Here’s something that trips up nearly every Auckland homeowner tackling a fencing project: that “100×100” timber post you’ve ordered? It’ll actually arrive measuring closer to 90x90mm once it’s been dressed and pressure treated. It’s not a mistake—it’s just how nominal sizing works, and knowing this upfront can save you real headaches when planning gate posts and gaps.
For Auckland’s damp climate, getting the treatment rating right matters just as much. H4-rated 100×100 posts are the standard for ground contact, offering decades of resistance to rot and borer. In this guide, I’ll walk you through the exact specifications, proper burial depth, and where the 100×100 shines compared to smaller alternatives—so your fence is built to last.
- H4 treatment is essential for ground contact: In Auckland’s damp climate, H4-rated 100×100 posts are mandatory for any post going directly into soil or concrete—they resist rot and borer for decades.
- Nominally 100×100 means actually 90×90: After dressing and pressure treatment, your “100×100” post measures approximately 90x90mm—factor this into gate posts and any gap calculations.
- Burial depth follows the one-third rule: For standard 2.4m fence posts, concrete at least 600-750mm deep—this gives the stability needed against Auckland’s seasonal wind loads.
- Stronger than 90×90, ideal for boundary and structural work: The 100×100 posts handle heavier fence panels and light retaining walls up to 900mm better than smaller alternatives, with recommended post spacing of 2.0-2.4m on-centre.
Understanding the 100×100 Timber Post: Why “100×100” Isn’t Actually 100x100mm
One of the first things that catches Auckland homeowners out is the difference between nominal and actual dimensions when selecting fence posts. A 100×100 timber post refers to its nominal size—essentially what it was called when first milled. By the time it’s been kiln-dried, planed, and dressed for fencing, you’re typically looking at an actual finished dimension of around 90x90mm. This isn’t a trick or a compromise; it’s standard industry practice across New Zealand, and understanding it upfront saves headaches down the track.
The slight reduction in size matters when you’re planning your fence layout and calculating post spacing for fence panels. If you’re working from plans or council requirements, always work from the actual finished dimensions. The seasoned team at The Timber Fencing Guys always confirms actual measurements with clients before installation, ensuring your boundary fence post specifications align with what’s actually delivered.
Nominal vs Actual Dimensions Explained
When you see “100×100” on a quote or product label, mentally reserve 5-10mm on each face for dressing. Your finished post will measure approximately 95x95mm undressed or 90x90mm fully dressed and ready for installation. This tolerance affects how your rails slot into the post notches and should guide your hardware selection. Trust us—measuring before purchasing hardware has saved many an Auckland tradie a second trip to the suppliers.
H4 Treatment Ratings: What Auckland Homeowners Need to Know
Auckland’s climate presents specific challenges that overseas guides simply don’t address. Our clay-heavy soils hold moisture against buried timber for extended periods, and our high winter rainfall means posts can remain saturated for weeks. This is precisely why H4 rated timber posts for Auckland fencing aren’t optional—they’re essential for longevity. A post that might survive five years in drier parts of New Zealand could fail within three years in a typical Auckland garden without adequate treatment.
The New Zealand building standards and BRANZ guidance both recognise that ground contact in our northern climate demands robust preservation. Cutting corners on treatment levels might save a few dollars upfront, but it almost always leads to premature failure, especially for boundary fence applications where replacement means disturbing neighbours’ gardens and potentially council-approved structures.
H3 vs H4: Selecting the Right Treatment Level
H3 treated timber post ratings are designed for above-ground use—think fence palings, rails, and pergola beams where timber stays consistently dry. Once you’re burying timber in the ground, even in well-drainedakohanga sites, you need H4 treated timber post minimum. The additional chemical retention in H4 provides essential protection against fungal decay and insect attack that occurs when moisture consistently contacts untreated or lightly treated pine.
For Auckland’s often damp suburban sections with poor air circulation, we frequently recommend stepping up to H5 for critical structural posts. The marginal cost difference becomes negligible when you calculate the expense of premature replacement—including labour, materials, and the disruption of removing and reinstalling fence panels.
The Science Behind Pressure Treated Radiata Pine
New Zealand’s pressure treated radiata pine post stock represents some of the best value structural timber available. The treatment process forces preservative compounds deep into the wood fibres under pressure, creating uniform protection throughout the post’s cross-section. This means that if the post surface gets scratched during installation, the underlying timber remains protected. Visual grade radiata pine takes treatment well and provides a clean, consistent appearance that stains or paints beautifully once installed.
Proper Installation: Burial Depth and Concreting Guidelines
Even the finest 100x100mm fence post will fail prematurely without correct installation. Auckland’s expansive clay soils add another consideration—during wet winters, clay swells against concrete footings, and during dry summers, it contracts. This seasonal movement creates pressure on posts that no amount of concrete can fully prevent. Understanding how deep to set a 100×100 fence post in ground is crucial for long-term stability.
The One-Third Rule for Fence Post Stability
The traditional trades rule remains sound: bury one-third of the total post length for permanent fence installations. For a 2.4m post used as a standard boundary fence, that means burying approximately 800mm and leaving 1.6m above ground. This gives adequate leverage against wind loads while keeping the buried section deep enough to resist frost heave and seasonal soil movement. For taller fences or exposed sites in areas like the Waitākere Ranges, consider burying 40% of the post length for additional security.
Concreting Best Practices for 100×100 Posts
Professional concreting techniques make a measurable difference in Auckland conditions. We recommend using a post mix or 20MPa concrete with adequate drainage aggregate, poured to create a slight dome that sheds water away from the post base. Never leave a concrete collar sitting proud of the ground where it can trap moisture against the timber. The post should emerge cleanly from the concrete with no gaps, and we always apply an additional coat of timber treatment to the buried section before concreting—call it belt and suspenders, but this extra step has extended countless fence lives across Auckland.
Comparing Post Sizes: When to Choose 100×100 Over Alternatives
Understanding the 100×100 vs 90×90 timber post comparison helps Auckland homeowners make informed decisions. The extra 10mm in a 100×100 post provides approximately 20% more timber cross-section, translating to meaningfully greater resistance to bending and twisting under load. For standard residential fencing with medium-weight panels, either size performs adequately, but the 100×100 demonstrates its superiority in windy locations, taller installations, and any application where posts face sustained lateral pressure.
100×100 vs 90×90: Which Post Size for Your Fence
For a typical 1.8m high boundary fence in suburban Auckland, 90×90 posts handle the load adequately if your section has reasonable wind protection and you’re using standard palings or lightweight panels. However, once you add features like trellis tops, decorative caps, or plan to grow climbing plants against the fence, stepping up to 100×100 provides the structural margin that prevents failures during Auckland’s occasional winter storms. Our complete Auckland homeowner’s guide to NZ post sizes covers these decisions in greater detail for different fence configurations.
100×100 vs 100×75: Structural Considerations
The 100×75 dimension—sometimes called a “narrow” post—serves specific purposes well. These work excellently as intermediate stiffener posts in long fence runs where you want to reduce panel span without the visual weight of full square posts. They’re also popular for side gate frames and as vertical battens in decorative fence designs. However, for primary boundary fence post applications where structural integrity matters most, the full 100×100 section provides superior resistance to the twisting forces that develop over years of seasonal timber movement.
Best Uses for 100×100 Timber Posts in Auckland
The 100×100 timber post genuinely earns its reputation as the workhorse of Auckland fencing. Its dimensions provide the strength needed for demanding applications while remaining economically priced compared to larger hardwood alternatives.
Boundary Fence Applications
Answering the common question about what size timber post for a boundary fence in Auckland—for most residential applications, 100×100 represents the sweet spot. It accommodates standard 75mm post spikes without excessive waste, accepts 100mm fence brackets where needed, and provides sufficient mass to resist wind loading across Auckland’s varying terrain. A well-concreted 100×100 post will outlast many fence panel replacements, making it the economical choice for permanent boundary solutions.
Light Retaining Wall Construction
For light retaining wall construction, 100×100 posts serve as vertical members in small garden walls up to approximately 600mm high. They must be treated to H5 level given their ground contact in potentially damp soil conditions, and should be spaced at no more than 1.2m centres to handle lateral soil pressure. Proper drainage behind the wall remains critical—the posts themselves are only part of a system that includes aggregate backfill and drainage membrane.
Gate Posts and Heavy Panel Support
Gate posts and heavy panel support demand the robustness that only 100×100 posts provide. Single gates up to 1.2m wide mounted on 100×100 posts with heavy-duty strap hinges perform reliably, and double gates benefit equally from the additional timber mass that resists sagging over time. For motorised gates, structural steel sleeves inserted over the timber posts provide the ultimate mounting surface for automation hardware while maintaining the timber appearance at the property boundary.
Available Lengths and Grade Selection
Standard lengths from 1.8m to 3.6m cover virtually every Auckland fencing scenario. For standard 1.5m to 1.8m residential fences, the 2.4m length handles standard burial depths while minimising waste. Taller privacy fences up to 2.1m require 3.0m posts buried 900mm, and specialized applications like acoustic barriers or security fencing often need the full 3.6m lengths.
Visual Grade vs Structural Grade Timber
Visual grade timber carries fewer natural characteristics like knots and resin pockets, providing a cleaner appearance for fence posts that form part of your property’s street appeal. Structural grade timber—often designated as “structural” or “SDG”—meets rigorous bending strength requirements and works perfectly well for fencing where posts are concealed within panels. Both grades receive identical treatment, so the choice comes down to appearance preferences and budget. For most Auckland boundary fences where posts sit between panels, structural grade delivers excellent value without visible quality compromises.
Whether you’re planning a straightforward boundary replacement or a more complex fencing project, our timber fence resources provide further guidance for making the right specifications for your Auckland property. Getting the post size, treatment level, and installation approach right first time means your fence serves your whānau reliably for decades.
Choosing the right 100×100 timber post for your Auckland property means understanding more than dimensions—it’s about matching treatment levels to our unique climate, installation depth to our clay soils, and post specifications to your fence’s actual demands. The difference between a fence that lasts decades and one that fails prematurely often comes down to these seemingly small decisions made at the planning stage.
Whether you’re replacing a boundary fence in Mt Roskill or building a new installation in Pukekohe, getting your specifications right first time saves significant expense and disruption. Explore timber fence resources or contact The Timber Fencing Guys to ensure your project starts with the right foundation.
