Timber decks and jetties by the water

A deck over the river, a jetty on the lake, steps down to a swimming hole: timber is the natural choice beside the water in New Zealand. It is also where timber has the hardest life. Splash, damp air, shade, ground contact and the occasional flood keep wood wet for long periods, and wet wood is what rot fungi and wood-boring insects need.

Why treated timber exists

Most framing and decking timber sold here is radiata pine. The NZ Timber Preservation Council, which represents the timber treatment industry, explains that radiata pine has little natural durability, so it is treated with preservatives to meet Building Code requirements and last outdoors. It also takes up treatment unusually well.

Treated timber is labelled with a hazard class, an “H” followed by a number. The number tells you the conditions the timber is treated for. Using a lower class than the job needs is one of the most common reasons outdoor timber fails early.

Hazard classes in plain language

The Council’s guide sets out the classes. Simplified, for outdoor work near water:

Class	Treated for	Typical outdoor uses (per the Council)
H3.1	Weather above ground, non-structural, painted and vertical	Cladding, fascia boards, exterior joinery
H3.2	Weather above ground, or sheltered but at risk of trapped moisture	Decking, handrails, palings, deck joists and bearers
H4	Direct ground contact or immersion in fresh water	Fence posts, garden edging, landscaping timber
H5	Severe conditions: continuous wetting in ground contact or fresh water in critical uses	House piles and poles, retaining walls, freshwater marinas
H6	Sea water or estuarine ground, regularly immersed	Saltwater wharf piles, marinas

So, as a rough guide from the Council’s descriptions: deck boards and the frame above ground are usually H3.2; anything in the ground is at least H4; piles and anything structural in wet ground or fresh water usually needs H5; and timber in sea water or estuary mud is H6. Your designer, builder or council will confirm what the Building Code requires for a particular job, especially for anything structural or over water.

Rot: what starts it

Rot fungi need moisture, air and food. Treated timber resists them, but several things give them a way in:

• Cut ends and holes. Treatment doesn’t always reach the core of thick timber. Cutting, notching or drilling on site can expose less-treated wood. Seal cut ends as the supplier recommends.
• Water traps. Flat tops on posts, joints that hold water, boards laid tight against each other, and leaf litter on the deck all keep wood wet.
• Poor airflow. Decks built low to the ground, or enclosed with no ventilation, stay damp underneath.
• Wrong class. Above-ground timber used in ground contact rots quickly.

Borer

The insect most people mean by “borer” is the common house borer, also called the common furniture beetle. Wikipedia’s summary describes a small beetle, originally from Europe and now worldwide, whose larvae bore through wood for three to four years before emerging as adults through small round exit holes. Damp, untreated or poorly treated timber suits it, which is another reason to use the right class and keep timber dry. Fresh holes and fine powder below them suggest an active infestation; old, weathered holes may be long finished. If you are unsure, or the damage is in structural timber, get it checked by a builder or a timber pest specialist. Borer Wise has a short checklist for telling whether holes are still active.

Upkeep that pays off

• Clean the deck of leaves, moss and dirt, especially between boards, so water drains and the timber dries.
• Re-oil or re-stain on a regular cycle, following the product’s instructions. Coatings slow the wetting and drying that leads to cracking.
• Check fixings. Use fixings rated for the conditions; near water, especially salt water, ordinary fixings corrode quickly.
• Look underneath once a year for soft timber, fungus, borer holes and loose connections, particularly at the bottom of piles and where joists meet bearers.
• After a flood, let the structure dry, clear debris from around piles, and check for movement or scour around foundations.

Jetties and work over water

Jetties, boat ramps and structures in or over rivers and lakes are a different matter from a garden deck. They are usually structural, often need building consent, and work in a riverbed or lakebed may need resource consent from the regional council. Talk to the council early, and use a builder with experience of work over water.

The short version

Choose the hazard class for the wettest part of each piece of timber, keep water moving off and away, and look underneath once a year. A deck by the river can last a long time if the wood gets a chance to dry out.