Reinforced slab-on-ground concrete floors that combined the floor and the foundation became popular during the late 1970s as they were considered to be cheaper (on flattish sites), termite, borer and vermin-proof, fire-resistant, draught-proof, and did not deteriorate.

They could distribute loads to the ground over the whole slab, thus creating a lower load per square metre than a floor supported on piles, making them particularly suitable where the soil bearing capacity of the ground was lower. As with particleboard flooring (see floors), it provided a flat level surface on which to construct and erect wall frames.

Concrete slab floor construction was often used for internal garages or lower levels of split level houses, with suspended floors used for other parts of the house.

Construction details

Excavation

Construction started with the removal of topsoil and excavation for footings. Footings were designed in accordance with the NZS 1900 Model Building Bylaw. In 1978 NZS 3604 became the standard for slab on ground floors.

Integral or separate slab

Construction was typically:

• foundation wall cast integrally with the slab as an edge thickened slab (Figure 1)
• constructed as a separate concrete (Figure 2) or concrete masonry (Figure 3) perimeter foundation wall with the slab laid later.

Figure 1: Concrete floor slab and integral footing.

Figure 2: Concrete floor cast into rebate in foundation wall.

Figure 3: Concrete masonry foundation wall and concrete floor.

The advantage of having the foundation footing integral with the slab was that less formwork was required as the edge thickening was cast with a 45° slope at the junction of the underside of the slab and the footing.

The separate perimeter foundation wall was more suitable where a taller foundation wall was required or a garage was being provided in split-level designs. The construction also allowed independent movement between the wall and the slab, and as hardfill was retained by the foundation walls, it made placement easier.

Footings

Foundation footings had the same requirements for size, depth and reinforcement as the foundation footings for suspended timber floors. The height of the finished floor level was typically a minimum 150 mm above the level of permanent paving and 225 mm above the level of unprotected ground.

Hardfill and DPM

A minimum 75 mm thick layer of compacted hardfill was placed over the entire floor area. This was covered with a 25 mm layer of sand or 50 mm site concrete (weak concrete) to provide a smooth base, over which was placed a continuous damp proof membrane (DPM).

The DPM typically consisted of polythene sheet, and it prevented ground moisture being absorbed by the concrete and then entering the building.

Reinforcing

Mild steel reinforcing mesh was placed over the DPM and tied to the foundation footing reinforcing.

Concrete

Finally, the concrete for the slab (and footing if combined) was placed, compacted, floated, then mechanically steel trowelled to a level, smooth finish. The concrete was then cured by keeping the slab wet for seven days.

Pipes

All in-slab waste pipe locations had to be accurately positioned and tested before the slab was poured.

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Finishes

When concrete was used, wall-to-wall carpet was a popular floor covering in the bedrooms, living room and sometimes the dining room. Linoleum, vinyl sheet, vinyl tiles, quarry (clay) tiles or cork tiles were commonly used in wet areas such as the kitchen, bathroom and laundry.