Costs of concrete versus timber for domestic floors
Most new houses are built on a concrete slab ground floor. Some surveys indicate that the national average is 80% of new houses built on concrete, although there is some regional variation. The main reason for the dominance of concrete is cost. But on some sloping sites, timber floors can be cheaper.
On a flat site in any of the main centres, concrete flooring is up to $20 per metre cheaper than a timber floor. Indeed concrete costs would need to rise substantially before timber would become cost competitive in most locations, (see Figure 1).
Hillside sites
However, Figure 2 shows the ground slope at which a timber floor becomes cheaper than a concrete slab for a particular layout of house. The options compared in Figure 2 are a timber floor on timber piles or a concrete slab on a bench cut into the hillside. The critical slope, above which timber floors are cheaper than concrete depends on a variety of factors, but Figure 2 shows one of the pivotal factors, namely the cut/disposal (excavation) rate for the bench. If the cut material can be disposed of on-site, then the cur/disposal cost rate is low and the concrete slab is competitive on quite steep slopes.
But it is more likely that the cut material will need to be disposed of off-site. In the main centres, a rate of $25 per m2 would be typical and the critical slope is around 12 degrees, At this slope, the maximum height of the timber piles is about 2 m for the example show, and is well within the scope of braced piles in NZS3604, the light timber frame standard, which provides for piles up to 3m. The example allows for the cost of enclosing the subfloor with baseboards in the timber option. If enclosure is not required, then further costs are saved on the timber option and the critical slope (above which timber foundations are cheaper than concrete slab) reduces to 9 degrees.
Timber floors on flat sites
Returning to flat sites, the designer may choose a timber floor, despite the cost disadvantage, for a variety of reasons, including:
• a suspended timber floor provides more clearance against flooding
• a preference for the slight give underfoot of a timber floor compared to a slab
• additions to an existing house so that the same floor level is carried through.
NZS3604 provides for a variety of solutions of pile, bearer and joist sizes and spans. Figure 3 indicates the costs of five choices of layout using commonly available sizes of bearer and joist. It is apparent that the total floor cost varies widely between the choices — up to 22% difference between the cheapest and dearest options.
Much of the cost difference arises in the joist price. It is preferable to use the smaller joist sizes, particularly below 200 mm, as then the cost of joist nogs is avoided. This is offset to some extent by the need for more piles, but generally the smaller joist sizes give the most economic timber floor solution. Figure 3 is for a particular floor layout of 17.4 m x 8.7 m, and 102 m2 floor area. The same result holds with other layouts, for example, a square 12.2 m x 12.2 m floor plan with a 150 m2 floor area. This shows that bearer and joist sizes in Options 2, 3 and 5 are still the cheapest layouts.
The above examples indicate potential cost savings in foundations and floor framing that are possible through careful design and layout by builders and designers. The costs in Figures 1, 2, and 3 are for a particular size and layout of the floor plate and designers need to carefully consider their size circumstances in assessing the cost effectiveness of various designs.
* Ian Page is the BRANZ Economist.
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