Retaining wall calculator

Enter your wall's length and the height of ground it will hold back, and the materials tab counts your posts (and how long each needs to be, including the buried part), the rails to span between them, and the drainage kit. The budget tab turns the same wall into an installed-cost range from NZ cost guides, for timber, concrete block and gabion.

The materials list assumes the classic Kiwi timber post-and-rail wall: H5 posts concreted into the ground, H4 rails stacked behind them, scoria and a drainage pipe at the back. It is a planning list, not a design. For anything over knee height near things you care about, the design step is real.

Walls almost never snap. They lean, slowly, and the push that leans them is water. Dry soil is heavy; saturated soil is heavier and slipperier, and it presses on the wall with a force that grows every wet winter. A wall built without drainage is not holding back dirt, it is holding back a slow flood, and the flood always wins.

The standard NZ fix costs a fraction of the wall and is the least skippable part of the build:

• Free-draining backfill. A blanket of scoria or drainage metal behind the wall, not the clay you dug out, so water falls to the bottom instead of pressing on the timber.
• A perforated drainage pipe (Novaflo is the name you will hear) along the base at the back, laid on a slight fall and run to a proper outlet, not to a dead end.
• Filter cloth between the soil and the scoria, so twenty years of silt does not clog the drainage layer into a dam.

Weep holes or gaps between rails let the last of it out the front. If a wall you are pricing has no drainage line in the quote, that is not a saving, it is the reason the cheap quote exists.

The Building Act draws the line at 1.5 metres of retained ground, but the half everyone misses is the second condition: no surcharge. A surcharge is any load on the ground above the wall beyond the ground itself. A driveway is a surcharge. A parked ute is a surcharge. A shed, a pool, sloping ground rising above the wall, or another wall stepping up behind it, all surcharges.

So a 1.2m wall holding up the flat lawn is usually exempt. The same 1.2m wall holding up the neighbour's driveway needs consent, and an engineer, because vehicles push sideways on the ground far harder than lawn does. Height alone tells you nothing; ask what is sitting on top.

One more wrinkle from the other direction: in rural zones a separate exemption runs to 3m of retained ground, but only with a chartered engineer's design behind it. And any wall with a drop of a metre or more may need a barrier along the top under the Building Code, consent or not.

Timber is the cheapest to build and the easiest to DIY, which is why most NZ residential walls are timber. The trade-off is life: treated timber in wet ground is a 15-to-20-year structure, and it looks like a fence, because it is one.

Concrete block or keystone costs more per metre and wants a proper footing, but it shrugs off moisture, lasts generations, and reads as landscaping rather than infrastructure. If the wall is part of the view from the kitchen window, block usually earns its premium.

Gabion baskets (rock-filled wire cages) sit in between on cost, drain through themselves by nature, and go up fast on awkward sites. The look is a taste call: love-it-or-hate-it industrial. They also want depth front to back, so they eat more of a narrow section than a timber wall does.

Whichever way you lean, the drainage rules in section 02 do not change. Gabion aside, every wall still wants scoria, pipe and cloth behind it.

Consent triggers aside, some situations should send you to an engineer even when the law technically leaves you alone. A wall near the boundary, where failure lands on someone else's property. Ground that is already moving, cracking or weeping water. Clay that turns to soup every winter. A wall stepping up a slope in tiers, because tiers load each other and quietly become one tall wall in the eyes of physics. And anything holding ground near your house footing.

An engineer's design on a residential wall is usually hundreds to a few thousand dollars. The wall it protects is tens of thousands, and the excavator coming back to rebuild a failed one costs more than both. Cheap insurance, bought early.