Concrete Posts and Gravel Boards: When to Specify Them, Sizing, Spacing and Mixed-Timber Construction

Quick Answer: Concrete posts and concrete gravel boards are the longest-life UK fencing foundation, with a typical service life of 40–60+ years against 15–25 for treated timber. Standard slotted concrete posts (also called "morticed concrete posts") are made to BS 1722-3 and BS EN 12839 with two slots that accept timber overlap panels, close board pales or concrete gravel boards. Standard sizes: 2.4m, 2.7m and 3.05m total length, 100mm × 125mm section. Post spacing is 1.83m centres for 6ft panels. Set in 1:2:4 concrete to 600mm depth for 1.8m fences, 750mm for 2m+ fences. Combining concrete posts with timber panels or close board pales is the highest-value compromise: rot-proof structure with replaceable timber, ideal for boundaries near drainage or in clay soil.

Summary

Concrete posts solve the single biggest weakness of timber fencing: post rot at ground level. Pressure-treated softwood posts buried in concrete last 15–25 years before the foot fails. Concrete posts last the life of the boundary, often outliving two or three sets of panels. The trade-off is weight (a 2.4m concrete post weighs ~50 kg), cost (£25–60 each vs £8–20 for timber), and aesthetics — concrete is a more industrial look than smooth timber.

For most contractors, the right framing is: concrete posts and gravel boards as the structural skeleton; timber panels or pales as the replaceable skin. This combination gives the customer a 40+ year boundary with affordable 10–15 year skin renewals — the lowest lifetime cost of any common fencing system.

This article covers concrete post specification, installation, the standard slotted-post system used across the UK, and where concrete is the wrong answer. Where the fence is timber close board on concrete posts, also read timber close board fencing for the pale and rail spec.

Key Facts

• BS 1722-3 — Specification for strained wire fences
• BS EN 12839 — Precast concrete products — elements for fences
• Standard slotted concrete post sizes — 2.4m (8ft), 2.7m (9ft), 3.05m (10ft) total length
• Section dimensions — 100mm × 125mm for standard; 125mm × 150mm for heavy duty
• Post weight — 2.4m post approximately 50 kg; 2.7m approximately 55 kg; 3.05m approximately 65 kg
• Slot dimensions — 36mm wide × 90mm deep, on two opposite faces
• Standard post spacing — 1.83m centres for 6ft panels (5'6" panel + 12mm tolerance each side)
• Concrete strength — minimum C35/45 (cubes 35 N/mm², cylinder 45 N/mm²) for fence posts
• Reinforcement — typically 4× 8mm mild steel rebar with stirrups
• Concrete gravel boards — 1830mm × 150mm × 50mm; weight ~22 kg
• Hole depth — 600mm for 1.8m fences; 750mm for 2m+ fences
• Concrete mix for setting — 1:2:4 (cement:sharp sand:20mm aggregate) or postcrete equivalent
• Hole diameter — 250–300mm for standard posts; 300mm+ for heavy duty
• Cement specification — CEM I or CEM II to BS EN 197-1
• Repair-grade concrete — for haunching: 1:3 cement:sand
• Service life — 40–60+ years with proper concrete cover to reinforcement
• Carbonation depth — 1–2mm/year in atmosphere; reinforcement at 30mm+ cover lasts the design life
• Re-use — concrete posts can be reused if extracted carefully; check for cracks and rebar exposure

Quick Reference Table

Detailed Guidance

When concrete posts are the right choice

Specify concrete posts where:

• Service life expectation 25+ years — concrete posts outlive multiple sets of panels
• Heavy clay or waterlogged ground — timber posts rot quickly in saturated soil; concrete is unaffected
• Boundary near drainage or watercourse — same reason
• High wind exposure — concrete posts have higher fixed-end resistance than timber
• Pet containment — dogs scratch and chew timber posts; concrete is indifferent
• Difficult-access for future repair — getting a digger in to dig out a rotten post 15 years later is expensive; better to install once
• Boundary with shared maintenance — concrete avoids future disputes over who pays for post replacement

When concrete posts are the wrong choice

Avoid concrete where:

• Aesthetic priority — for high-end landscape design, oak or composite posts look better
• Listed Building / Conservation Area — concrete may be restricted by planning conditions
• Ground conditions unknown — buried services or made ground; concrete posts are hard to remove if you hit something
• Future re-routing planned — easier to extract timber posts later
• Tight access — 50–65 kg posts are awkward in narrow rear gardens; budget for two-person handling

Installing slotted concrete posts

The sequence:

1. Set out at 1.83m centres (for 6ft panels) along the line
2. Dig holes to depth from the table above; 300mm diameter typical
3. Place 50mm pea-shingle base in each hole for drainage
4. Stand the post in the hole, support with two timber braces and clamps; check plumb on two faces, check the post height (top of post should be the same as adjacent posts ±5mm)
5. Pour 1:2:4 concrete or postcrete to 50mm below ground level; finish with a slope away from the post
6. Slide the gravel board into the slot from the side, between two adjacent posts
7. Repeat for next post — note that the second post position must align with the gravel board length (1830mm) and the panel width (1830mm) — measure both before pouring

For long runs, set every fourth or fifth post first as a reference, then infill — this gives a straighter line than setting one at a time.

Concrete gravel boards

The standard concrete gravel board is 1830mm × 150mm × 50mm, weighing about 22 kg. It slides into the post slots from the side. Once both posts are set, the gravel board cannot be removed without lifting it past the slot top.

The board sits with its underside at ground level — the 150mm height of the gravel board is above ground. Timber panels or close board pales then sit on top of the gravel board, with their underside protected from ground contact.

Mixing concrete posts with timber panels

The standard combination for residential boundaries:

• Concrete posts (slotted) at 1.83m centres
• Concrete gravel board between each pair of posts
• 1830mm × 1830mm overlap, lap or close board panels sitting on top of the gravel board, sliding into the post slots
• Capping rail along the top (optional but recommended)

Panels are replaceable from above without disturbing the posts or gravel boards. Service life: 10–15 years for overlap panels, 15–20 years for close board, 40+ for the posts and boards. Net annual cost is the lowest of any boundary system.

Mixing concrete posts with close board

The slot accepts arris rails as well as panels. To convert a concrete-post system to close board:

• Use morticed concrete posts or slotted with custom rail brackets
• Fit three arris rails between posts (for 1.8m fences)
• Pale-fix as standard for close board
• Concrete gravel board still in the slot at the base

The hybrid gives close-board strength on a 40+ year skeleton.

Curing and re-fitting

Once posts are set:

• 24 hours minimum before fitting panels (wet-mix); 30 minutes for postcrete
• 48 hours minimum before backfilling significantly disturbed soil
• Avoid loading the post (panel weight + wind) for the first 24 hours

In cold weather (<4°C), use rapid-set products or insulate concrete with hessian and frost protection.

Handling concrete posts safely

A 2.7m concrete post weighing 55 kg presents manual-handling risks per HSE guidance:

• Two-person handling on the ground
• Mechanical lift (skip loader, post-puller, mini-digger with quick-hitch) for over-shoulder lifting
• Avoid carrying over uneven ground — drop risk
• Wear safety boots and gloves — concrete edges are sharp
• PPE — eye protection when cutting (extremely rare; usually the post fits as-is)

Cutting concrete posts

If you have to cut down a post:

• Diamond-tipped angle grinder; mark cut line with chalk
• Wear eye and respiratory protection — silica dust is hazardous
• Cut from all four sides incrementally; do not cut through rebar in a single pass
• Re-expose rebar 50mm and bend or trim as needed
• A cut post lacks the original engineered cover to rebar; expect reduced service life if exposed to weather. Best practice: order the right length, do not cut.

Frequently Asked Questions

How long do concrete fence posts last?

The structural concrete lasts 60+ years if it's properly cured to BS 8500 specification. The failure point is corrosion of the internal rebar — which only happens if carbonation reaches the rebar. With 30mm cover, that takes 30–60 years. Practically: longer than the building lifespan in normal residential use.

Can concrete posts crack?

Yes, in three scenarios:

1. Frost heave — water trapped against the post freezes and cracks the surface. Drains the post slot; avoid water pooling at the base
2. Impact damage — being struck by a vehicle or heavy equipment
3. Insufficient cover to rebar — corrosion expands the steel, cracking the concrete from inside

For domestic boundaries, none of these are common.

Are concrete posts cheaper than timber?

Per post, no — concrete is roughly 2–3× the timber unit cost. Over 25 years, concrete is cheaper because timber posts need replacement once or twice in that period (£60–120 per replacement including labour). The break-even is around year 15.

Can I paint or stain concrete posts?

Yes — masonry paint (Sandtex, Dulux Weathershield) bonds to clean, cured concrete. Stain doesn't work well; the concrete is too porous and gives a patchy result. A masonry paint coating extends the visual life and slows weathering but isn't needed for structural durability.

What's the difference between "morticed" and "slotted" concrete posts?

In practice they're often used interchangeably. Strictly:

• "Slotted" — vertical slots on the post face that accept panels or boards sliding down from above
• "Morticed" — square holes in the post face that accept rectangular arris rails

Most modern posts are slotted to take either, with optional brackets for arris rails where needed.

Can I reuse old concrete posts?

Yes, if undamaged. Check for visible cracking and exposed rebar. A post with intact concrete cover and no surface cracks can be reset and reused. A post with corroded rebar showing should be scrapped.

Regulations & Standards

• BS 1722-3 — Specification for strained wire fences

• BS EN 12839 — Precast concrete products — elements for fences

• BS 8500-1 / -2 — Concrete — Complementary British Standard to BS EN 206

• BS EN 206 — Concrete — Specification, performance, production and conformity

• BS EN 197-1 — Cement specification

• BS EN ISO 1461 — Hot dip galvanised coatings

• Construction (Design and Management) Regulations 2015 (CDM) — relevant for commercial fencing projects (notifiable if >30 days / 500 person-days)

• HSE manual handling guidance — Manual Handling Operations Regulations 1992; concrete posts at 50 kg exceed safe single-person lifting

• BSI — BS 1722-3 / EN 12839 — fencing standards

• HSE — Manual Handling Operations — handling concrete posts safely

• Planning Portal — Fences, gates and walls — height limits

• Concrete Society — Concrete Durability — service life and carbonation

• Construction Products Association — product standards

• timber close board fencing — pales and rails fixed to concrete posts

• fence post installation depth — calculating post depth in different conditions

• fence repair replacing posts — converting from timber to concrete in repair

• planning permission fences walls — height and consents

• fencing regs — overarching boundary rules