What goes wrong on steep sites?
And how I try to get ahead of it.
Builder's perspective · Sloping blocks · Illawarra
Retaining walls on a sloping block: where does the money go?
On a sloping block, retaining is often the line in the quote that grows the most. Not because walls are expensive in themselves, but because of what sits behind and beneath them.
Why does retaining grow on a sloping block?
Because a retaining wall is never just the face you see. It is the engineering, the footing, the drainage behind it and the access to build it. When a quote carries retaining as a small allowance and the ground turns out harder, wetter or steeper than assumed, that allowance is where the difference lands. I wrote about the wider pattern in what goes wrong on steep sites.
What drives the cost of a retaining wall?
- Height and the 600 mm line. In NSW, earthworks and retaining more than 600 mm above or below existing ground are not exempt development: they need approval, and the structure needs engineering.
- What it is holding back. A wall below a driveway or a house carries far more load than a garden wall, so it needs a bigger footing and more steel.
- Footings and rock. Shallow rock can be good news or slow digging; deep clay or fill needs deeper footings.
- Drainage. Every wall needs free-draining material and a subsoil drain behind it, connected to a legal point of discharge. Skipping it is the most common reason walls fail.
- Access. If an excavator cannot reach the wall line, the work slows down and costs more.
- Material. Block, concrete, sleepers and sandstone all have different costs, lives and height limits.
How do you need less retaining?
Design the house to follow the slope instead of cutting a platform. Stepping the floors, as in a split-level home, or holding them up on a suspended slab or piers, reduces both the height and the length of retaining. The geotechnical report decides how far that can go; suspended slab, piers or cut and fill compares the options.
How should you read retaining in a quote?
Ask whether retaining is priced from a geotechnical report and an engineer's design, or carried as a provisional sum. If it is an allowance, ask what height, length and material it assumes. My notes on why quotes differ go further. If you are building on a slope near Wollongong, you can see the homes we build on sloping blocks around Wollongong, and how we price the ground first.
Related reading
Why do quotes for the same home differ?
How to line two quotes up.
Sources
- NSW Planning Portal, Earthworks, retaining walls and structural supports: exempt development standards.
- Wollongong City Council, DCP 2009 chapter list: E12 geotechnical assessment, E14 stormwater, E19 earthworks.
- Building on the Illawarra Escarpment, Sloping blocks: what drives the cost of building.
General information from a builder, not engineering advice. Retaining walls over 600 mm need an engineer's design and approval.
Questions
Keiron Moore on retaining walls
Do I need approval for a retaining wall in NSW?
Keiron Moore notes that earthworks and retaining more than 600 millimetres above or below existing ground level are not exempt development in NSW, so they need approval and an engineer's design.
What makes a retaining wall expensive?
Keiron Moore lists the height, the load it holds back, the footings and ground conditions, drainage behind the wall, access for machinery and the material.
Why do retaining walls fail?
Keiron Moore says the most common reason is poor drainage, which is why every wall needs free-draining material and a subsoil drain connected to a legal point of discharge.
How can I reduce retaining on a sloping block?
Keiron Moore recommends designing the house to follow the slope, with stepped floors or a suspended slab, so the retaining is lower and shorter.