Getting the floor zone right

Here’s one for the architects out there:  When you’re designing a house for a client – in this case, a house that has two or more storeys – what are some of the architectural features that you first look to square away and “put in the bank”?   Is it providing natural light?  Cross-ventilation?  The materiality?  How about the spatials?  That is, things like room sizes/layouts and the floor-to-ceiling heights?  In planning for these things, do you ask yourself, “How thick should my floor be?”

In terms of a room’s sense of space and atmosphere, ceiling height plays a huge role.  Australian building codes prescribe a minimum ceiling height of 2400mm for habitable rooms, but at the high-end, luxury corner of town, 2700mm (9 foot in the old scale) is the established norm, if not merely the starting point.

At some point in your project, a structural engineer will join your team to help establish and frame the structural spatials of the house.  The engineer will be charged with the task of designing the floor structure that will support all the upper storey floors, walls, and roof loads, and then successfully suspend them over the lower storey(s).  And, given that the depth (or thickness) of those structural elements will have a direct influence on the available floor-to-ceiling height, the engineer will be given the directive, “Make it all as thin as possible.”

As a structural engineer, I can reliably confirm that when architects bring new projects to the table, one of the first things we’re asked to design and establish is the floor zone.  That is, the thickness of structure required (joists, beams, concrete slabs….whatever) that makes up the suspended structure:


Floor zone

Based on what I’ve observed in the industry, I can only assume that at some point in time, maybe 50 years ago or so, all of the lecturers and professors in the Architecture schools and faculties at universities around the country must have collaborated and conspired together to establish and agree that the floor zone of a house should be 300mm.  (That’s 12 inches for our American friends).   For, no matter what the size of the rooms, or the layout/transfer of upper-floor walls to lower-floor walls, nearly every set of approved architectural drawings I receive for new projects show a floor zone of 300mm.   This universal assumption of 300mm is then used to set the RL’s (Reduced Levels) of each of the upper floor levels, and the floor-to-ceiling heights, which then drive the final height of the roof, which is squeezed below the local council’s development control limit.  With the RL’s then fixed and locked in with the Development Application-approval, the engineer is often then handed the poisoned chalice of a 300mm floor zone which s/he then must get to work.

But let’s look closely at what a 300mm floor zone actually means.  In the case of a timber-joisted floor, the reality of typical construction is as follows:

Floor zone - joist construction

Let’s do some quick arithmetic:  (Relax, it’s the only maths we’ll do for today).  300mm floor zone – 12mm carpet/underlay – 19mm structural particleboard – 19mm battens – 13mm gyprock ceiling leaves just 237mm for the actual structural joist that has to do all the work and span across the room.   So let’s round up and assume we can squeeze in a 240mm deep floor joist.

Now the strength and spanning capacity of a 240mm deep timber joist is a reasonably fixed and finite property.  Assuming a luxury residence where we want better than the bare minimum performance expectations in terms of floor bounce/vibration, and assuming typical installation at 450mm centres, and supporting typical floor loads only (i.e. not supporting loadbearing walls that support upper levels or a roof), a 240mm deep LVL joist will satisfactorily span around 4.4m in a domestic situation.  That’s it!  If the span over the room below is greater than 4.4m, a different structural framing & flooring solution is required or a deeper joist will be required, and thus a floor zone greater than 300mm will be required.

Now back in the 1960’s and 1970’s – when this magical dimension of 300mm was seemingly set in stone – residential architectural fashion favoured smaller, compartmentalised living (i.e. separate formal dining rooms, enclosed kitchens, etc), 4.4m timber spans, supplemented with the occasional steel beam were more readily achievable.  However, 50-60 years on, we’re now in a very different time and subject to very different architectural fashions, objectives, design features, and building criteria and limitations:

  • Open plan living is now de rigueur, with combined kitchen/dining/living areas creating lower-storey room spaces significantly larger than 4.4m.
  • Engineering design codes have changed significantly in the last two decades, arguably becoming more stringent and limiting acceptable levels of vibration, bounce, and deflections in floors. The concrete design code, in particular, has been revised several times, resulting in floor slabs now typically being thicker than what was permissible in the 20th century.  In short, floors need to be stronger and stiffer than what may have been the case in the past.
  • Architectural features and desirables have changed, for example, setting down the wet areas to create flush floors at the bathroom door entries; heavier and larger-sized feature floor tiles that are less tolerant to floor deflections; recessed lighting that notches into the structure and reduces its strength; better thermal and acoustic insulation; concealed/ducted air-conditioning and other services now need to be incorporated into the structure; heavy stone bench tops; larger bathtubs; etc.

The executive summary is that we’re asking structural floors to do far more than they used to do in the past.  They’re spanning further; they’re carrying greater loads; they’re required to deflect less….and yet the architectural community is seemingly still applying the same expectations on structural depth that applied 50 years ago under very different circumstances.   In the case of high-end, luxury houses with large rooms, full-brick construction, and highly-specified finishes, the old rule of thumb of a 300mm floor zone is now rarely achievable.

Of course, good engineers will incorporate and employ beams into their designs to help break down the spans and support the floor.  However, depending on the loads and spans involved, you’ll still be stretching to squeeze it all in to 300mm.  We’ve established above that by the time all the finishes/claddings are taken into account, the available zone left for structure is 237mm.  How, then, does the steel beam fit in if the engineer is obliged to use a 250mm deep universal beam?  Or if the concrete floor slab has to be 260mm thick to support the masonry walls over and a wet-area setdown?  Something has to give….

Floor zone - wet area setdown

Whilst we’re at it, and as touched on above, it’s worth emphasising that the Australian Standard governing concrete slab design, AS3600, has changed tremendously in the last three decades, resulting in floor slabs becoming significantly thicker than what was previously achievable.  As the behaviour and realised effects of long-term creep deflection and shrinkage strains are becoming better understood, each subsequent edition of AS3600 has revised the formulae for stiffness and deflection that affect concrete slab design.

As a quick demonstration, consider a 1st Floor concrete slab in a house, required to span 5.0m “simply supported” over a living room below.  The slab is subject to domestic live load only (150kg/m2) and we’ll assume a 50kg/m2 super-imposed dead load to allow for finishes (e.g. timber flooring on top, and a gyprock ceiling underneath).  For the sake of this exercise, we’ll apply a final, total deflection limit of Span/250, and we’ll assume 32MPa concrete.

Using the 2001 edition of AS3600, the slab would be 200mm thick.   (Simplified method: Ief = (0.1-13.5p)/bd3)

Using the 2009 edition of AS3600, the slab would be 225mm thick.  (Simplified method: Ief = [(0.055(f’c)1/3 – 50p] bd3

And, finally, using the 2018 edition of AS3600, if one were to use the Bischoff equation to accurately calculate Ief, the slab would be 240mm thick!

The purpose of the above is not to present a case study in engineering but, rather, to simply demonstrate and emphasise that the architectural community cannot continue to cling to rules-of-thumb or spatial expectations that may have applied in years gone by.  It is not, as some purport, a case of engineers becoming increasingly conservative or being less skilled.  (In fact, the opposite is true:  Today’s engineers are armed with software analysis tools that allow us to model and derive stresses and design actions far more accurately and efficiently than the hand-methods that were available to our predecessors…thus allowing us to push and drive materials harder and more effectively).

So what’s the solution and where to from here?  The answer is simple:  Engage with your engineer earlier in the process, before you start locking in ceiling heights and floor RL’s.  There is, in NSW at least, a tendency not to engage the engineer until after the Development Application (DA) is approved by Council.  If the DA was lodged with RL’s and ceiling heights that cannot subsequently be achieved, then it’s going to be an awkward and painful process for everyone – including the fee-paying client – if plans and levels need to be back-tracked and re-submitted.  Accordingly, engage your engineer at the start to undertake a preliminary concept design and to give preliminary advice on likely structural layouts and depth requirements.   Alternatively, if you know in advance that your floor will be timber-joisted, then look up established span tables (they’re widely available) and determine whether your joists will be 200, 240, or 300mm deep for the spans involved.  And then account for all your finishes, claddings, and services to be concealed above and below that.

300 is a pretty special number.  It’s the perfect score in tenpin bowling.  It’s a common bar spacing dimension for reinforcement in concrete slabs.  It was supposedly the number of Spartan warriors that held off the Persians at the battle of Thermopylae.  And sometimes – but only sometimes – it’s an achievable floor zone.



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