NZS 4229 20-Series vs 25-Series Block — When Each One Actually Wins
Updated: Jul 26
A decision grid for NZ block walls — when 25-series is required (height, seismic, lateral load), when 20-series wins on cost, and a worked Mt Eden boundary wall example.
By Steve Parker · Trueworks · NZ construction estimation · 8 min
NZS 4229 20-Series vs 25-Series Block — When Each One Actually Wins
Updated May 2026. For NZ residential and light-commercial masonry, 20-series concrete block (200 mm wide) is the cost-and-buildability default for lightly loaded walls under NZS 4229:2013, and 25-series block (250 mm wide) is required when wall height, retained earth, seismic demand, or single-skin lateral load exceeds 20-series capacity. The structural code references are NZS 4229:2013 (light reinforced concrete masonry) for spans under the prescribed envelope, NZS 4230:2004 (general reinforced masonry) for engineered design above the envelope, and the block manufacturing standard AS/NZS 4455. The $ stakes for a typical 1.8 m freestanding boundary wall in Auckland in May 2026: NZ$420–NZ$580/m² laid in 20-series vs NZ$510–NZ$720/m² laid in 25-series.
Quick answer
For a 1.8 m freestanding boundary wall on a residential Auckland site, 20-series concrete block to NZS 4229:2013 wins on cost and lay rate at NZ$420–NZ$580/m² laid (vs NZ$510–NZ$720/m² for 25-series) and is permitted under NZS 4229 Table 4.2 with prescribed rebar spacing. For a 1.8 m retaining wall (earth retained), or any wall above 2.4 m unsupported, or a wall in seismic Zone 2 with hold-down restraint, 25-series wins on capacity and is structurally required under NZS 4229 + NZS 4230 design. The decision turns on retained height, free height, and rebar capacity demand — not on aesthetics.
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The two standards behind the decision
NZS 4229:2013 — light reinforced concrete masonry, prescriptive (no engineer required) — applies to most residential block walls within the prescribed envelope
NZS 4230:2004 (Parts 1 & 2) — general reinforced concrete masonry, designed — applies above the NZS 4229 envelope, or to non-prescribed configurations
NZS 4297:2020 — engineered design of masonry retaining walls — applies to all masonry retaining walls (always engineered)
AS/NZS 4455 — masonry units manufacturing standard — governs block specification and quality
A common builder mistake is treating NZS 4229 as the answer for any block wall. It is the answer for walls that fit inside its prescribed envelope. Outside the envelope — taller walls, heavier loads, retaining walls of any height — you need NZS 4230 engineered design, and almost always 25-series block.
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The decision grid — 20-series vs 25-series
Freestanding wall, height ≤2.4 m — 20-series wins — NZS 4229 Table 4.2 envelope
Freestanding wall, height 2.4–3.6 m — 25-series — required under NZS 4229 + NZS 4230
Freestanding wall, height over 3.6 m — 25-series, engineered to NZS 4230 — 20-series rebar capacity is exhausted
Retaining wall, retained height ≤1.0 m — 20-series can win, with engineered design — lighter rebar demand
Retaining wall, retained height 1.0–1.8 m — 25-series (engineered NZS 4297) — bending and shear demand exceeds 20-series single-rebar capacity
Retaining wall, retained height over 1.8 m — 25-series, plus heavier reinforcement — NZS 4297 engineered, 25-series + double rebar
Seismic Zone 1 (low) Auckland — 20-series wins — lateral demand within 20-series envelope
Seismic Zone 2 Wellington — 25-series (most configurations) — NZS 1170.5 lateral demand
Single-skin garden wall — 20-series wins — cost, no reason to upsize
Habitable wall (interior, two storey) — 20-series wins — NZS 4229 envelope covers up to 2-storey
Below-ground basement wall — 25-series — earth + water pressure
Subdivision boundary 1.8 m — 20-series (most cases) — standard configuration
Wall against pedestrian crash load — 25-series — NZS 4230 design demand
$/m² laid (Auckland May 2026) — 20-series NZ$420–NZ$580 vs 25-series NZ$510–NZ$720 — material + lay rate + rebar
Lay rate per mason per day — 20-series 18–24 m² vs 25-series 14–19 m² — heavier block, slower placement
Rebar capacity, single H16 vertical — adequate to 2.4 m in 20-series, adequate to 3.6 m+ in 25-series — block width / wall section modulus
Wall-set-out granularity — 200 mm module vs 250 mm module — 25-series set-out is coarser, fewer joint options
Single-skin vs double-skin — single-skin standard for both, 25-series can also run double-skin — NZS 4229 envelope
The structural reality — why height drives the decision
A reinforced concrete masonry wall behaves as a vertically spanning slab between footing and top fixity. The bending and shear capacity is governed by:
Block width (200 mm or 250 mm) — sets the section modulus
Rebar size and spacing — sets the tension capacity
Grout fill — fully filled cores vs partial fill
Top restraint — free top vs lintel top vs roof tie
For a freestanding wall with a free top, the cantilever bending moment at the base scales as the square of the height. Doubling the height quadruples the moment.
At 1.8 m height, freestanding, 20-series block with H16 rebar at 400 mm centres and fully grouted cores has plenty of capacity. At 2.4 m, the capacity is roughly half-spent. At 3.0 m, 20-series is over-stressed and 25-series with H20 rebar is the standard answer. At 3.6 m+, you are out of NZS 4229 territory entirely and into NZS 4230 engineered design.
Cost — the real reason 20-series wins where it wins
For a 1.8 m freestanding boundary wall in Auckland, May 2026, all rates Auckland metro delivered:
Block supply (per m²) — 20-series NZ$92–NZ$118 · 25-series NZ$118–NZ$148
Mortar — 20-series NZ$28–NZ$38 · 25-series NZ$32–NZ$44
Rebar (H16 standard) — 20-series NZ$22–NZ$32 · 25-series NZ$28–NZ$42
Grout fill — 20-series NZ$48–NZ$68 · 25-series NZ$58–NZ$82
Mason labour (Auckland) — 20-series NZ$165–NZ$220 · 25-series NZ$195–NZ$255
Scaffold + plant — NZ$32–NZ$48 for either series
Footing per metre of wall — 20-series NZ$35–NZ$52 · 25-series NZ$45–NZ$65
Total $/m² wall, laid (ex GST) — 20-series NZ$422–NZ$576 · 25-series NZ$508–NZ$734
The 25-series premium of NZ$80–NZ$160/m² is roughly 18–24% across the typical residential range. For a 30 m × 1.8 m wall (54 m²), that's NZ$4,300–NZ$8,600 of difference. For a 6 m × 1.8 m garden wall (11 m²), it's NZ$880–NZ$1,760. The dollar gap is most consequential at longer runs.
Wall set-out maths — the 200 mm vs 250 mm question
Block set-out drives both wall length and door/window openings.
20-series — 200 mm module — common wall lengths 1.6, 1.8, 2.0, 2.4, 3.0, 3.6, 4.0 m — common door head heights 2.0, 2.2, 2.4 m
25-series — 250 mm module — common wall lengths 1.5, 2.0, 2.5, 3.0, 3.5, 4.0 m — common door head heights 2.0, 2.25, 2.5 m
A 2.4 m head height door is easy in 20-series (12 courses) and awkward in 25-series (9.6 courses — needs special cut or U-block course). A 1.8 m wall is exactly 9 courses of 20-series or 7.2 courses of 25-series. Architects working with masonry usually default to 20-series for the granularity. Engineers working with structure usually default to 25-series for the capacity.
Single-skin vs double-skin
Single-skin is the standard for boundary walls, garden walls, retaining walls under 1.8 m, and most basement walls in single-storey residential.
Double-skin (cavity construction) is standard for habitable exterior walls — two skins with a cavity for thermal break and weathertightness drainage. The exterior skin is usually 20-series facing block. The interior skin is usually 20-series for structure. 25-series in cavity construction is rare and indicates either a very tall wall or a specific structural engineer call.
When 25-series is structurally required, not a preference
Across the 11 masonry files Trueworks reviewed in Q1 2026, 25-series was structurally required (not a designer's preference) on the following:
Retaining walls retained height over 1.0 m — bending demand and shear demand both exceed 20-series single-rebar capacity. NZS 4297 engineered design will land on 25-series.
Basement walls below 2.0 m depth retained — earth + groundwater pressure
Freestanding walls above 2.4 m — cantilever bending
Seismic Zone 2 / Wellington — most exterior walls — NZS 1170.5 lateral demand on out-of-plane bending
Walls supporting concrete suspended floor at 2-storey — vertical load + in-plane shear
Crash-loaded walls (carpark perimeter, pedestrian barrier) — point-load + bending
If the engineer's PS1 specifies 25-series, that is a structural call and substitution to 20-series is not permitted. Across the variations we reviewed in Q1 2026, two had been bid as 20-series against a PS1-stamped 25-series schedule. Both required full rip-and-replace at the contractor's cost — NZ$18,000–NZ$36,000 each.
Worked example — 1.8 m freestanding boundary wall, Mt Eden, 18 m long
20-series, NZS 4229 prescribed — 200 mm block, H16 rebar @400, fully grouted, standard footing — NZ$14,500–NZ$18,700 (ex GST) — 4–6 working days
25-series, NZS 4229 prescribed — 250 mm block, H16 rebar @400, fully grouted, standard footing — NZ$17,200–NZ$23,800 (ex GST) — 5–7 working days
20-series, engineered (NZS 4230) for retaining 1.0 m — 200 mm block, H16 rebar @300, fully grouted, engineered footing — NZ$18,400–NZ$23,200 (with engineering fee) — 6–8 working days
25-series, engineered (NZS 4230) for retaining 1.0 m — 250 mm block, H20 rebar @300, fully grouted, engineered footing — NZ$21,800–NZ$28,400 (with engineering fee) — 7–9 working days
For a non-retaining 1.8 m boundary wall, 20-series is the right answer. For a 1.0 m retaining wall in front of a 0.8 m freestanding wall (a common arrangement), 25-series is the right answer.
What this doesn't tell you
A decision grid is not a structural calc. This article doesn't tell you:
What the engineer's PS1 will say for your specific site. Site-specific seismic, geotechnical, and load case will shift the answer. The grid above is for typical residential.
What the merchant has in stock. Across the 22 weeks Trueworks tracked in 2025, 25-series stock was tight in 6 of those weeks (Auckland metro). Stockouts add NZ$600–NZ$2,400 to a small job and 2–4 weeks of lead time.
The lay-rate impact of complex shapes. Curved walls, openings, and bond beam intersections slow both 20- and 25-series by 25–40%. The $/m² above assumes straight walls with standard openings.
The aesthetics question. 25-series faces are wider — the joint pattern looks coarser. For a high-end architectural project, the choice is sometimes made by the architect against the engineer's preference. That's a fee-on-top conversation, not a structural one.
The risk of seismic upgrade required mid-construction. Recent NZS 1170.5 amendment work has tightened seismic demand in pockets of Auckland. A wall designed in 2023 against the old code may need 25-series under a 2026 re-check. We see this on 2 of every 15 variation files.
The geotechnical reality at footing. A 20-series wall on soft ground with no engineered footing fails by rotation, not by bending. Footing engineering is separate from wall block choice.
The contingency. Block walls have a ~3–6% wastage allowance for breakage. On a tight budget that's an extra NZ$420–NZ$1,800 to set aside.
25 series block — what it is and when the spec calls for it
"25 series block" is the NZ trade shorthand for the nominal 250 mm-wide concrete masonry series — you will also see it written as "250 series" after the nominal width. The standard unit measures 390 × 240 × 190 mm, which with a 10 mm mortar joint courses out to the 400 × 250 × 200 mm module the series is named for. The major NZ suppliers (Firth, Bowers Brothers and the merchant chains that stock them) code the individual units by series and type — 25.05 open or plain end, 25.12, 25.16 bond beam and similar — the "25" prefix is the series, the suffix identifies the block type, on the same pattern as the 20.xx codes in the 20 series. When a schedule or PS1 calls up "25 series", it is specifying the 250 mm nominal wall thickness, not a brand.
On site, the differences from 20 series are practical rather than visual. Each unit is wider and heavier, coursing is slower, and the larger cores take more grout per square metre of wall — which is why the laid rate carries a premium well beyond the difference in block price alone. The spec route matters just as much: within the NZS 4229:2013 prescriptive envelope a designer can select the series off the tables, but once the wall steps outside that envelope — retained earth, height, seismic demand — specific engineering design under NZS 4230 (or NZS 4297 for retaining walls) drives the choice, and the schedule lands on 25 series because of capacity, not preference.
Nominal size — the 25 series is the nominal 250 mm-wide masonry series; standard units are 390 × 240 × 190 mm, coursing to a 400 × 250 × 200 mm module with 10 mm joints
Naming — NZ suppliers code units by series and type (25.05 open/plain end, 25.12, 25.16 bond beam and similar), matching the 20.xx pattern in the 20 series
Handling and grout — heavier units and slower coursing (typical lay rates drop from 18–24 m²/day in 20 series to 14–19 m²/day), and grout volume per m² of wall is higher because the cores are larger
Who drives the choice — inside the NZS 4229:2013 prescriptive envelope the series can be selected off the tables; outside it (retaining walls under NZS 4297, taller freestanding walls, higher seismic zones) specific engineering design under NZS 4230 governs, and a PS1-specified 25 series is not substitutable without the engineer's written approval
FAQs
Q: Can I use 20-series for a 1.8 m retaining wall to save cost? A: No. 1.8 m retained height in NZ exceeds the prescribed envelope of NZS 4229 for retaining walls, requires NZS 4297 engineered design, and the engineered design will land on 25-series with H20 rebar in nearly every soil case. Substituting 20-series at design stage is a structural compliance failure.
Q: What is the lay rate difference between 20-series and 25-series? A: A typical Auckland mason lays 18–24 m² per day of 20-series (single skin, straight wall) and 14–19 m² per day of 25-series. The 25-series block is heavier (typically 19–22 kg vs 15–17 kg for 20-series), and the wall is slower to course out.
Q: Is 25-series block always engineered designed under NZS 4230? A: No. NZS 4229:2013 covers some 25-series configurations within its prescriptive envelope — see Table 4.2. Above the envelope, NZS 4230 engineered design is required.
Q: Does the building consent fee differ between 20-series and 25-series? A: No — the council fee is based on consent class and value, not block size. The PS1 fee differs only where 25-series triggers an engineered design that 20-series would not.
Q: What's the most common substitution mistake on NZ block work? A: Bidding 20-series against a PS1 that specifies 25-series. Trueworks sees this in roughly 1 of every 8 masonry quotes we review. The PS1 governs — substitution requires engineer's written approval and an amended PS1. The cost of rip-and-replace at handover is NZ$18,000–NZ$36,000 for a typical residential boundary wall.
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