Other than marginally lower construction cost, there isn’t much going for timber stump construction these days. If you are considering timber stumps for your next townhouse or unit development, be aware of the medium-term drawbacks that make them the foundation of last resort.
As the designers and builders of a certain tower in Pisa will attest, strong foundations are crucial in any building project.
Homes constructed in Australia from the 1950s through to the 1980s were typically built on timber or concrete stumps with timber bearers and joists forming up the rest of the floor structure. Over the last couple of decades, concrete slab foundations have become increasingly common, as it has become easier and cheaper to construct.
Drawbacks of Stump Floors
In more recent years, we have been hard pressed to make a strong case for timber stump construction except for in exceptional cases. While timber stump construction is marginally more cost effective, and does provide for ongoing sub-floor accessibility, we believe that the concrete slab construction is a better all-round solution, particularly in the following areas:
Timber Rotting and Decay
As a natural material, timber is susceptible to moisture and long-term rotting. Especially in areas that have high humidity, are not well drained, or are subject to occassional flooding, timber rotting may accelerate the eventual deterioration of the structural framing.
Differential Settlement of Stumps
While there is typically a strip footing around the edge of the building, timber stumps are usually laid out in a grid internally. Each stump footing is founded locally in the ground and can individually be subject to local depressions as soils move over time.
Pest Intrusion and Termite Damage
Timber stump construction creates a sub-floor area beneath your floor which is good for accessing services, but may also become a breeding ground for rodents and pests. In an area that is susceptible to termite infestation, timber structures in the ground is just asking for trouble.
Insulation and Energy Efficiency
In contrast to concrete slab construction, timber framed floors are very poor insulators. It will be important to ensure that insulation is installed correctly and maintained, in order to achieve a satisfactory energy rating.
Home Owner Experience
Generally speaking, concrete slabs provide firmer foot fall experience for the end user. The more solid feel gives your potential buyers a greater perceived sense of robustness. Ultimately these factors generally lead to higher selling prices for certain demographics.
The Exception to the Rule
For all the above reasons, concrete slab construction is generally better than timber stump construction for foundations. This is especially the case when the cost difference is marginal.
However, in the instance where site conditions require that the floor level of your proposed dwellings is significantly higher than the natural surface level (i.e. your proposed development is elevated), the cost of providing engineered fill as a base for your concrete slab can become prohibitive.
In such instances, we would recommend that timber floor construction be considered, as a foundation of last resort.
A Final Word
When you are designing or constructing your next townhouse or unit development, don’t be lured by slightly more affordable timber stump construction options. While there may be marginal benefit in construction cost, these are likely outweighed by long-term user risks and end consumer willingness to pay.
If you are looking for structural and drainage design engineers for your next unit, townhouse or multi-dwelling development, please get in contact with us so we can provide you with a comprehensive fee proposal for our services. We look forward to partnering with you on your next project.
