As a company designing and building houses for the past 40 years, we can tell you with certainty that the Earth isn’t flat. Rises and depressions of varying degrees are in every landmass. Looking at a topographical map of Australia, you’ll notice that roughly two-thirds of the country sits on an elevated area or next to it.
As a result, sloping blocks are as common as flat sites. They pose a challenge to designers and builders, as standing or walking on an incline isn’t exactly comfortable. The extra procedures builders have to do make building on a sloping block cost hefty, with one estimate adding between AUD$40,000 and AUD$100,000 on top of the baseline price.
But why does it cost that much? More importantly, is the extra cost worth it?
Two Major Considerations
Home builders agree that they often consider two factors in determining the cost of building on a sloping block. The first is the gradient, which they calculate by getting the rise and run of the lot. For example, a lot with a 500-mm rise and 10,000-mm run gets a gradient of 1:20 or 5%.
Home builders generally consider a gradient of less than 10% shallow, while a gradient of over 20% is steep. They also point out that the cost of sloping block homes can significantly increase for slopes of 15% and above due to more work being done and more safety concerns. Depending on the project, the cost increase can be between 30% and 50%.
Some projects have built homes on gradients of up to 50% but necessitate complex foundations. This increases the amount of building materials needed, such as concrete and steel. Additionally, the foundations will be buried deeper underground for stability.
The second consideration is whether the land is an upslope or downslope relative to the street or road. Upslope construction is more complex than downslope because it requires displacing more earth to make the site easier to build on. Either way, the displacement requires trucks and other heavy equipment.
Even with robust foundations and proper cut and fill, homes on sloped blocks are still vulnerable to natural occurrences. If anything, homeowners should be more aware of how the land redirects rainfall from roof to drainage.
Martin Brook, associate professor of applied geology at the University of Auckland, said urban development is as much of a factor in worsening natural disasters as climate change. Citing the recent landslides in Auckland brought about by record rainfall, he warned that the weight of any building could weaken the soil over time, regardless of the soil’s geological properties.
It won’t take record rainfall for any developed slope to give way, so paying attention to a house’s runoff management is a must. You’d want to keep water away from the foundations to minimise the risk of a landslide and, ultimately, structural failure. But you also don’t want it to spill over to your neighbour or the road, lest you risk flooding.
Two of the most common solutions are retaining walls and soakaways. Retaining walls keep the water out of the structure, while soakaways allow pooling water to permeate back to the ground. They’re a must for cut-and-fill and split-level site preparation.
Meanwhile, soakaways are excavations partly filled with rocks or gravel. They’re usually made where runoff pools are, allowing water to permeate back to the ground. However, leaves and debris can also make their way and block the soakaway, so it must be protected with a filter.
Building a home is expensive enough, and making one on a sloping block adds to the expense. But is the latter worth it?
Sometimes, a homeowner won’t have a say in the matter. As mentioned at the beginning, about two-thirds of Australia sits on an elevated area. The closer you want your home to the highlands, the more likely you’ll have to contend with sloped land. Even in a predominantly flat area such as Sydney, there are bound to be sloping blocks.
However, there are ways to lessen the cost, such as building on sloped land bought for cheap. On top of that, it gives homeowners more options for implementing the features they want to see in a home. For example, sloping blocks enable home designs to add more windows, allowing people to see and appreciate their surroundings from more angles.
Another advantage is that it permits the addition of a walkout basement, granting direct access to the outdoors instead of moving through the house’s interior. It adds more living space to a home without expanding horizontally.
Building homes on sloped land isn’t easy, nor is it affordable. Builders must ensure the building doesn’t disturb the earth it sits on, even if it means adding several more items to their to-do list. But when done by the book, it’ll be a home anyone can be proud to own.