How to build a concrete countertop with an integrated sink
A cast concrete benchtop gives a kitchen or bathroom a durable, architectural centrepiece, while a formed basin removes the separate sink rim from the design. It can suit Australian cabinetry, tiled splashbacks and warm indoor-outdoor rooms when treated as a structural product rather than a decorative cover.
An integrated sink can be a basin-shaped recess cast directly into the concrete, or a purpose-made shell held in formwork while the top is poured around its flange. A formed recess creates the smoothest visual result but is harder to shape and waterproof. A cast-in sink system is usually more predictable for a first major DIY project because its bowl, drain and wet-area sealing are supplied as one unit.
Concrete is heavy. A 900 by 600 mm countertop that is 80 mm thick weighs about 104 kilograms before reinforcement, water and sink fittings are added. Large sink openings and long unsupported spans increase bending stress, so cabinet load capacity, steel supports and reinforcement need careful attention before any concrete is mixed.
The work requires accurate formwork, controlled mixing and patient curing. Use metric dimensions, wear eye protection, gloves and suitable respiratory protection for dry cement, and keep power tools away from damp areas. Australian weather can change quickly, so curing must suit the local conditions rather than follow an assumed timetable.
Plan the worktop and basin
Set the finished dimensions from the cabinet layout, tiled splashback and appliance clearances. Check wall alignment, corner conditions and any service access beneath the sink. A 600 mm-deep top is common in Australian kitchens, but the cabinet size, sink bowl and cooking appliances should determine the final measurements. Mark the front overhang on the cabinet boxes so the reinforcement and supports align with the design.
The smoothest integrated basin is a true recess with a continuous fall towards the waste, a waterproof membrane and compatible detailing around every penetration. The alternative is a sink specifically labelled for cast-in-place installation. Follow its fixing template rather than scaling dimensions from a product photograph. Record the waste position, tap location, bowl depth and required support before building the mould.
Build rigid formwork and reinforcement
Construct a positive mould from sealed 18 mm plywood or a purpose-made countertop formwork system. Brace the sides closely enough to prevent bowing under concrete pressure, checking them with a spirit level before the pour. Apply a release agent only to surfaces that will remain form faces, and seal joints so wet concrete cannot leak into cabinetry. A basin former must be stable, smooth and correctly supported.
Place steel in the lower half of the slab, typically using 10–12 mm reinforcement bar or suitably rated mesh. Maintain the designed concrete cover, commonly around 25–40 mm, and provide adequate lap lengths at every joint. Frame large sink openings with additional reinforcement and support beneath the bowl. Keep steel clear from drains and waste fittings, since a misplaced bar can obstruct plumbing or make coring for a mixer impossible. Synthetic concrete fibres may control fine surface cracking but do not replace structural steel.
Prepare the cabinet and plumbing
Level and brace the cabinets, then install solid supports beneath the sink and along any long front edge. A reinforced concrete top may weigh more than the cabinet was designed to carry, particularly where several people lean on it during food preparation. Add adjustable legs, a steel subframe or structural cleats where necessary, and keep the supporting cabinetry sealed against water and movement.
In Australia, a kitchen-sink waste is commonly 50 mm while a bathroom basin often uses 40 mm, although the selected products and local requirements determine the correct size. Set the waste and any deck-mounted mixer position precisely before casting. Check the National Construction Code, plumbing requirements and AS/NZS 3500, then consult the local council or certifier where wet-area work is involved. A licensed plumber should handle concealed waste connections and any work requiring certification.
Mix and cast the concrete
Calculate the concrete volume from the finished dimensions and deduct the sink opening before adding a material allowance. The example above contains 43.2 litres before waste. Check the published yield of each bag rather than assuming a fixed amount per 20 kg bag, and allow roughly a 10% material take-off for the take-off quantity. Small projects are often supplied as bagged concrete, while a strong 25–32 MPa structural or decorative mix is more suitable than mortar or topping compound.
Add clean water exactly as directed by the manufacturer and mix until the concrete is uniform and workable. Do not stiffen the mixture by adding extra water after it begins setting, because extra water increases shrinkage and weakens the finished surface. Pour the concrete evenly into the form, working from one end towards the other. Rod or vibrate around the edges, reinforcement and basin opening to remove trapped air, while avoiding damage to the sink shell or formwork. Strike the top to level, allow bleed water to disappear, then float and trowel the surface.
Finish the surface and cure properly
Cover the finished top with a clean sheet of plastic supported just above the concrete, and use damp hessian beneath it where the product specification calls for damp curing. Follow the stated curing period, commonly several days, while treating 28 days as the point at which full strength is developing. Do not seal the surface tightly against the plastic, as trapped moisture can cause mottling. Remove temporary supports only when the concrete and supplier guidance allow it.
Climate matters across Australia. Perth’s hot, dry conditions and west-facing rooms can dry a surface rapidly, while Brisbane’s humid weather can extend surface drying times. Sydney and Melbourne can bring rapid temperature changes and drying winds. Shade the work from direct sun and wind, maintain gentle surface moisture without washing out cement, and protect the pour from heavy rain, traffic and impact.
Fit the mixer, waste and splashback
Fit the sink waste with the gasket or sealant specified by the sink manufacturer, then connect the trap and check for leaks with water. A basket strainer and overflow must sit at the correct height, and the waste must remain removable for cleaning. Work around any accessible cabinet joints that may move with temperature or building settlement rather than locking the rigid concrete assembly against them.
For a deck-mounted mixer, use a cast-in threaded insert where possible. If the hole must be drilled later, map the reinforcement first and use the mixer supplier’s cutting jig; many Australian mixers require a hole around 35–38 mm, but the specified fitting is the authority. Avoid tightening against uncast screw holes in plain concrete. Fill the movement gap between the benchtop and splashback with sanitary silicone rather than rigid grout, as the joint must accommodate small movements without cracking.
Seal, use and maintain the benchtop
Allow the concrete to reach the cure level required by the sealing system, then apply a compatible sealer approved for food-preparation surfaces when the benchtop will be used as a kitchen work surface. A penetrating sealer can reduce staining and water absorption, but it does not turn concrete into a waterproof membrane. Observe application, drying and ventilation instructions, and recoat when water no longer beads on the surface.
Use trivets for hot pans, wipe up acidic spills promptly and clean with mild detergent rather than abrasive pads. Fine pinholes can sometimes be filled with a compatible polymer repair mortar after curing, but deep cracks, repeated edge chips or movement-related cracking warrant investigation before the top is coated. Grinding repairs may release respirable silica, so use suitable respiratory protection, eye protection and wet or controlled extraction methods. Reinspect seals around the waste and splashback during ordinary cleaning.
Start with a full-size cardboard or plywood template, then have the cabinet supports and plumbing positions checked before buying steel, formwork or concrete. Order the published dry-mix quantity with the calculated allowance, protect the work area for curing and arrange the plumber’s attendance before the cabinet is closed in.
A properly supported mould, correctly positioned reinforcement, careful concrete placement and an uninterrupted curing period will provide the strongest foundation for a durable concrete countertop with a clean integrated appearance.