H4 vs H5 Timber

The severity step

The exact boundary comes from current standards, compliance documents and the selected product's scope. Do not infer it from diameter, colour or the fact that one merchant lists a pole under landscaping.

Retaining walls are systems, not rows of treated poles

A retaining member must resist soil and water loads through a designed layout. Pole diameter, spacing, embedment, wall height, soil conditions, surcharge and drainage act together. Replacing an H5 pole with an H4 post because the visible diameter is similar changes one of several critical specifications.

MBIE's Schedule 1 guidance describes a building-consent exemption for some retaining walls up to 1.5 m of retained ground only where the wall does not support surcharge. The exemption does not remove Building Code obligations and is not an engineering design. A driveway, building or sloping ground near the wall can create surcharge.

Questions for an H5 quote

1. What species, preservative system, hazard class and producer identification will be supplied?
2. Is the member structurally graded or machine-peeled to a documented diameter/taper range?
3. Which lengths and small-end diameters are actually available?
4. What field-cut or drilling restrictions apply?
5. Which fasteners and connectors are compatible?
6. How will the pack be handled, unloaded and stored without damage?

A quote that says only 'H5 poles' cannot be reconciled to a design. Keep the treatment certificate or producer documentation with the order and confirm every substituted diameter or length.

Do not upgrade one attribute and downgrade another

An H5 member may satisfy a treatment requirement but fail the structural schedule if it is too small, ungraded or incorrectly spaced. Conversely, a strong structural member with H4 treatment does not satisfy an H5 durability specification. Treatment and structural capacity are independent pass/fail gates.

• Fence in ordinary ground contact: H4 may be the documented product class; still design the fence.
• Small garden edging: confirm soil contact, intended life and food-growing context rather than defaulting to a retaining-wall pole.
• Engineered retaining system: follow the exact H5/species/diameter schedule and drainage details.
• Fresh-water application: obtain product-specific documentation; do not extrapolate from a land-based post.

Consequences determine the level of verification

A replaceable garden stake and a pole supporting retained ground may both touch soil, but failure has different consequences. H5 purchasing normally belongs to a higher-verification workflow: engineered member schedule, treatment documentation, controlled diameter or section, drainage design and inspection. Price should be compared only after those requirements match.

Round poles need special dimensional care. A nominal diameter may refer to a range, and taper means the small end can govern. The designer's schedule may state small-end diameter or another measurement convention. Ask the supplier to quote using that same convention rather than assuming '200 mm pole' is exact along its length.

• Confirm whether peeling or machining has affected the treatment envelope.
• Identify which end is placed in the ground under the design.
• Do not drill or notch outside producer and engineer limits.
• Plan lifting so the treated surface is not deeply gouged.

Service life is not an H-number warranty

Treatment standards define hazard-class performance requirements; a project service life also depends on species treatability, processing quality, member detailing, installation damage, soil and water conditions, maintenance and design. Ask what warranty or durability statement applies to the exact branded product and keep its conditions.