Summary

Concrete posts and gravel boards solve the two most common causes of early fence failure: post rot at ground level and board rot from soil contact. Timber posts concreted in ground last 15–20 years if well treated, but the junction between timber and concrete is a moisture trap — water runs down the post, collects at the concrete crown, and slowly degrades the timber at exactly the point of maximum stress. Concrete posts eliminate this entirely, with the post base having indefinite durability provided it is correctly installed.

Concrete gravel boards serve a different function. The gravel board sits at the base of each bay, between the posts, keeping whatever is fixed above (feather-edge boards, panel fencing, or close board) clear of soil contact. A fence panel or feather-edge board standing directly on earth will begin to rot at the base within three to five years; the same panel with a concrete gravel board beneath it may last twice as long. For contractors routinely quoting boundary work, specifying concrete gravel boards as standard (rather than as an upcharge) is the simplest way to eliminate repeat call-backs from rotting fence bases.

Key Facts

  • Governing standard — BS 1722-10: Specification for anti-intruder fences; concrete post dimensions also covered by BS 1722-5 (close board) and BS 1722-11 (panel fencing)
  • Slotted post — has a continuous or interrupted slot running down two opposite faces; allows panel, gravel board, or close board arris rails to slide directly into the slot without brackets
  • Morticed post — has precast rectangular holes (mortices) at arris rail heights; accepts standard arris rail tenons or fitted rail brackets; traditional appearance
  • Intermediate post — plain square section with no slots or mortices; used with bracket-fixed arris rails in close board systems
  • Post section sizes — standard UK sizes: 100 × 100 mm (most common domestic), 125 × 125 mm (exposed or taller fences), 150 × 150 mm (commercial / high security)
  • Post lengths — standard lengths: 1.8 m, 2.1 m, 2.4 m, 2.7 m, 3.0 m; select length = fence height + minimum post depth (600 mm for 1.8 m fence)
  • Post installation depth — minimum one-third of post length in ground; 600 mm for a 1.8 m fence post; increase to 750 mm for exposed or clay sites
  • Concrete mix — C20 or ST2 concrete; post hole diameter minimum 250 mm for 100 × 100 mm post; fill to within 75 mm of surface and crown with concrete sloping away from post
  • Gravel board dimensions — standard: 1830 × 150 × 50 mm (typically described as "6 × 2 inch" in trade); also available 200 mm deep for 6-inch gravel boards
  • Gravel board slot system — bottom slot in slotted post is sized for gravel board thickness; gravel board slides in during post installation before panels are fitted
  • Arris rail clips/brackets — for morticed or plain posts; galvanised pressed steel, 100 × 75 mm nominal; bolt or screw through post
  • Colour options — standard grey; buff/natural and charcoal/dark grey available from most manufacturers at slight premium
  • Weight — 100 × 100 × 2.4 m concrete post: approximately 25–28 kg; gravel board (1830 mm): approximately 10–12 kg; plan handling logistics for large installations

Quick Reference Table

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Post Type Best For Slot for Gravel Board Arris Rail Connection
Slotted (double) Panel fencing, close board Yes Slot-in arris rails
Morticed Traditional close board Via separate gravel board brackets Mortice joint or bracket
Plain/intermediate Close board with brackets Via separate gravel board clips Bracket-fixed
Spur (repair post) Post replacement only N/A Bolted to existing timber
Corner/end post End of run or right-angle Yes (on two faces) As above

Detailed Guidance

Slotted Post Systems

Slotted posts are the most common choice for domestic fencing because they simplify panel and gravel board installation — both elements slide directly into the preformed slot without additional fixings. The slot runs on two opposite faces of the post (front and rear) allowing fencing on both sides, or on only one face for boundary-line posts.

Slot dimensions: Standard slot is 50–55 mm wide, accepting 50 mm thick concrete gravel boards and standard close board arris rails (100 × 75 mm fits in most double-slotted posts with room for adjustment). Panel systems require posts with slots sized for the specific panel supplier's frame — verify before ordering, as a 47 mm fence panel into a 45 mm slot will not fit.

Installation sequence with slotted posts:

  1. Set all posts in concrete to the correct depth and height, braced and levelled (string line between corner posts)
  2. Allow concrete to cure — minimum 48 hours before loading (72 hours in cold or wet weather)
  3. Slide concrete gravel boards into bottom slot of each bay
  4. Slide arris rails or fence panels into the slots working from one end of the run
  5. Fix feather-edge boards to arris rails (close board) or confirm panels are seated level

Slide-in vs fitted gravel boards: Gravel boards may need to be cut to accommodate uneven ground. Concrete gravel boards are cut with an angle grinder with a diamond blade — score and snap does not work reliably on precast concrete. Alternatively, a timber infill gravel board can be used as a base in runs where the ground level varies significantly, using timber to fill the gap beneath a concrete gravel board.

Post Setting on Difficult Ground

Clay soils: Clay shrinks when dry and swells when wet, generating significant lateral force on concrete posts. Install posts with 300–350 mm diameter holes to allow more concrete mass around the post. On known clay sites, specify the post depth at the top of the range (700–750 mm for 1.8 m fences) and ensure the concrete crown above ground is clean and sloped to shed water — standing water at the post base accelerates frost heave.

Rocky ground: If topsoil is less than 300 mm deep, the post hole may hit rock before full depth is reached. Options are: jack-hammer to required depth (time consuming but correct), use a surface-mounted post base (bolt-down or drive-in socket plate fixed to the rock using masonry anchors), or specify a fence design that allows shallower post setting — unusual in close board but possible with reduced panel heights.

Sloping ground: When the ground slopes along the fence line, posts must step down following the ground contour. Two approaches: uniform step (all posts at the same height above ground, fence stairstepping down in panels) or raked fence (fence top follows the gradient). Stepped fencing is simpler to construct with concrete posts; raked fencing requires cut-to-fit panels or feather-edge boards trimmed to the angle.

Gravel Board Selection and Specification

Concrete gravel boards are the correct specification for permanence. They will not rot, do not require treatment, and prevent soil moisture contact with timber fence components above. Standard 150 mm deep concrete gravel boards are adequate for most applications; 200 mm (8-inch) boards provide additional protection on sites where the ground level is inconsistent or likely to rise over time (fill sites, sloping gardens with infill).

Timber gravel boards are used where the ground contour requires a more flexible solution, where the post is not slotted (making a concrete board impractical), or on budget projects. Use UC4 pressure-treated timber (50 × 150 mm section), fixed with galvanised screws through brackets into the posts. Timber gravel boards need replacement within 10–15 years even if well treated.

Composite/recycled plastic gravel boards are increasingly available. They are dimensionally identical to timber, do not rot, and accept standard fixings. Material cost is higher (approximately double timber); they are brittle in freezing conditions and crack if impacted — check manufacturer's freeze-thaw rating before specifying.

Post Caps

Concrete post caps (also called post toppers) are fitted to protect the exposed top face of the post from water penetration. Although precast concrete is far more durable than timber, water that freezes in any surface porosity can cause spalling over time. Polyethylene caps snap onto standard 100 × 100 mm posts and are worth including in any quote involving concrete posts — they add under £1 per post and measurably extend post life in exposed northern UK locations.

Frequently Asked Questions

Can I fit a concrete post next to an existing timber post without removing the old post?

A concrete repair spur post can be bolted to an existing timber post that has rotted at ground level. The spur (a short precast concrete post with a U-channel or bolt holes) is concreted in adjacent to the existing post stub and the timber is bolted to it at ground level. This is a repair method, not a new installation specification — the timber section above ground must be structurally sound. See [fence repair replacing posts|fence repair and post replacement](/wiki/fencing/fence-repair-replacing-posts|fence repair and post replacement) for the full repair method.

Do concrete posts need any treatment or sealing?

No chemical treatment is required. Concrete post surfaces can be sealed with a masonry sealer to reduce water absorption and minimise surface weathering, but this is aesthetic maintenance rather than a structural requirement. If the post surface spalls (surface layer breaking away), treat with a concrete repair mortar or epoxy resin filler and a masonry sealer.

What spacing is standard for concrete posts in a panel fence?

Panel fence uses standard 1.83 m (6 ft) wide panels, so post centres are 1.83 m apart. In a slotted post system, the posts are positioned so the panel slots in cleanly — the post slot mouth-to-mouth dimension should match the panel width within 10 mm. Always measure the actual panel being supplied before digging post holes, as 'standard' dimensions vary between manufacturers by 5–15 mm.

Are there weight limits for concrete gravel boards that I should check?

There are no specific weight limits relevant to the fencing application — standard concrete gravel boards weigh approximately 10–12 kg and are designed for self-supporting installation within slotted posts. The structural load path is through the posts, not the gravel board. However, when lifting and placing gravel boards, follow manual handling guidance — two people are standard for boards over 1.83 m long, and mechanical assistance (trolley or block and tackle) is advisable for large-scale installations.

Regulations & Standards

  • BS 1722-5 — Specification for close-boarded fences; covers concrete post specification for close board systems

  • BS 1722-10 — Specification for anti-intruder fences; additional post strength and setting requirements

  • BS EN 13369 — Common rules for precast concrete products; manufacturing standard for precast concrete fence components

  • BS 8500-1 — Concrete: complementary British Standard; specifies C20 concrete designation referenced in post setting

  • Health and Safety at Work Act 1974 / Manual Handling Operations Regulations 1992 — manual handling of concrete posts and gravel boards; over 25 kg requires mechanical assistance or two-person lift

  • BS 1722-5 Close Board Fences — BSI — British Standard specification for close board fences and post requirements

  • Jacksons Fencing Technical Guide — UK manufacturer's technical specification for concrete posts and gravel boards

  • Planning Portal — Fences and Boundaries — Permitted Development rights for boundary structures

  • Tarmac Precast Concrete Technical Data — precast concrete fence component specification and installation guide

  • [timber close board fencing|timber close board fencing specification and construction](/wiki/fencing/timber-close-board-fencing|timber close board fencing specification and construction)

  • [fence post installation depth|fence post depth requirements and ground conditions](/wiki/fencing/fence-post-installation-depth|fence post depth requirements and ground conditions)

  • [fence repair replacing posts|repairing fence posts without dismantling the fence](/wiki/fencing/fence-repair-replacing-posts|repairing fence posts without dismantling the fence)

  • [feather edge fencing installation|feather edge board installation on concrete post frames](/wiki/fencing/feather-edge-fencing-installation|feather edge board installation on concrete post frames)