Here’s something that catches plenty of Auckland homeowners off guard: that “100×100” timber post you’ve been planning around? It won’t actually measure 100 by 100 millimetres once it’s been dressed and machined. After treatment and dressing, you’re looking at a finished size closer to 90 by 90mm—and if you didn’t account for that in your measurements, your fence line is going to feel it.
Getting the right post size matters more than most people realise. In Auckland’s climate—with our wet winters and persistent borer pressure—choosing correctly between H4 and H5 treatment levels, and matching post dimensions to your fence height and wind exposure, can mean the difference between a fence that stands solid for decades and one that needs expensive repairs within a few years.
This guide walks you through your timber post 100×100 options for Auckland properties, covering actual dimensions, treatment requirements, and how to select the right post for your specific conditions so you can build with confidence.
- Nominal versus actual dimensions matter: A “100×100” post typically finishes at 90x90mm after dressing, so factor in the final dressed dimensions when planning your fence line and gap spacing.
- Ground contact demands H4 treatment minimum: In Auckland’s wet conditions, posts set directly into soil or concrete must be H4 treated to resist rot and borer; H5 is recommended for retaining walls or high-moisture areas.
- Post size should match fence height and exposure: For typical 1.8m paling fences in Auckland’s wind zones, 100x100mm posts are the standard choice, while 125x125mm or larger suits taller fences or wind-exposed sites.
- Concrete depth follows post height: Set your 100×100 posts at 600-750mm deep in firm ground for a 1.8m fence, increasing depth for taller fences or loose/volatile soils common across Auckland.
The Nominal vs Actual Size Trap That Catches Most Auckland DIYers
If you’ve been shopping for a timber post 100×100 and noticed something puzzling at the checkout, you’re not alone. This is probably the most misunderstood thing about timber fencing in New Zealand, and it causes no end of headaches on job sites across Auckland.
Here’s the deal: when a retailer lists a post as “100×100”, they’re quoting the nominal sawn size—that’s what the timber measured before it was processed. But once that rough-sawn timber goes through the dressing machine to create a smooth, uniform surface, each face loses several millimetres. A 100×100 timber fence post typically ends up closer to 90-95mm by 90-95mm after dressing. That 5-10mm difference doesn’t sound like much until you’re trying to fit standard brackets, calculate exact spacing for your rails, or space your posts precisely for a 6-metre fence run.
Forum threads across New Zealand trade sites are full of frustrated homeowners asking why their “100mm” posts don’t fit their 100mm brackets. The answer is always the same—it’s not a manufacturing defect, it’s just how timber dressing works. At The Timber Fencing Guys, we make sure our customers understand this upfront, so you can plan your project correctly from the start.
Standard Timber Post Sizes and What They’re Actually Used For
Understanding the real dimensions of timber fence post sizes NZ starts with knowing which size suits which application. The most common square timber posts you’ll encounter in Auckland are:
- 75x75mm (nominal) – Dressed down to roughly 65-70mm, these work well for lightweight ornamental fences, garden borders, or short decorative panels. They’re not suitable for anything over 1.2m in height or where wind load is a factor.
- 100x100mm (nominal) – The workhorse of residential boundary fences. Dressed to approximately 90-95mm, this is your go-to size for standard 1.8m high timber paling fences throughout Auckland’s suburbs.
- 125x125mm (nominal) – Dressed to around 115-120mm, these substantial posts are ideal for taller fences (2.1m+), properties in higher wind zones, or where you’re installing heavy gates. The extra mass provides significantly better resistance to wind load and gate weight.
- 150x150mm and 200x200mm – These are specialist sizes typically reserved for retaining walls, structural applications, or where post sizes must match specific architectural requirements.
Why a 100×100 Post Actual Dimensions After Dressing Matter for Your Project
When calculating how many posts you need for a fence run, measure your actual planned post spacing rather than working from rough estimates. A standard 1.8m high paling fence with two rails typically has posts spaced 2.4 metres apart on centre. Knowing your posts will actually be 90-95mm wide rather than 100mm matters when you’re fitting rail brackets, especially if you’re using metal post shoes or adjustable brackets. Always verify your bracket specifications against the dressed dimensions, not the nominal size printed on the tag.
Matching Post Size to Fence Height, Wind Load, and Auckland Soil Conditions
Auckland’s building environment presents specific challenges that directly affect what size post you need. The city sits across multiple wind zones, with coastal and elevated suburbs experiencing significantly higher wind loads than inner-city areas. The New Zealand Building Code compliance documentation through BRANZ provides guidance on these wind zone classifications, and your local council will confirm which zone applies to your property.
As a practical rule for Auckland conditions: a 1.8m high timber paling fence in a standard wind zone needs a minimum 100×100 timber post 100×100, set at least 600mm deep in concrete. For properties in high-wind zones or for fences over 2.1m, step up to 125x125mm posts with a minimum concrete depth of 750mm. Auckland’s volcanic soils, particularly in areas like Mount Eden, One Tree Hill, and the North Shore, can be excellent for post installation due to their good drainage and stability—but always check for clay layers that may retain moisture and accelerate rot at the concrete interface.
The question of how deep to concrete 100×100 fence posts depends on your fence height and wind exposure, but Auckland’s volcanic subsoil generally allows for good concrete keying. A minimum of one-third of the total post length should be underground—so a 2.4m post needs roughly 800mm in the ground, with 600mm being an absolute minimum for short fences in sheltered positions. The hole should be at least 300mm diameter to allow proper concrete placement around the post, with no voids or air pockets.
H4 vs H5 Treatment: Why Ground Contact Ratings Matter
All timber posts for ground contact applications in New Zealand must be properly treated, and understanding the hazard class system is essential for longevity. H4 treatment (minimum 4kg/m³ penetration) is the standard requirement for posts in ground contact for residential boundary fences. H5 treatment (minimum 6kg/m³) provides additional protection and is required for posts in permanent ground contact where there’s increased moisture exposure, salt spray proximity, or where the timber will be partially submerged periodically.
For most Auckland boundary fence applications, H4 treated radiata pine posts provide excellent durability when correctly installed with proper concrete encapsulation above ground level. However, if you’re building near the coast in areas like Mission Bay, Takapuna, or the Gulf Harbour, H5 treatment gives you that extra margin of protection against salt exposure. The cost difference is typically modest—around $3-5 per post—but the extended service life is well worth it.
Square vs Round Posts: Making the Right Choice
Both shapes come in standard dimensions, and the choice often comes down to aesthetic preference and your planned installation method. Square timber posts offer a traditional look that pairs naturally with paling fence construction, are easier to brace and align during installation, and provide flat surfaces for rail fixing. Round posts have a more contemporary appearance and are popular for rural properties, decorative fencing, or where a softer aesthetic suits the property style.
For most Auckland residential applications, square 100×100 posts remain the most practical choice. They handle standard bracket systems more predictably, are easier to keep perfectly vertical during concrete setting, and the flat faces make cutting and fitting rails straightforward for both professionals and experienced DIYers.
Whether you’re tackling a full fence replacement in Papakura, installing a new boundary fence in Ponsonby, or adding a gate entrance in Henderson, getting your post sizes right from the start saves expensive corrections later. The Timber Fencing Guys supply quality H4 treated posts in all standard sizes, dressed to consistent dimensions so your planning calculations will be accurate.
Conclusion
Getting your timber post sizes right from the start means understanding the difference between nominal and dressed dimensions, matching your post choice to Auckland’s wind zones and soil conditions, and selecting the appropriate treatment level for your location. Whether you’re building in calm Grey Lynn or wind-exposed Takapuna, these fundamentals apply across the city.
Take the time to plan carefully before purchasing—your future self will thank you when the installation goes smoothly and your fence stands solid for decades. Browse our full range of timber fence post sizes NZ or contact The Timber Fencing Guys for expert advice on your Auckland project.
