Location Location – How your suburb impacts the cost of your project

If you’re planning a major “alterations & additions” project for your home, or even building an entirely new house from the ground up, you’ll be amazed at how much your suburb and location impacts the structural engineering of your project.   Before you’ve even thought about the size of your new living room or what sort of tiles you want in your bathroom, the simple address of your property has already had a huge influence on the structure (and cost!) of your building project. 

The suburb you live in, and where your property is situated within that suburb, is critical in many of the engineering and construction decisions that need to be made, and the resulting specifications for the materials.  So much so, that when a structural engineer sits down to start the design work on your house, the first thing he or she will do is look at where the house is located.  That’s because your house’s location will drive the design and specifications of things like:

  • The foundations, i.e. how deep, thick, and strong/stiff your footings need to be. (Which impacts cost!)
  • The concrete specification and strength, i.e. how durable it needs to be. (Which impacts cost!)
  • The mortar specification for your bricks, i.e. how strong and durable it needs to be. (Which impacts cost!)
  • The windows, i.e. how thick the glazing needs to be to resist wind loads. (Which impacts cost!)
  • The steel, i.e. what sort of protection is needed to resist corrosion. (Which impacts cost!)
  • The framing, i.e. how deep, strong, robust, and heavily-braced the walls and roof rafters need to be to resist horizontal racking and uplift forces due to wind loads. (Which impacts cost!)

And all of the above items are just the basics that apply to every suburb.  In addition to the above, there are certain localities around Sydney that require additional consideration for very unusual and particular features that are unique to that area.  For example, the Pittwater region is a known area for slope stability and slip issues, meaning that additional studies, reports, and design is needed to ensure your house won’t slide down the hill!  Similarly, large areas in the south and south-west near the Camden district and towards Wollongong can be subject to mine subsidence because of the long-wall coal mining taking place (or planned to take place) underneath.

You’ll notice in the above list that the words “Which impacts cost!” appear each time.  And this is the key issue – there are requirements for both your house’s structural performance and its long-term durability that are driven by geographical factors.  These requirements influence the specifications and costs of the building materials, and how your engineer and builder have to go about their respective work.  We’ll try and sum these up concisely:

Durability and corrosion

As a very general comment, the closer you are to Sydney Harbour or the coastal beaches, the more expensive your building project could be.  Airborne salts from the harbour and ocean blow in the winds and cause damage/deterioration to many of the materials we build with, chiefly in the form of corrosion. 

If your house features exposed concrete (e.g. balconies, walls, roof slabs, driveways), then the strength of the concrete has to be increased to make it more durable and to help prevent water and salts from permeating through the concrete and triggering corrosion to the embedded steel reinforcement.  (This is actually what concrete cancer is.  You can read more about that concerning subject here). 

Picture of concrete cancer above a window frame
Houses closer to the ocean are at higher risk of developing concrete cancer if the concrete is not correctly specified to a higher level of strength and durability.

Similarly, any exposed structural steel elements, e.g. columns, decks, pergola posts, external stairs, outrigger roof awnings, etc, will be prone to corrosion – in other words, rust!  An appropriate protective coating will need to be specified for the exposed steel, often defaulting to hot-dipped galvanising.  Maintenance could also be an issue for you, as even the best-performing protective coatings only have a lifespan of 10-15 years in the more aggressive locations, i.e. within 100m of a surf beach.

And, in the same vein, other materials such as the mortar in your bricks or the brick cavity ties also need to be upgraded to cope with the more aggressive locations near the coast.  If your property is in greater western Sydney, i.e. towards Penrith, then these are less relevant issues and the requirements can be more relaxed (and thus cheaper).

Wind

Wind can be one of the trickier forces an engineer needs to design for, because – quite literally – every property is unique and different.  Factors that influence the wind loading on a house can be the local terrain and topography, the height of the building, how steep the land rises or falls in the approach to the site, the presence of trees (or lack thereof), whether the site is directly adjacent to open land (e.g. large parks or sports ovals), or adjacent to a body of water, and how much shielding the site gets from other similarly-sized houses or taller buildings.   

What this means is that, for example, properties on the northern beaches such as houses along the crest of Whale Beach or next to the beach in Narrabeen, will be subjected to significantly higher wind forces than, say, a terrace house in Paddington, or a semi-detached residence in Annandale.

Pictures of Whale Beach and Paddington
It shouldn’t surprise that the wind pressures on the houses above Whale Beach will be significantly higher than for the more sheltered dwellings in inner Paddington.

All of the above-mentioned local parameters get taken into account when determining the wind loads on your house, and this then impacts the design and construction of several of the building’s elements – chiefly the walls and the roof.  The walls need to have sufficient bracing (aka lateral restraint) to resist the horizontal racking forces imposed by wind; and the roof needs to be able to resist both the downwards and upwards wind pressures.  In most cases, it’s the uplift pressures that govern roof design, and this can drive how big the timber rafters need to be, or how closely they need to be spaced.  Your choice of roofing material also impacts this – concrete or terracotta roof tiles weigh more and will help negate the uplift pressures, whereas steel roofing (e.g. Colourbond sheeting) is obviously lighter and thus the timbers and the tie-down mechanisms need to be increased.  (And therein lies one of engineering’s many ironies…a roof that weighs less actually needs stronger structure!)

The horizontal wind pressures against a house will also influence the glazing design, i.e. your windows.  It can impact how thick the glass needs to be, and also the quality/strength of the frames, sills, and the seals around the glass panes.

Foundations

We’ve saved this until last, because it’s the big one and warrants a more detailed discussion.  Generally speaking, where possible, it’s good to build off rock!  With the exception of some suburbs in Sydney’s east, some of which feature extremely deep deposits of sand, the vast majority of the Sydney basin is underlain by bedrock.  However, there are three key variables that all differ very significantly across the city. 

  • How deep it is down to the bedrock, and whether it’s practical and economical to build off it.
  • How strong the bedrock is.
  • The properties and behaviour of the loose materials that sit on top of the bedrock.  In the case of Sydney, this means the sands and clays.  

Depth: If bedrock is reasonably close to the surface, say within one to 1.5 metres, then most engineers and builders will opt to “chase the rock” and build or pier down to it for your house’s footings.  The extra cost of excavating or piering down to rock can sometimes be offset by savings made with a reduction in the size of the footing beams and/or the volume of concrete needed overall.  Even if chasing rock does result in higher costs overall, some homeowners will feel this is worth the investment, as it means the house will be less prone to foundation settlement and building movement.  However, in many areas of Sydney where the bedrock is two, three, four, or more metres below the surface, it can be cost-prohibitive or simply not feasible to chase the rock, and the structure is instead “floated” on the shallower, softer material, i.e. the sands or clays.  This situation actually describes the majority of houses across Sydney.

Strength: Some suburbs are underlain by sandstone; many are underlain by shale (or a shale band that sits above the sandstone).  In the vast majority of cases, sandstone will be stronger than shale and offers better support for your house.  This is usually a good thing, because it means your footings don’t need to be as wide or as extensive as might otherwise be the case…although it can add significant construction cost if you’re excavating for a basement or a swimming pool.  (Rock is much more difficult and expensive to dig out than loose sand or clay!)  The strength of the rock isn’t too critical for lightweight or single-storey houses, but can start to come into play for larger two or three-storey houses, particularly those featuring concrete or masonry walls and suspended concrete slabs. 

Behaviour: The third variable, i.e. the properties and behaviour of the loose material (that is, the soils) on top of the rock, can have a huge influence on the design and construction of your house.  We speak of how reactive the soils are.  In simple terms, this is how much the soils change when there are changes in the moisture content in the ground.  Again, keeping it simple:  Sand is stable and doesn’t really change, regardless of the weather.  Clay, on the other hand, is the killer.  Clays will shrink when they dry out (as often occurs over a hot, dry summer), and they will expand and swell in size when there are prolonged periods of rain.  This shrink/swell behaviour can cause havoc with your house’s foundations as the structure rises and falls with the changes, thus triggering cracks in the walls, or causing doors/windows to jam.  It took Sydney’s home builders over a century to better understand these behaviours, but today we have design standards and codes (e.g. AS2870) that now tailor a building’s footings to better combat this movement. 

Building in Sydney: A raft slab under construction
The depth of the rib beams and how much concrete was required for this raft slab in Sydney’s north were directly governed by the moderately reactive clay at the property.

In very simple terms, houses that are founded on reactive clays need to have stiffer footings – this means they are deeper, bigger, stronger, and more heavily reinforced so they can resist the ground movement…all of which adds cost to your project.  The clays are categorised into different classifications across Sydney (e.g. “moderately reactive” and “highly reactive” are two such categories) and the reactivity of the clay under your house will drive how deep and closely spaced your footings need to be.  Most areas in Sydney’s inner west and on the north shore have moderately reactive clays overlying sandstone; the highly reactive clays are further west.  Some areas, particularly around Burwood/Strathfield, have extremely reactive clays, as do some pockets towards Parramatta and Blacktown.  The eastern suburbs fare much better – clays are rarely sighted east of Anzac Parade and, in fact, sandstone bedrock is usually close to the surface.  The exceptions are areas like Rose Bay, Double Bay, and some of the southern beaches areas, where the rock falls away and there can be upwards of 10-15m of (non-reactive) sands.   

It is for these reasons that it’s prudent and advantageous to commission a geotechnical investigation and report for your property early in the design phase.  A geotechnical investigation will carry out tests, probes, and boreholes on the property to ascertain the depths to the different materials and their stratum; it will assess the strength of those materials; and it will establish the reactivity of those materials.  Armed with this information, your structural engineer can then tailor the footing design to your site and optimise the system with respect to performance and efficiency.

– – – – – – – –

Notwithstanding Council planning and other regulatory requirements, all of the above sums up why you can’t take an engineering design or a set of house specifications from one site and just “plonk it” somewhere else.  A footing design that works in Woollahra is unlikely to work in Wahroonga.  Similarly, a concrete specification that’s appropriate for Burwood won’t be right for Bondi.  And a glazing specification for Mona Vale definitely won’t be right for Marrickville.

Cheers, 
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