Ask an architect what the differences are between the type of house they design and a standard project home offered by the volume builders, and they’ll give you a long (and worthwhile) list. Apart from the obvious factors and advantages of a bespoke design that is custom-tailored to the client’s needs, they might also cite the more efficient use of space and circulation; the attention to solar aspect and natural ventilation; the focus on insulation and colours; the attention to detail; and the vastly superior quality of the finishes, fittings, and overall aesthetic. Not to mention the fact that the custom builder who builds the architecturally-designed house will undertake and complete the construction in a very different manner to the volume builder that supplies the turnkey project home.
Project homes have their place, and whilst it’s easy for those with higher standards to be critical or dismissive, the reality is that the ubiquitous brick-veneer project home offers a relatively affordable and quality solution that has allowed hundreds of thousands of Australians to build their dream home. Head to the developing suburbs of any major city and you’ll see plenty of estates being occupied with the likes of what’s on offer out at Homeworld and other similar display centres.
Getting back to our main thrust: Architecturally, there’s no denying the differences that typify the contrast in quality and product between a garden-variety project home and a bespoke, architecturally designed home. But were you aware there is also a difference in the engineering of the two different house types?
For ease of assigning readily understood labels, let’s tag the two products here as project homes and high-end houses respectively. You’d be forgiven for thinking that the engineering of a project home is the same as a high-end home. After all, surely a steel beam will span the same distance regardless of whether the house cost $400k or $4M? Surely the floor joists don’t know or care whether they’re supporting a living room in Mosman or a living room in Kellyville?
In fact, it may surprise many to learn that the engineering of a project home is a very different exercise to that of designing a high-end house. We’ll give the reasons and provide a few examples shortly, but it’s important to first understand a few fundamentals…
- Project homes are more affordable because, amongst other things, they seek to achieve the absolute minimum. Yes, they also realise economies of scale through sheer volume and they realise efficiencies through “cookie-cutter” design and application. But it’s through the pursuit of achieving the bare minimum that the dollars are saved. Structurally, when it comes to the concrete slabs, the studs, the joists, and the rafters, there is no “fat” in the design. The member sizes are designed to within a millimetre of their capacity, so as to be as efficient as possible whilst still achieving the minimum strength and serviceability requirements of the relevant building and design codes.
- On a related front, the relevant design codes stipulate to designers the minimum requirements for strength and performance. For instance, the loading code (AS1170.1) tells the engineer the minimum design loads to apply to the structure. (As an example, a living room has to be designed for a minimum live load of 150kg/m2). Similarly, the Residential Timber Framed Construction code (AS1684) tells the engineer the maximum amount of deflection that is permitted (i.e. droop or sag), and how much vibration is permitted for a timber-joisted floor. These code-imposed limits – by intention – are not supposed to rule out or entirely prevent building movement. In other words, a floor can be designed and built entirely within the permissible limits of the codes, and yet still have a degree of flex and bounce that may be perceptible to the building’s occupants. Similarly, a house slab can exhibit a surprising amount of cracking and movement/rotation and yet still be well within what the codes permit. There is a trade-off between ideal performance and what is practical and affordable to build.
Project homes play this trade-off to perfection. (And that’s not a criticism…it’s a good thing). Houses – of any kind – may display and exhibit cracking, movement, floor bounce and other performance-based issues that may occasionally frustrate or upset the homeowner, but such movement should still fall within the permitted limits. To stipulate higher limits might reduce some of these issues or their likelihood of occurring, but it would also drive up the cost of construction, since all the structural elements would increase in size: The footings would be deeper; the slabs would be thicker; the beams, joists, and rafters would be bigger. No, our design codes actually do a fantastic job of ensuring our homes remain standing whilst providing what, ultimately, is viewed as an acceptable level of performance for the level of financial investment.
The fundamental difference with high-end houses is that their construction and “real life” usage often exceed these minimums. As do the expectations of their occupants. To put it bluntly: If you’ve just spent $3M building your house, you don’t expect your floor to bounce or your tiles to crack. And so there is an onus on the engineer to provide solutions that go beyond the minimum requirements of the code. This has led to the oft-misguided accusation that engineers “over design” their structures or are too conservative. But, again, the service life reality of a high-end house often exceeds the minimum requirements of the code and requires such an approach.
For example, 99% of project homes will utilise a waffle pod slab, which is well suited to brick-veneer construction and is an extremely economical solution. However, AS2870 (the Residential Slabs and Footings code) does not recognise or list waffle pods as a solution for full-brick construction on moderately reactive clays. Since most high-end houses feature full-brick construction (or, increasingly these days, off-form concrete walls), the high-end house must immediately turn to the more conventional and expensive raft slab or, alternatively, a strip footing system. To completely eliminate the risk of footing movement, some engineers may additionally design or suggest that the raft slab be further piered down to rock or more stable foundations if they are available on the site. Again, this simply works the dynamic on what is an acceptable level of performance for the level of financial investment – something that may be an option for a $3M high-end house, but not for the volume builder trying to deliver a $450k project home package.
Similarly, a standard project home will typically feature only one or more of three different floor finishes: Carpet, floating timber, or 10mm thick ceramic tiles that are direct-stuck. The waffle pod slabs and the timber-joisted floors can readily accommodate these. However, a high-end house might feature 20mm thick natural stone tiles that have to be laid on a 40mm thick screed bed. That’s an extra 120kg/m2 of dead weight on the floor structure that a standard project home would not satisfactorily support. Hence, the high-end house should feature thicker slabs or deeper floor joists to accommodate this.
Consider also amenity and appearance. Architects and homeowners LOVE flush floor levels when entering the wet areas. A high-end house will go to the effort and expense of creating a physical set-down in the structure so that, by the time the tiles and screed bed are laid to fall to the floor waste, the finished floor remains flush at the door entry. Creating this structural setdown can be a complicated and expensive detail at times, and the project builders simply do not entertain it. Walk through any project home and you’ll note there’s typically a 40mm step up in to the bathrooms and laundries – just perfect for you to stub your toe on in the middle of the night.
Project homes typically feature flexible finishes that can absorb and accommodate building movement. Internal walls will invariably be timber studs lined with gyprock. In contrast, a high-end house might feature full-brick construction, with the internal walls being rendered brickwork. Such a finish is highly brittle, and even the smallest amount of floor movement or deflection will induce cracking in the brickwork and render. Accordingly, the structure underneath the wall – be it a concrete slab or a dedicated steel beam – needs to be stiff enough to eliminate that risk of cracking.
Want some numbers to back this up? Let’s say a steel beam sits directly underneath an internal loadbearing wall and spans 4500mm. In the project home, the permissible deflection limit for the gyprock-lined stud wall is Span/250, and so the beam can deflect up to 18mm and still be within acceptable limits. However, if the same beam has to support a rendered brick wall in a high-end house, the permissible deflection limit to prevent cracking is Span/1000. Which means the deflection cannot exceed 4.5mm! And so getting back to the query we posed earlier – no, the beam doesn’t know or care whether it’s in Mosman or Kellyville – but it does have to be four times stiffer in this illustration, and will therefore be bigger. (Yes, we’re obviously ignoring the fact that the brick wall also weighs significantly more than the stud wall, which would drive a bigger beam in the first place, but our point remains valid: High-end houses require extra thought and engineering, and the structural outcomes will often be stronger and stiffer than what you’d typically see in a project home.)
The structural engineering of high-end houses is thus a specialist field. It takes experience and expertise to know how, where, and when engineering discretion needs to be applied, and when the engineering needs to step outside or go beyond the minimums or code requirements that would otherwise apply. It’s not something you pick up designing bridges or twenty-storey apartment buildings, and it’s certainly not something they teach at university.
As we’ve said on numerous occasions in other articles on our blog, the construction industry is definitely a field where you get what you pay for. That’s not to imply that cheap is nasty, but quality does cost. Building materials, building performance, and structural engineering are not commodities. If you’ve got the budget and desire to build a bespoke, architecturally designed house, then invest in a good architect, a good structural engineer, and a good builder. We can help with at least one of those.