How thick should an air conditioner concrete pad be in Australia?

A residential air conditioning concrete pad must be at least 100 mm thick to comply with standard domestic building practices. For heavier commercial split systems, ducted units, or reactive clay soils (Class H or E), increase the pad depth to 150 mm with SL72 reinforcing mesh set on 50 mm bar chairs to prevent vibration-induced cracking.

How long should a concrete pad cure before installing an AC unit?

Allow the concrete pad to cure for at least 48 to 72 hours before placing an air conditioner condenser on it. While modern concrete reaches structural serviceability for light loads within 3 days, full design compressive strength requires 28 days under AS 3600. Keep the slab damp during hot weather to prevent shrinkage cracking.

What size concrete pad do I need for a split system condenser?

The concrete pad should extend at least 100 mm to 150 mm beyond the unit casing on all four sides. A standard domestic condenser measuring 800 mm wide by 350 mm deep requires a finished pad size of approximately 1000 mm by 600 mm. This extra perimeter provides structural clearance for masonry wedge anchors and vibration-isolation mounts.

Do I need reinforcing mesh in a small concrete AC pad?

Yes, reinforcing mesh such as SL62 or SL72 is strongly recommended for outdoor concrete equipment pads. Reinforcement prevents cracking caused by ground movement, thermal expansion, and continuous mechanical vibration from the condenser compressor. Ensure the steel mesh sits on 50 mm plastic bar chairs to maintain specified concrete cover and avoid moisture ingress.

What concrete mix strength is required for an AC slab?

A minimum concrete strength of 20 to 25 MPa is required for residential equipment slabs under Australian standards AS 3600 and AS 2870. For DIY batches or small pours, use a quality commercial premix like dry-packaged concrete, mixed with clean potable water to a low-slump consistency to achieve maximum compressive strength and surface durability.

Can you pour an AC concrete pad directly on dirt?

No, concrete should never be poured directly onto loose soil or uncompacted turf. Excavate all organic topsoil to a minimum depth of 75 mm, install an edge timber formwork system, and lay a compacted 50 mm to 75 mm sub-base of crushed road base or recycled concrete aggregate. A compacted base prevents structural shifting and foundation settlement.

How to lay concrete pads for air conditioning units

Learn how to lay a concrete pad for an air conditioning unit. Expert trade guide covers excavation, sub-base compaction, formwork, and AS 3600 compliance.

Quick answer

An air conditioning concrete pad requires a minimum thickness of 100 mm using 20 to 25 MPa concrete poured over a 50 mm compacted road-base sub-base. The slab must extend 100 mm beyond the condenser footprint on all sides, include steel reinforcing mesh, and cure for 48 to 72 hours before loading.

Skill level: Intermediate
Time: 4–8 hours active work, plus 24–72 hours curing before loading depending on weather and mix
Trade: Building & Construction
Updated: 4 November 2026

Tools you will need

Stabila 196-2 LED - 1200mm Box Beam Spirit Level
Marshalltown MPB145D - 350mm x 100mm Magnesium Hand Float
Milwaukee M18 FUEL - 125mm Braking Angle Grinder
Kelso 100L - Heavy Duty Poly Tradesman Wheelbarrow
Cyclone Square Mouth - Post Hole and Trenching Shovel
Masterfinish by Holer 150mm - Edging Trowel
Bostik Blu-Seal - Concrete Curing Compound or 200um Polythene Sheeting

🧱 Concrete Pads for Air Conditioning Units — Professional Trade Guide

Why is a correctly laid concrete pad essential for air conditioning units?

Laying a concrete pad for an air conditioning condenser is a straightforward but important task that affects unit stability, drainage, vibration control, and long-term serviceability. On residential jobs in Australia, the pad must be properly founded, level, and sized to suit the installed unit while allowing clearances required by the manufacturer and local code requirements.

This guide covers site set-out, excavation, base preparation, formwork, reinforcement, concrete placement, finishing, and curing for a durable service pad suitable for typical domestic split system and package unit installations. Structural and footing variations should be confirmed where reactive soils, fill, or poor bearing ground are present, with engineering advice where required under local regulations and AS 3600.

What skill level is required to lay concrete pads for AC units in Australia?

Intermediate — Estimated time: 4–8 hours active work, plus 24–72 hours curing before loading depending on weather and mix

What specific tools are required for laying an AC unit concrete pad?

Tape measure, builder’s pencil, stringline and line level

1200 mm spirit level or laser level

Shovel, trenching spade and mattock

Plate compactor or hand tamper for small pads

Wheelbarrow or concrete mixer

Formwork tools: circular saw, drill/driver, screws or form pins

Screed board straightedge, magnesium float and steel trowel

Edging tool and jointer if required

Reinforcing mesh cutters or angle grinder

Rubber mallet and hammer

Bucket or measuring pail for batching water

Concrete vibrator pencil or tamping rod for larger pads

Broom for non-slip finish where required

What materials are needed for a durable AC unit concrete pad?

Concrete, typically 25 MPa premixed or site-mixed using 1:2:3 for general purpose work

Crusher dust or compactable road base for sub-base, typically 50–100 mm

Formwork timber, commonly 90 x 45 mm or similar straight boards

Reinforcing mesh, such as SL72 or light trench mesh as suited to pad size

Reinforcement bar chairs or packers to maintain cover

Polyethylene slip membrane optional on clean base where specified

Tie wire for reinforcement

Anchor bolts or masonry anchors if fixing down the unit base

Vibration isolation pads or anti-vibration mounts if specified by HVAC installer

Water for batching, finishing and curing

Curing compound or plastic sheeting

Expansion foam or isolation material if abutting masonry or slab edge

Safety Requirements

Wear PPE: safety boots, gloves, long sleeves, eye protection and hearing protection when cutting or compacting

Use P2 dust protection during excavation, dry cutting and handling cement products

Wet concrete is caustic; avoid skin contact and wash off immediately

Follow safe manual handling practices when moving cement bags, mesh and formwork

Use mechanical assistance or team lifting for large premix loads and compactors

Check for underground electrical, gas, water and communications services before excavation

Maintain safe access around the work area and isolate from other trades

If working adjacent to walls, raised ground or drops, provide compliant edge protection/scaffolding as required

Concrete work should align with AS 3600 principles, and any structural footing design or unusual ground conditions should be engineer-certified where required

Ensure condenser location, clearances and discharge positions comply with manufacturer requirements, NCC/state requirements and council rules where noise/setback controls apply

Step-by-Step Guide

Step 1: Assess location and confirm pad requirements

Confirm the condenser unit size, operating weight, fixing points and manufacturer clearance requirements before setting out the pad. For most residential units, the pad should extend at least 100 mm beyond the unit footprint on all sides, though larger overhang may be preferred for service access.

Check finished levels relative to adjacent paving, weep holes and drainage paths. The top of pad should generally finish above surrounding ground level by around 50–75 mm to reduce water ponding and sediment build-up under the unit.

On reactive clay, uncontrolled fill, or soft/wet ground, increase excavation and base build-up as needed or seek engineering input. If the condenser is to be wall-adjacent, ensure sufficient maintenance access and avoid blocking subfloor vents, drainage points or damp-proofing details.

Step 2: Set out the pad

Mark the pad location using stringline, spray marker or timber pegs. A common domestic condenser pad might be around 1000 x 500 mm, 1200 x 600 mm, or larger depending on unit size, but always size to the actual plant and maintenance zone.

Allow for formwork thickness and verify the pad sits square to the wall or fence line where presentation matters. Check the intended finished height using a laser or spirit level and confirm falls in the surrounding landscape direct water away from the dwelling.

Step 3: Excavate to depth

Excavate organic material, loose topsoil and any soft spots until competent founding is reached. Typical excavation depth for a small residential service pad is around 150–200 mm total, comprising 50–100 mm compacted base plus 75–100 mm concrete thickness.

Trim the base level and remove roots, debris and mud. If the subgrade is wet or unstable, undercut further and replace with compacted granular fill. Avoid pouring directly onto uncompacted topsoil, as settlement will cause rocking, cracking or water ponding.

Step 4: Prepare and compact the sub-base

Place compactable road base or crusher dust in layers and compact thoroughly. For small pads, a minimum 50 mm compacted base may be adequate on firm natural ground, while 100 mm is preferred where conditions are less reliable.

Compact in lifts no greater than 50 mm using a plate compactor or hand tamper. The base should finish level, dense and well-drained. Good sub-base preparation is critical to avoid differential settlement and corner cracking.

💡 On high-turnover summer jobs, keep a stock of road base and pre-cut forms on the ute or trailer. This saves mobilisation time for same-day condenser pad installs.

Step 5: Install formwork

Construct formwork to the finished pad dimensions using straight timber secured with stakes or pins. Set the top of forms to finished level, checking both length and crossfall.

For air conditioning pads, the finished top should generally be level to support the condenser frame correctly. Any surrounding surface drainage should be handled outside the pad rather than by sloping the pad itself, unless the manufacturer allows slight fall.

Brace the forms so they cannot spread during placement. Apply release agent if desired for cleaner stripping. Recheck diagonals and level before pouring.

Step 6: Place reinforcement

Install light reinforcing mesh where required, especially for pads larger than small proprietary service pads or where ground reliability is mixed. A slab thickness of 100 mm with centrally placed SL72 mesh is common good practice for a durable pad, though many very small pads may be poured unreinforced if ground conditions are excellent.

Support the mesh on bar chairs or packers so it sits in the upper-middle zone, maintaining concrete cover of about 30 mm minimum from edges and underside for this type of non-exposed residential work. Do not leave reinforcement lying on the base.

Where anchor bolts will be cast in, mark their positions from the condenser base rail dimensions before placement.

Step 7: Mix or order concrete

For consistency and productivity, premixed 25 MPa concrete is preferred when batching multiple pads or when access permits. For small isolated pads, site-mixed concrete may be used with a general purpose ratio of 1:2:3.

If higher performance or more structural duty is required, use a structural mix around 1:1.5:2.5 or an engineer-specified mix. Keep water addition controlled; overly wet mixes are easier to place but reduce strength, increase shrinkage and create laitance on the surface.

Only mix what can be placed and finished promptly in current weather. During hot summer conditions, stage labour and have curing materials ready before the pour starts.

Step 8: Pour and compact the concrete

Place the concrete evenly into the forms, avoiding segregation. Shovel or chute the mix in layers and work it into corners and around reinforcement.

Rod, tamp or lightly vibrate the concrete to remove air pockets, especially at edges. Do not over-vibrate small pads, as this may cause excessive bleed water and aggregate settlement.

Slightly overfill the forms, then strike off with a straightedge using a sawing motion across the top of forms. Recheck level immediately after screeding.

Step 9: Finish the surface

Once screeded, bull float or magnesium float the surface to close it up. For air conditioning units, the finish should be flat, dense and level rather than highly polished.

Edge the perimeter to reduce chipping. A steel trowel finish is acceptable if the unit will fully cover the pad, though a light broom finish can also be used where the pad remains exposed and accessed by service personnel.

Avoid finishing while bleed water is present. Premature trowelling traps moisture and weakens the wearing surface. If cast-in fixings are required, install them accurately while the concrete is still workable, checking centre-to-centre dimensions against the unit rails.

Step 10: Cure the pad properly

Begin curing as soon as the surface can be protected without marking. Apply curing compound or cover with plastic sheeting to retain moisture.

In hot, windy Australian summer conditions, cure for at least 3–7 days where practical. Prevent rapid moisture loss, as this is a common cause of surface dusting and early shrinkage cracking in small pads.

Do not allow HVAC installers to load the pad too early. As a practical guide, light loading may be possible after 24–48 hours in favourable conditions, but best practice is to allow longer where scheduling permits, particularly for cast-in anchors.

Step 11: Strip forms and complete surrounding area

Remove formwork carefully after the concrete has gained enough strength to avoid edge damage, typically the next day. Backfill around the pad with free-draining material and grade the adjacent surface away from the slab.

Clean up splatter, remove waste material and check the final pad for level, edge quality and dimensions. Confirm clearances for pipework, condensate drains and electrical isolator positioning remain suitable for the HVAC contractor.

Step 12: Install fixings or hand over for condenser installation

If the condenser is to be mechanically fixed, drill and install masonry anchors after sufficient cure, unless cast-in anchors were used. Fit anti-vibration pads where specified.

Before handover, verify the pad is stable, level and free of rocking or hollow corners. A properly formed pad reduces call-backs for vibration noise, cracked pipework supports and poor drainage under the unit.

Common Mistakes to Avoid

1.Pouring directly onto topsoil or unprepared ground without a compacted sub-base

2.Making the pad too small, leaving no perimeter allowance for fixings, vibration mounts or service access

3.Using an overly wet mix, which reduces strength and increases shrinkage cracking

4.Leaving reinforcement on the ground instead of supporting it on chairs with proper cover

5.Loading the pad too early before adequate curing and strength gain

6.Finishing below surrounding ground level, causing water ponding and sediment accumulation

7.Ignoring manufacturer clearance requirements and local noise/setback constraints

Pro Tips

1.Use 100 mm slab thickness as a reliable standard for most residential condenser pads unless site conditions or specification dictate otherwise. It provides better durability than ultra-thin pads with minimal extra material cost.

2.Keep the top of pad about 50–75 mm above finished ground and grade the surrounds away. This significantly reduces splashback, corrosion risk and maintenance issues.

3.For repetitive builder work, pre-make common form sizes such as 1000 x 500 mm and 1200 x 600 mm to speed up set-out and keep dimensions consistent across lots.

4.Where vibration or acoustic complaints are likely, coordinate early with the HVAC contractor on anti-vibration mounts and location relative to lightweight bedroom walls.

5.Hiring a plate compactor is more economical than purchase for occasional one-off pads, but if you’re doing multiple summer installs each week, ownership usually pays for itself quickly in labour and call-back reduction.

Recommended Tools

Belle Plate Compactor — reliable compaction for sub-base preparation on repetitive small slab work

Stabila 1200 mm Spirit Level — accurate level control for formwork and finished pad setup

Ox Magnesium Float — good control for small pad finishing without overworking the surface

Makita Angle Grinder with diamond blade — useful for trimming mesh and cutting form stakes or anchors

Rapidset or Ramset masonry anchor kit — practical for post-installed condenser hold-down fixings where required

Frequently asked questions

How thick should an air conditioner concrete pad be in Australia?: A residential air conditioning concrete pad must be at least 100 mm thick to comply with standard domestic building practices. For heavier commercial split systems, ducted units, or reactive clay soils (Class H or E), increase the pad depth to 150 mm with SL72 reinforcing mesh set on 50 mm bar chairs to prevent vibration-induced cracking.
How long should a concrete pad cure before installing an AC unit?: Allow the concrete pad to cure for at least 48 to 72 hours before placing an air conditioner condenser on it. While modern concrete reaches structural serviceability for light loads within 3 days, full design compressive strength requires 28 days under AS 3600. Keep the slab damp during hot weather to prevent shrinkage cracking.
What size concrete pad do I need for a split system condenser?: The concrete pad should extend at least 100 mm to 150 mm beyond the unit casing on all four sides. A standard domestic condenser measuring 800 mm wide by 350 mm deep requires a finished pad size of approximately 1000 mm by 600 mm. This extra perimeter provides structural clearance for masonry wedge anchors and vibration-isolation mounts.
Do I need reinforcing mesh in a small concrete AC pad?: Yes, reinforcing mesh such as SL62 or SL72 is strongly recommended for outdoor concrete equipment pads. Reinforcement prevents cracking caused by ground movement, thermal expansion, and continuous mechanical vibration from the condenser compressor. Ensure the steel mesh sits on 50 mm plastic bar chairs to maintain specified concrete cover and avoid moisture ingress.
What concrete mix strength is required for an AC slab?: A minimum concrete strength of 20 to 25 MPa is required for residential equipment slabs under Australian standards AS 3600 and AS 2870. For DIY batches or small pours, use a quality commercial premix like dry-packaged concrete, mixed with clean potable water to a low-slump consistency to achieve maximum compressive strength and surface durability.
Can you pour an AC concrete pad directly on dirt?: No, concrete should never be poured directly onto loose soil or uncompacted turf. Excavate all organic topsoil to a minimum depth of 75 mm, install an edge timber formwork system, and lay a compacted 50 mm to 75 mm sub-base of crushed road base or recycled concrete aggregate. A compacted base prevents structural shifting and foundation settlement.

Written by Bricko, the Building & Construction specialist at Aus.Tools.