Retaining wall learnings

An example of compliant retaining walls, constructed by Onlandscapes

In October 2024, a complaint was filed against David Driver, a Brick and Blocklaying Licensed Builded Practioner (LBP). Driver was engaged to build a deck and retaining wall.

Following the complaint, an investigation into Driver’s work found that a 1.7m high timber retaining wall had been built with no drainage behind it, which left it at risk of collapse, and which supported the neighbour’s concrete wall.

Other issues included the use of 50x50mm and 75x25mm landscaping pegs and 100x75mm fence posts in place of 125x125mm H5 treated piles.

“Given the height of the wall and the surcharge, the wall should have been specifically designed and have a building consent for its construction,” said the Board.

The LBP Board noted further issues with Driver’s work, including:

• Undersized bearers with unsupported joins.

• A completely unsupported corner of the deck relying on a 100x50 joist cantilevering 720mm “which is already sagging and at risk of collapse".

• Sub-floor framing supported by the underlying ground itself in places.

• Kwila decking timber installed tight rather than with the recommended 4-6mm ventilation gaps.

As a result, the Board fined Driver $3,500 and costs of $1,750 and recorded its decision on a public register for three years.

Surcharges ignored

Ruling on a similar issue, a Ministry of Business, Innovation and Employment (MBIE) determination highlighted the importance of correctly calculating retaining wall surcharges at the design stage. It found that a new masonry block retaining wall on a property in Tauranga could not establish compliance with Building Code Clause B1 Structure. Following an inspection by the authority, a failed inspection report was issued, as the authority was concerned the retaining wall was surcharging the neighbour’s retaining walls.

The original design plans and structural calculations for the retaining wall failed to consider its impact on the existing retaining walls, explained the authority.

In the determination, Andrew Eames, MBIE Principal Advisor Determinations, wrote: “I must consider whether the original design of the new retaining wall relied on the existing retaining walls on other property to achieve Building Code compliance.

“I have received no plans and specifications to suggest the original design for the new retaining wall relied on Building Code compliance being demonstrated using any element of the neighbour’s existing retaining walls,” said Eames.

The design also had several elements criticised, including the consent’s failure to address the passive (horizontal) load imposed on the lower-level ground on the neighbour’s side of the property boundary, among other things.

Therefore, Eames said he was forced to conclude that the retaining wall did not comply with Building Code B1 Structure.

A hedge is not a barrier

Another noteworthy determination for landscapers is one that ruled that a hedge is not a barrier. The determination was sought by the owners of a property in Timaru, after Timaru District Council (TDC) refused to issue a Code Compliance Certificate (CCC) for their building work due to the possibility of a fall from the retaining walls on their property.

The property features two retaining walls, which create a sudden 1.1m change in level. The change in level led to the property failing its final inspection, as the walls were deemed non-compliant with Building Code F4 – Safety from falling.

Following the initial failure, the property owners installed a temporary barrier, which was deemed non-compliant during a subsequent inspection in 2021. That led to discussions between TDC and the property owners about whether a hedge could be a suitable barrier.

“The authority (council) was of the view that a hedge might be a suitable barrier once it’s established enough to fit the barrier requirements, but that this may take years,” wrote MBIE Lead Determinations Specialist Peta Hird.

After the 2021 inspection, a hedge of Akapuka bushes (Griselinia lucida) was grown to approximately 1.05m high and 0.7m deep. In 2024, a further inspection was carried out. However, the hedge was deemed non-compliant as it could not be counted as a barrier. Hird said a barrier must reduce the likelihood of accidental falls, which the hedge did not achieve.

“The hedge, as it currently stands, limits access to the top of the retaining wall along its length. However, it is reasonably foreseeable that future owners of the property may change the landscaping [of] the backyard, including the removal of the hedge, with the effect of increasing the likelihood of an accidental fall from the upper retaining wall.

“I therefore conclude there is a reasonable probability that people will, or could be, on or in close proximity to the top of the upper retaining wall, such that there is likelihood of accidental fall.” Hird added that, given a hedge is likely to be impacted by seasonal growth patterns and have variable lifespans, it could not be counted as a building.

“As the hedge is not a building, its planting, growth and maintenance is not building work under the Act. The hedge does not satisfy the requirements of the Building Code [...] and I confirm the authority's decision to refuse to issue a code of compliance.”