How we build a timber retaining wall

For lower walls under 1.2m in height, we have developed a strong standard design using H5 treated poles and tongue and groove retaining timber for extra strength and good looks.

A retaining wall has to be strong enough to hold back existing ground. That single requirement determines the size and strength of the poles and timber, and the specifications of any footings, foundations and anchors that may be involved. It is why two walls that look identical from the lawn can be very different jobs below it.

We only use quality timber and robust galvanised fittings. Poles in ground contact are H5 treated - the treatment level intended for timber that stays permanently in the ground. Using anything lighter in that position is the single most common reason a retaining wall fails early.

Drainage

Depending on circumstances, drainage may be a consideration to remove water build-up behind your wall. It is worth taking seriously. Water trapped behind a wall adds load the wall was never designed to carry, and that load arrives exactly when the ground is at its softest. Drainage metal and a drainage pipe behind the timber, taken to a sensible outfall, is a small part of the build cost and does more for the life of the wall than almost anything else you can spend money on.

Council consents and professional advice

Council consents and engineering design are needed for all walls over 1.5m in height.

If a wall has a driveway above it, or a building platform close at hand, this will affect the loading placed on your wall and will need professional input. The extra weight of a car, a shed or a house sitting near the top of a wall is called a surcharge, and it can push a wall that would otherwise be exempt into needing a design and a consent even though the wall itself is not tall.

The rough rule of thumb. A wall under 1.5m, with nothing heavy above it and nothing else being retained by it, is generally exempt. A wall over 1.5m, or a wall of any height carrying a surcharge such as a driveway or a building platform, generally needs engineering design and a building consent.

That is the rule of thumb, not the ruling. Christchurch City Council, Selwyn and Waimakariri each apply it to your specific site, and land damaged in the earthquake sequence can be treated differently again. We will tell you honestly which side of the line we think your job sits on, and if it needs an engineer we will say so at the quote stage rather than half way through the build.

Christchurch ground

Retaining in this city is not the same as retaining anywhere else in New Zealand. Parts of the east are on liquefaction-prone land where the ground behaves very differently in a seismic event. Port Hills sections bring slope, rock and access problems in the same job. Flat western suburbs are usually the straightforward end of it. All of this affects pole depth, spacing and whether an engineer needs to be involved - and it is another reason we would rather stand on the site than quote from a photo.

Timber or block?

We build in timber, and for most residential jobs timber is the sensible answer: it is cheaper, it goes up faster, it copes with an awkward or sloping site far more easily, and a well-drained H5 pole wall has a long life. Concrete block and crib systems have their place on tall walls and heavily loaded ones. If we think your job genuinely wants something we do not build, we will tell you that too.

Wall and fence in one job

Most retaining walls end up with a fence on top of them, and a lot of them end up with a deck alongside. Having the same crew do the wall, the fence and the deck means the post spacings line up, the timbers match, and nobody is standing around waiting for someone else’s trade to finish. It is normally cheaper than booking three separate contractors, and it is one of the practical advantages of using a business that does all three.

Phone us today for free advice and costing as you make more use of your existing land area.

Timber Retaining Walls in Christchurch and Canterbury