Retaining Wall Checklist For Brisbane Quotes

Working out a surcharge load before you quote

Most of the disagreement between two retaining wall quotes comes from four things: how the height was measured, whether anything loads the soil above the wall, what drainage was allowed for, and how the crew expects to get material onto the site. Settle those four and the quotes become comparable. The list below walks through them in the order they actually come up.

Method and sources

The compliance points come from Brisbane City Council's published retaining wall requirements and the Queensland Development Code, which is where the five approval triggers and the subsoil drainage requirement come from. The certification points follow the Form 15, Form 12 and Form 16 definitions used in Queensland building approvals. The pricing points reflect how south-east Queensland retaining walls are quoted, on face area rather than lineal metres.

One assumption runs through the whole list: that you are dealing with a residential wall on your own land. Boundary walls, body-corporate land and commercial sites bring in questions this list does not cover. It is also general information rather than advice on your particular site.

Before you ask anyone for a price

• Measure the height from the low side, including any fill

  Height is measured on the wall plus retained cut or fill above natural ground level, meaning the ground line before anyone cut into it or tipped fill on it. Measuring only the visible face is how people end up believing a 1.3 metre wall is under the approval line.

• Measure the length, then work out the face area

  Walls are priced on face area, height times length, so a quote in dollars per lineal metre cannot be compared with one in dollars per square metre without converting it first.

• Write down what sits on the ground above the wall

  A driveway, shed, pool, house or second wall is a surcharge, a load pressing on the soil the wall holds. It triggers building approval at any height and changes the engineering completely.

• Measure the distance to the nearest building or wall

  Inside 1.5 metres of a house, shed or another retaining wall, the two structures interact and building approval applies regardless of how low the new wall is.

• Check whether the wall touches the pool fence line

  If the wall forms part of a pool barrier it is assessed as part of that barrier, which brings its own compliance requirements on top of the retaining approval.

• Add the fence height to the wall height

  A combined wall and fence reaching 2.0 metres or more is a trigger on its own. It catches plenty of jobs where neither the wall nor the fence looked tall.

While you are getting quotes

• Ask for the drainage to be itemised, not implied

  Subsoil drainage is mandatory under the Queensland Development Code. If it is not a visible line on the quote, you cannot tell whether it was priced, and it is the most commonly cited cause of wall failure.

• Confirm the agi pipe discharges to approved stormwater

  An agi pipe is the perforated drain at the base of the wall. It has to go somewhere legal. Discharging onto a neighbour or letting it run to nowhere puts the water straight back into the soil behind the wall.

• Ask what backfill goes behind the wall

  Free-draining gravel wrapped in geofabric, a filter cloth that stops clay silting up the drain, behaves completely differently to the clay that came out of the cut. Clay backfill against the wall face is a slow-motion failure.

• Ask whether excavation and spoil removal are included

  Cutting into a Brisbane ridge and carting the spoil away is often a four-figure line on its own. Headline per-square-metre rates almost never include it, and two quotes will differ wildly on it.

• Check whether the price includes GST

  Published material rates are usually quoted excluding GST. A quote that looks ten per cent cheaper sometimes just is not showing it yet.

• Get the access assumption in writing

  Whether a machine can reach the wall line, or every sleeper is carried down a side path, can move the labour component more than the choice of material does.

Before you sign anything

• Check the contractor's QBCC licence yourself

  QBCC licensing is triggered by the dollar value of the building work, not by wall height. The licence register is public and takes a minute to search.

• If approval applies, confirm who is arranging the engineering

  A wall needing approval generally needs structural design to AS 4678, the Australian standard for earth-retaining structures, by an RPEQ engineer, meaning a Registered Professional Engineer of Queensland, certified on a Form 15. It should be clear on the quote whether that sits with you or with the contractor.

• Ask what certification you will hold at the end

  Form 15 covers the design, Form 12 covers installer or aspect certification, and Form 16 is the building certifier's inspection certificate. Knowing which you should end up with makes the paperwork checkable rather than mysterious.