Direct answer
A pergola post base bracket does three jobs at once: it transfers the post load into the slab, it holds the end grain of the post clear of standing water, and it survives the corrosion environment created by the fasteners and the treated timber around it. Most wholesale complaints about post bases trace back to the second and third jobs, not the first.
Before you place a bulk order, confirm four numbers: the inner opening (must match your actual milled post section, not the nominal name), the base plate footprint and anchor hole spacing (must clear your slab edge distance), the steel thickness, and the coating class. Everything else is secondary.
Why nominal post sizes cause the most returns
The single most common sourcing error is ordering by nominal name. A “6×6” post is not 6 inches across.
| Post description | Actual milled section | Notes |
|---|---|---|
| US nominal 4×4 (S4S) | 88.9 mm (3.5 in) | Dressed four sides |
| Metric 90 × 90 mm | 90 mm (3.54 in) | Common in EU/AU |
| US nominal 6×6 (S4S) | 139.7 mm (5.5 in) | Dressed four sides |
| Metric 150 × 150 mm | 150 mm (5.91 in) | Common in EU/AU |
| Rough-sawn 6×6 | ≈152.4 mm (6 in) | Undressed, varies by mill |
Jiansen’s LTZJ post base series is engineered around metric stock. The main line uses a 92 mm (3.62 in) inner opening, which is 90 mm plus a 2 mm assembly clearance. That same opening takes a US nominal 4×4 at 88.9 mm with roughly 3.1 mm total clearance — tight enough to sit square, loose enough to install without forcing.
The large-post line uses a 153 mm (6.02 in) inner opening, designed for metric 150 mm stock with 3 mm clearance. Note the conversion carefully: 153 mm is 6.02 in, not 6 in. If you are buying for a US 6×6 market at 139.7 mm, a 153 mm opening leaves 13.3 mm of slop — visually loose and prone to shimming complaints. For that market, specify the 143 mm custom opening (139.7 + 3.3 mm), which is available from 50 pieces with no tooling charge because the profiles are laser cut.
Procurement rule: never write “fits 6×6” on a spec sheet without stating which 6×6 you mean. It is the fastest way to generate a return.
What the base plate actually tells you
Buyers routinely confuse two dimensions that appear on the same drawing.
- Inner opening — the socket the post sits in. This is the fit dimension.
- Base plate footprint — the outer plate that contacts the slab. This is the anchoring dimension.
On the LTZJ-000 post base, the inner opening is 92 mm while the base plate footprint is 152.5 mm (6 in) with a 130 mm inner frame and 32 mm anchor hole spacing. Those two “6 inch” numbers are unrelated, and mixing them up produces spec sheets that no installer can use.
The footprint matters because of concrete edge distance. ACI 318 Chapter 17 governs anchoring to concrete, and post-installed anchors carry minimum edge distance and spacing requirements to prevent splitting failure — for cast-in-place anchors, a common rule of thumb is at least 1.5 times the anchor diameter from the edge, with post-installed anchors typically requiring more. If your customer is anchoring near a slab edge or on a narrow footing, the base plate footprint and hole spacing determine whether the job is even code-legal. Always publish both numbers.
| Model | Inner opening | Base plate | Hole spacing | Thickness | Fasteners supplied |
|---|---|---|---|---|---|
| LTZJ-000 | 92 × 92 mm | 152.5 mm (inner frame 130) | 32 mm | 3 mm | 8 screws + 4 expansion anchors |
| LTZJ-X1 (85° angled) | 92 × 92 mm | 152.5 mm (inner frame 130) | — | 3 mm | 8 screws + 4 expansion anchors |
| LTZJ-005 (large post) | 153 × 153 mm | 181 × 214 mm | 32 mm | 3 mm | 8 screws + 4 expansion anchors |
| LTZJ-W1 (light duty) | 92 × 92 mm | 150 mm (inner frame 125) | 30 mm | **2.3 mm** | 8 screws + 4 expansion anchors |
The W-series is a deliberately lighter 2.3 mm product for light structures and should be quoted as its own line — not as a discount substitute for the 3 mm main series.
Standoff height: the detail that fails inspections
A post base that sits flush to the slab traps water against the end grain of the post. That is where rot starts, and it is why building codes address it directly.
The 2021 International Residential Code, section R317.1.4 (Wood Columns), requires that wood columns exposed to the weather be approved naturally durable or pressure-preservative-treated wood — with an exception permitting untreated columns where they are supported by concrete piers or metal pedestals projecting 1 inch (25 mm) above a concrete floor, or 6 inches (152 mm) above exposed earth where the earth is covered by an approved impervious moisture barrier.
For a wholesale buyer, that translates into two practical questions:
- Does the base lift the post off the slab at all? If your customer is building on a patio slab, the 1 inch (25 mm) standoff is the governing number.
- Is the installation over exposed earth? Then the requirement jumps to 6 inches (152 mm) above grade with a moisture barrier — which usually means a concrete pier, not just a bracket.
Many imported post bases are flat-bottom designs with no standoff at all. They are perfectly serviceable when the customer is also specifying treated timber, but they will not carry an untreated post under the R317.1.4 exception. If you are selling into the US residential market, say so plainly in the listing rather than letting the installer discover it at inspection.
Listing rule: state the standoff geometry as a measured dimension. Do not imply code compliance the product does not deliver on its own — compliance depends on the timber species, treatment, and site condition as much as on the bracket.
Corrosion: match the coating to the timber, not to the price
Modern copper-based wood preservatives (ACQ, CA) are significantly more corrosive to steel than the older CCA chemistry they replaced. This is the single most under-specified variable in outdoor bracket sourcing.
The two coating standards you will see quoted are different things:
| Standard | What it covers | Typical spec |
|---|---|---|
| **ASTM A653** | Zinc coating applied to steel **sheet** before forming | G90 (0.90 oz/ft²), G185 (1.85 oz/ft²) |
| **ASTM A153** | Hot-dip galvanizing applied to **hardware and fasteners** after fabrication | Class D — minimum average 1.0 oz/ft² (305 g/m²) zinc |
Two implications for procurement:
- Sheet coating and fastener coating are separate decisions. A G185 bracket paired with electroplated screws will fail at the screws. Connector manufacturers publish coating-selection guidance precisely because the fastener, not the bracket, is usually the first component to corrode in treated-timber assemblies.
- Post-fabrication galvanizing covers cut edges. Sheet galvanized to G90/G185 is coated before the blank is cut and formed, so the sheared edges and punched holes expose bare steel. For coastal or high-exposure projects, specify hot-dip galvanizing after fabrication, or move to stainless.
Ask your supplier three questions before you commit to a container:
- Which standard and class — A653 G-number, or A153 Class D after fabrication?
- Are the supplied fasteners coated to the same class as the bracket?
- For stainless requests, which grade — 304 for general outdoor, 316 for coastal salt exposure?
A specification checklist for wholesale post base orders
Use this as the body of your RFQ. Every line is a number your supplier should be able to answer without hedging.
- Inner opening, stated in mm with the inch conversion to two decimals (e.g. 92 mm / 3.62 in)
- Target post section, stated as the actual milled dimension, not the nominal name
- Base plate footprint and inner frame dimension
- Anchor hole spacing and hole diameter
- Steel thickness — and confirm whether the quoted line is the main series or a light-duty variant
- Standoff height above the slab, as a measured dimension
- Coating standard and class for the bracket
- Coating standard and class for the supplied fasteners
- Fastener count per unit and whether expansion anchors are included
- Packing weight and carton dimensions for freight calculation
- MOQ and tooling cost for any custom opening
On the Jiansen LTZJ line, the main series is 3 mm and ships with 8 screws plus 4 expansion anchors per post base; carton data is published per model so you can calculate landed cost before sampling. Custom openings such as the 143 mm US 6×6 fit start at 50 pieces with no tooling charge.
How the post base relates to the rest of the frame
A post base is one of four connection types in a typical pergola kit, and they must share a dimensional family or the build will not go together:
- Post base — post to slab
- Corner and tee connectors — post to beam, at 3-way, 4-way, 5-way or L configurations
- Beam brackets — beam to post face, sized to the beam thickness rather than the post
- Angled connectors — for sloped or non-orthogonal frames
Beam brackets are sized on a different axis. The Jiansen beam brackets use a 47 mm opening, dimensioned for metric 45 mm beam stock with 2 mm clearance. A US nominal 2× beam at 38.1 mm will sit loose in that opening, so for US framing you should confirm a narrower custom option rather than assuming the metric part will work.
For load path, fastener sizing and span logic across the whole frame, see our companion guide on heavy duty pergola bracket load ratings and bolt specs. The full connector range is listed under metal pergola brackets.
Frequently asked questions
What size post base bracket do I need for a 4×4 post?
For a US nominal 4×4, the actual milled section is 88.9 mm (3.5 in), so a 92 mm (3.62 in) inner opening gives about 3.1 mm total clearance — enough to install square without forcing. The same 92 mm opening also suits metric 90 × 90 mm stock with 2 mm clearance. Order by the milled dimension, never by the nominal name.
Will a 6×6 post base fit my 6×6 post?
Not necessarily. A US nominal 6×6 measures 139.7 mm (5.5 in), while a metric 150 mm post measures 150 mm. A 153 mm (6.02 in) opening is designed for the metric post and will leave 13.3 mm of slop around a US 6×6. For US framing, specify a 143 mm custom opening, which is available from 50 pieces with no tooling charge.
How high should a post base lift the post off concrete?
IRC R317.1.4 permits untreated wood columns where they are supported on concrete piers or metal pedestals projecting 1 inch (25 mm) above a concrete floor, or 6 inches (152 mm) above exposed earth with an approved impervious moisture barrier. Confirm the standoff geometry as a measured dimension before you list a product as suitable for untreated posts.
What coating should I specify for treated timber?
Copper-based preservatives such as ACQ and CA are more corrosive to steel than older CCA chemistry. Specify hot-dip galvanizing to ASTM A153 Class D — a minimum average of 1.0 oz/ft² (305 g/m²) of zinc — or sheet galvanized to ASTM A653 G185, and make sure the supplied fasteners carry the same class. A high-spec bracket with under-spec screws fails at the screws.
What is the difference between the base plate and the inner opening?
The inner opening is the socket the post sits in and determines fit. The base plate footprint is the outer plate that contacts the slab and determines anchoring. On the LTZJ-000 the inner opening is 92 mm while the base plate is 152.5 mm with 32 mm hole spacing — two unrelated numbers that are frequently confused on spec sheets.
Does anchor edge distance affect which base plate I can use?
Yes. ACI 318 Chapter 17 sets minimum edge distances and spacings for anchoring to concrete to prevent splitting failure. If the installation is near a slab edge or on a narrow footing, the base plate footprint and anchor hole spacing decide whether the anchor pattern is achievable, so publish both dimensions in your catalogue.
What is the minimum order for a custom inner opening?
For the Jiansen LTZJ line, custom openings such as the 143 mm US 6×6 fit start at 50 pieces with no tooling charge, because the profiles are laser cut rather than stamped from a die. Lead time and price differential should be confirmed per project.
Summary
Post base selection is a dimensional problem before it is a structural one. Get the inner opening right against the actual milled post section, publish the base plate footprint and hole spacing so the installer can check edge distance, state the standoff as a measured number rather than a compliance claim, and match the coating class across bracket and fasteners to the preservative chemistry in the timber.
Jiansen’s LTZJ post base range covers 92 mm and 153 mm openings in 3 mm steel, a 2.3 mm light-duty W-series, and custom openings from 50 pieces. Full dimensional data, carton weights and fastener counts are published per model.
Sources
- 2021 International Residential Code, R317.1.4 Wood Columns — standoff requirements for wood columns on metal pedestals
- American Galvanizers Association, Hot-Dip Galvanizing Small Parts, Hardware and Fasteners to ASTM A153
- Simpson Strong-Tie, Guidelines for Selecting Materials & Coatings — ASTM A153 Class D zinc coating weight
- MiTek, Corrosion Protection technical bulletin — G90 / G185 galvanization requirements under ASTM A653
- ACI 318 Chapter 17, Anchoring to Concrete — edge distance and spacing provisions (overview)
