness depends on the load placed on it. That makes 75 mm useful as a lower-end reference for genuinely light, non-load-bearing work rather than a universal patio specification.
A 75 mm slab leaves less depth for reinforcement positioning, concrete cover, local surface variation and construction tolerance than a thicker slab. It also contains 25% less concrete than a 100 mm slab over the same area. For these reasons, many residential patio projects are planned thicker than the absolute lower-end reference even when they carry only pedestrian loads.
Around 100 mm is a common practical planning thickness for many residential slabs and outdoor flatwork because it gives more concrete depth than a 75 mm slab while remaining straightforward to form and place. Cement Australia's DIY slab guide even uses 100 mm formwork as an example when discussing slab setup.
However, 100 mm should not be presented as a structural guarantee. A 100 mm patio poured over uncompacted fill can still crack or settle. A 100 mm patio carrying a heavy concentrated load may require local thickening, special reinforcement or a separate footing. The slab design must fit the use.
A thicker slab may be considered where the patio will carry heavier or concentrated loads, where the ground is less reliable, where local edges or beams are required, or where a structural designer specifies additional depth. The entire patio does not always need to be equally thick: some designs use a general slab thickness with thickened edges, pads, beams or independent footings at load points.
Examples that deserve closer review include large masonry fireplaces, masonry outdoor kitchens, spa pools, pergola or roof columns, substantial retaining elements, heavy storage, machinery and any location that may be driven over by a vehicle. These loads should not be added to a light pedestrian patio after the slab is poured without checking whether the slab and foundations can carry them.
| Patio Use | Planning Thickness Context | Load Level | Design Note |
|---|---|---|---|
| Light Pedestrian Patio | 75–100 mm Is A Common Planning Conversation | Light | 75 mm Is Only A Lower-End Non-Load-Bearing Reference |
| General Residential Patio | About 100 mm Is Commonly Considered | Light–Moderate | Base, Joints And Reinforcement Still Matter |
| Heavy Outdoor Furniture / Kitchen | 100 mm+ May Be Considered | Moderate | Check Concentrated Loads And Footings |
| Spa / Masonry Fireplace / Columns | Project-Specific Design | Heavy / Concentrated | May Need Local Footings Or Thickening |
| Vehicle Access | Do Not Treat As A Pedestrian Patio | High | Use Driveway/Pavement Design Guidance |
This table is deliberately qualitative. A number by itself cannot account for soil movement, drainage, slab dimensions, reinforcement and load distribution. Where the patio has structural loads or forms part of a building system, use the project documentation rather than a generic web table.
Concrete is strong in compression but it does not bridge unsupported ground indefinitely. If the soil or base settles unevenly, part of the slab can lose support. Bending stresses then increase and cracking can develop. A stable base helps distribute loads and provides a consistent platform for construction.
Cement Australia's DIY slab guide recommends allowing additional depth for road base or crusher dust, placing a compacted bedding layer and emphasises that a firm, well-drained base is important for reducing cracking. The exact base type and thickness should suit the site rather than being copied mechanically from a small-garden-slab tutorial.
Small DIY guides often show around 50 mm of compacted bedding below a simple slab, but real projects may need more excavation, better-quality road base, geotechnical preparation or removal of unsuitable fill. The base requirement depends on soil, drainage, previous disturbance and the load.
A patio placed over recently backfilled ground beside a new house can be more vulnerable to settlement than one placed over undisturbed competent soil. If the ground has been filled, confirm compaction requirements before forming the slab.
Water is a major contributor to patio problems. Saturated subgrade can lose support, expansive soils can move with changing moisture, and poor surface falls can send water toward the building. The patio should be planned as part of the site drainage rather than simply poured level because a level slab looks tidy.
Surface drainage should direct water to an appropriate point without creating hazards or damaging neighbouring property. The required falls depend on the layout, finish, drainage system and local building requirements. Patios adjoining a house also need careful treatment at door thresholds, damp-proofing zones, weepholes and wall interfaces.
Outdoor concrete commonly needs positive drainage away from buildings unless a designed drainage system handles water differently. The actual slope should work with doors, finished floor levels, drains and local construction requirements. Do not raise a patio against wall systems or cover drainage openings simply to make the slab thicker.
A small change in thickness can make a significant difference to the concrete order. Because volume equals area multiplied by thickness, increasing a patio from 75 mm to 100 mm adds one-third more concrete compared with the 75 mm quantity.
| Patio Area | 75 mm Thick | 100 mm Thick | 125 mm Thick |
|---|---|---|---|
| 10 m² | 0.75 m³ | 1.00 m³ | 1.25 m³ |
| 20 m² | 1.50 m³ | 2.00 m³ | 2.50 m³ |
| 30 m² | 2.25 m³ | 3.00 m³ | 3.75 m³ |
| 40 m² | 3.00 m³ | 4.00 m³ | 5.00 m³ |
| 50 m² | 3.75 m³ | 5.00 m³ | 6.25 m³ |
These are geometric volumes before any practical allowance, thickened edges or beams. Once the final slab dimensions have been selected, the Concrete Measurement Calculator can help estimate the volume required.
Concrete Volume (m³) = Length (m) × Width (m) × Thickness (m)
Millimetres must be converted to metres before multiplying. A 75 mm patio is 0.075 m thick, 100 mm is 0.100 m and 125 mm is 0.125 m. If the patio contains several shapes, divide it into manageable sections and calculate each volume separately.
Reinforcement requirements depend on the patio design. Mesh can help control crack widths and improve the slab's ability to distribute tensile stresses, but reinforcement does not stop concrete from shrinking or guarantee a crack-free patio. It also works properly only when it is positioned where the design requires it.
Cement Australia's small-slab guide notes that reinforcing mesh, where required, should be supported on bar chairs and fully encased rather than left sitting directly on the compacted base. This is an important principle: mesh at the bottom because workers walked it down may provide very different performance from correctly supported reinforcement.
Some DIY guidance shows reinforcement supported within the slab rather than on the ground, but exact reinforcement depth is a design issue. Cover requirements depend on exposure, reinforcement type and specification. Do not use a generic “middle” rule when structural drawings specify a different position.
Fibres can improve certain fresh and hardened concrete properties, but not every fibre product is a direct structural replacement for conventional mesh or reinforcing bars. The fibre type, dosage and design intent matter. If a supplier proposes fibre-only reinforcement, ask whether it is specifically designed and documented for the patio application.
Concrete shrinks as it loses moisture and experiences temperature movement. Control joints create intentional weakened planes so shrinkage cracks are more likely to form in planned locations. Joint layout should consider slab shape, dimensions, corners, columns, penetrations and interfaces with existing concrete.
Cement Australia's DIY slab guide recommends adding control joints on larger slabs and uses 1.5 m intervals in its simple small-slab tutorial. That specific spacing should not be copied blindly to every patio, but the underlying lesson is sound: joint planning is part of slab design, not a cosmetic step added after cracking begins.
A patio that meets a house, column, existing slab or other fixed element may need an isolation or movement joint so the new concrete can move independently. Pouring hard against another structure can create restraint and encourage random cracking or transfer loads in unintended ways.
Joint type, material and location should suit the detail. A decorative saw cut is not automatically equivalent to a full-depth isolation joint.
Long narrow slabs, re-entrant corners, L-shapes, sharp changes in width and openings can concentrate stress. Joint layout becomes especially important in these geometries. Where practical, divide the patio into more regular panels and avoid creating thin “fingers” of concrete around garden beds or posts.
Some patios use thickened edges where the slab borders soft ground or supports a load, but edge thickening should have a purpose. If a post, wall or roof load requires a footing, simply making the slab edge a little thicker may not be enough. Structural loads should be transferred through a designed footing or slab system.
A pergola, patio roof or verandah post can carry substantial wind and gravity loads. The concrete supporting that post may need an isolated footing or engineered thickening that is much deeper than the general patio slab. Do not assume a 100 mm patio can function as a post footing simply because the post sits on top of it.
Water is heavy. A spa can create a large concentrated load once the shell, water and occupants are included. The manufacturer may specify foundation requirements, and those requirements should control the slab design. A general pedestrian patio thickness is not an appropriate substitute.
Lightweight modular outdoor furniture is very different from brickwork, blockwork or stone benchtops. A masonry fireplace or barbecue can impose concentrated loads along a relatively small footprint. If heavy permanent features are planned, design the supporting slab or footing before the concrete is poured.
A pedestrian patio should not be assumed suitable for vehicle traffic. Vehicle loads can create higher stresses and repeated loading, especially near slab edges. If the area may be used for parking or access, treat it as a driveway or pavement design problem rather than a patio.
CCAA publishes a Residential Concrete Driveways and Paths data sheet specifically addressing residential paving design and construction. For concrete quantity once dimensions are known, use the Concrete Driveway Calculator.
Thickness and compressive strength are separate design variables. A stronger concrete mix does not automatically compensate for an inadequately supported slab, poor jointing or insufficient thickness. Likewise, a thick slab made with poor concrete and inadequate curing can perform badly.
Where a specified strength is required, follow the project documentation and use appropriately supplied concrete. Cement Australia advises that projects requiring a specified final concrete strength should use professionally supplied concrete rather than relying on packaged DIY concrete to certify a structural strength.
Bagged concrete is convenient for small pours, but a patio quickly becomes a large volume. Cement Australia's DIY guidance suggests bagged concrete for very small jobs, site mixing for intermediate quantities and ready-mixed concrete for larger pours where access allows. A continuous patio pour is often easier to place consistently with ready-mix once the volume becomes substantial.
If you use bagged concrete, use the exact yield printed on the product data sheet. The Concrete Bag Estimator can estimate whole bags once the slab volume and manufacturer yield are known.
At 75 mm thick, a 20 m² rectangular patio has a geometric volume of 1.50 m³. At 100 mm, it requires 2.00 m³. At 125 mm, it requires 2.50 m³. These figures exclude thickened edges, footings and any contingency allowance.
A 30 m² patio requires 2.25 m³ at 75 mm, 3.00 m³ at 100 mm and 3.75 m³ at 125 mm before waste or thickened sections. A 25 mm thickness increase over a 30 m² area adds 0.75 m³ of concrete, which can materially affect cost.
Increasing thickness increases concrete volume, excavation depth, formwork depth and potentially reinforcement or finishing labour. However, choosing the thinnest possible slab solely to save concrete can be false economy if the slab is not suitable for its load or ground.
A sensible cost comparison should include the entire patio: excavation, base material, compaction, formwork, reinforcement, concrete, pump or barrow access, finishing, joints and curing. Concrete-only cost is just one part of the project.
Soil behaviour varies widely. Sand, clay, uncontrolled fill and reactive ground behave differently when moisture changes or loads are applied. Patio work close to a new house can also encounter trenches and service backfill that are less compact than surrounding natural ground.
If the subgrade is questionable, removing weak material and replacing it with properly compacted base can be more effective than simply adding concrete thickness. Where movement is significant or the patio is structural, obtain geotechnical or engineering advice.
An external patio and the house slab can move differently. Tying them together without design can transfer movement or create cracking. Leaving them completely unrelated can also create a drainage or level problem if detailing is poor. The interface should account for movement, damp-proofing, finished floor levels, drainage openings and any structural connection.
Pool surrounds can experience settlement, wet conditions and movement at the pool-shell interface. Slip resistance and drainage are also important. The slab, joints and finish should be planned with the pool structure rather than treated as a standard garden pad.
A level patio on a sloping site may require excavation on the high side and retaining or fill on the low side. Thickening the concrete does not replace proper earthworks. Uncontrolled fill at the low edge can settle, while unsupported edges may crack.
Significant level changes may require retaining structures or engineered fill. Consider finished drainage and surrounding ground levels before setting the formwork height.
Formwork establishes the slab dimensions, edges and finished levels. Cement Australia's slab guide recommends using formwork timber that matches the intended slab thickness so the surface can be screeded level with the top. Pegs should be firm and low enough not to obstruct the screed.
Curved patios, steps, rebates and thickened edges need more detailed formwork. Check dimensions, diagonals, falls and levels before concrete arrives; errors become expensive after the pour begins.
Concrete should be placed without excessive segregation and consolidated so it fills around reinforcement and against formwork. Poor compaction can leave honeycombing or voids. On a patio, the crew also has to keep up with screeding, edging and finishing before the concrete becomes too stiff.
A very smooth steel-trowelled finish can become slippery outdoors. Cement Australia's DIY guidance notes that a broom or wooden-float finish can provide texture for outdoor pathways. Patio finish should consider slip resistance, appearance, cleaning, drainage and whether the surface will be exposed to rain.
Curing controls early moisture loss and supports cement hydration. Cement Australia's DIY slab guide recommends beginning curing the same day after finishing and describes keeping the surface moist for seven days in its small-slab example. Project specifications may require different methods or durations.
Hot, dry or windy weather increases evaporation. CCAA's hot-weather concreting guidance notes that high temperatures accelerate hydration and surface moisture loss. Plan enough labour, concrete supply and curing materials before the pour starts rather than trying to solve rapid drying afterward.
Cement Australia's DIY guide says its example slab can be walked on after three days and should avoid heavy loads for at least seven days. That is product-and-guide-specific advice, not a universal structural loading schedule. Concrete strength development depends on the mix, temperature, curing and load.
Heavy furniture, masonry construction, spa installation or other substantial loads should wait until the project requirements are satisfied. If early loading matters, confirm the required strength with the concrete supplier or engineer.
The difference is 0.025 m³ of concrete per square metre. Over 10 m² that is 0.25 m³. Over 40 m² it is 1.00 m³. This extra material can noticeably affect cost, but it also shows why thickness selection should be made before ordering, not adjusted casually on pour day.
Increasing from 100 mm to 125 mm adds 25% more concrete volume. A 40 m² patio changes from 4.00 m³ to 5.00 m³ before any extra allowance. If only local heavy-load areas need additional support, a designed thickened pad or footing may be more efficient than thickening the entire patio.
Overlaying existing concrete can create level, bonding, drainage and movement problems. The old slab must be stable and suitable for the proposed system, and raising finished levels must not compromise doors, wall drainage or damp-proofing. In some cases removal and replacement provides better control.
If an old slab needs to come out first, see the Concrete Removal Cost Guide for demolition, disposal and recycling factors.
Many simple ground-level pedestrian patios are routine residential flatwork, but engineering becomes more important when the slab carries structural posts, roof loads, masonry, retaining loads, a spa, significant fill, unusual ground conditions or interfaces with the building structure.
Local building approval requirements also vary. Cement Australia recommends obtaining the necessary local-authority approvals before starting DIY concrete work. Check your council, building surveyor or relevant authority where the patio is part of a larger building or structural project.
CCAA maintains Australian technical publications covering concrete construction, residential paving, slabs, finishes and hot-weather concreting. Its Guide to Concrete Construction 2020 provides contemporary industry guidance on concrete composition, performance and site practices. Cement Australia also publishes practical slab guidance for small DIY projects.
These resources are useful for understanding good practice, but a web guide cannot replace structural drawings, a geotechnical report, council conditions or product-specific instructions. Use the most specific project document available.
For a light non-load-bearing slab, 75 mm is a published lower-end reference. Around 100 mm is a common practical planning thickness for many residential patio slabs because it provides more depth for normal construction and outdoor use. Neither number should be treated as automatically correct.
The best patio thickness is the thickness that works with the actual load, base, soil, drainage, joint layout, reinforcement and project details. If the patio will support heavy or structural loads, design those loads first and calculate the concrete quantity second.
A Durable Patio Is A System. Thickness Works Only When The Other Parts Support It.
Know Whether The Patio Carries People, Furniture, Masonry, Posts, Spas Or Vehicles.
Stable, Well-Prepared Support Reduces Differential Settlement And Bending.
Good Joint Layout Helps Manage Shrinkage And Movement As Concrete Changes.
Protecting Early Moisture Helps The Concrete Develop Better Surface And Strength Properties.
Quick Answers About 75 mm, 100 mm, Reinforcement, Bases, Joints And Patio Loads.
Cement Australia gives 75 mm as a minimum reference for a non-load-bearing slab, while also advising professional advice because required thickness depends on load. A patio may need more depending on use and site conditions.
It may be suitable only for some light non-load-bearing work. It should not be treated as a universal patio thickness.
Around 100 mm is a common practical planning thickness for many residential patios, but base, ground, loads, joints and reinforcement must still be suitable.
It depends on the design. Where mesh is required, it should be properly supported and positioned rather than left on the ground.
Small DIY slab guides often show around 50 mm of compacted bedding, but actual base requirements depend on the soil, fill, drainage and load.
Larger slabs generally need planned joints to manage shrinkage and movement. Joint spacing and layout should suit the slab geometry and project requirements.
Do not assume so. A filled spa can create a heavy concentrated load. Follow the spa manufacturer and structural foundation requirements.
A pedestrian patio should not automatically be used for vehicle traffic. Vehicle use should be treated as driveway or pavement design.
The geometric volume is 2.00 m³ before any allowance, thickened edges or footings.
Outdoor concrete normally needs positive drainage, but the exact fall and drainage detail should suit doors, walls, drains and local requirements.
Curing requirements depend on the concrete and specification. Cement Australia's small-slab guide describes moist curing for seven days as a DIY example.
Sometimes overlays are possible, but the existing slab, bonding, movement, drainage and finished levels must be suitable. Removal may be a better option where the old slab has failed.
After The Final Patio Thickness, Shape And Any Thickened Sections Are Confirmed, Use The Relevant ConcreteCreek.com Tool Or Guide.
Convert Patio Dimensions And Thickness Into Concrete Volume.
Open CalculatorEstimate Packaged Concrete Bags From Volume And Product Yield.
Open EstimatorPlan Demolition And Disposal If An Existing Patio Must Be Removed.
Read GuideLearn About Concrete Ingredients, Curing, Strength And Durability.
Read GuideUse Project Drawings, Applicable Standards And Qualified Advice For Structural Or High-Load Patio Work.
Provides The 75 mm Non-Load-Bearing Minimum Reference And Advises Professional Thickness Guidance Based On Load.
View Slab GuidancePractical Guidance On Formwork, Base Preparation, Reinforcement, Joints, Finishing And Curing.
Read Slab GuideAustralian Industry Guidance On Concrete Materials, Performance And Site Practices.
Open CCAA GuideResidential Paving Guidance Relevant Where Outdoor Concrete Carries More Than Light Patio Loads.
Browse CCAA Data Sheets