Fence Posts: UC4 Timber vs Concrete vs Metal Spikes for UK Gardens

Your fence posts will rot and snap at ground level fifteen years before the rest of the fence fails. It's the most common cause of fence failure in UK gardens. Six panels of perfectly good timber, toppled by three rotten posts. And the cost of fixing it isn't just posts and panels. It's digging out lumps of twenty-year-old concrete with a sledgehammer, on your hands and knees, next to your neighbour's prize dahlias, while they watch through the kitchen window.

Post choice is the single decision that determines whether your fence lasts ten years or fifty. Get it right once and you never think about it again. Get it wrong and you'll be doing the whole thing over before your next MOT cycle.

What they are and what they're for

A fence post is the vertical support that carries fence panels, gates, and trellis along a boundary line. UK fence posts come in three materials: pressure-treated timber, precast concrete, and galvanised steel spikes. The post takes the full lateral wind load on the panel and transfers it into the ground, which is why how the post is set matters as much as what it's made of.

The critical spec for any timber post in ground contact is its use class rating. UC4 (Use Class 4, defined in BS EN 335) means the timber has been pressure-treated with enough preservative to resist rot in permanent ground contact. UC3 means above-ground use with occasional wetting. A UC3 post buried in soil looks identical to a UC4 post, costs about the same, and rots at the groundline in five years or less. The treatment penetration is fundamentally different. BS 8417, the UK timber preservation standard enforced by the Wood Protection Association and referenced in NHBC Part 3.3, requires UC4 for any post set in the ground and sets a minimum desired service life of fifteen years.

Many fence posts sold at garden centres are only UC3-treated, suitable for above-ground use only, despite being marketed for "outdoor use." Outdoor use does not mean ground contact. Ask for the UC rating in writing before you buy. If the merchant cannot confirm UC4, walk away.

Fence posts pair directly with fence panels. Neither does anything useful without the other. Order them together, from the same supplier where possible, so the post spacing matches the panel width.

Types and specifications

Three post types cover 99% of UK domestic fencing. Each has a narrow band of situations where it's the right answer.

A few things that table doesn't capture. The £10 trade price on UC4 timber is from Bradfords Building Supplies, who stock an incised 100x100x2400mm post for £9.98 when the equivalent at Wickes is £27. Elliotts Builders Merchants sits in the middle at £19.90. Homeowners consistently overpay at Wickes and B&Q when two phone calls to local builders' merchants saves £15 per post, which on a 10-post boundary is £150, enough to buy the Postcrete and the gravel board with change to spare.

The lifespan numbers on concrete posts come with a caveat. A well-made precast concrete post from an established manufacturer (FP McCann, Supreme, Allen Concrete) will outlast you. A cheap wetcast concrete post from an online fencing supplier can crack at the groundline after fifteen years as the rebar inside corrodes and expands. The 25-50 year figure assumes reinforced precast with proper concrete cover to the rebar. Ask the merchant for the manufacturer.

Metal spikes are honestly marketed as a quick, cheap solution and they deliver on that promise for the first two years. Then the ground freezes and thaws, the spike tilts, the post rocks loose in the socket, water pools in the metal cup around the post base, and the timber starts rotting from inside the spike outwards. The spike itself will sit in your soil for decades. The fence it holds up will not.

Post size and cross-section

For a standard 1.83m (6ft) fence panel, the minimum post cross-section is 75x75mm timber. In practice, almost every UK installation uses 100x100mm (4x4 inch) because the extra thickness slows rot progression and resists wind load far better. The cost difference between 75x75 and 100x100 is under £5 per post. Always specify 100x100.

Post length is calculated from fence height plus burial depth. For a 1.83m (6ft) fence you need a 2.4m post: 1.83m above ground, 0.6m in the ground, with a small margin for levelling and a gravel board. Taller fences need proportionally longer posts: a 1.83m fence with a 150mm gravel board needs a 2.4m post; a 2m fence needs 2.7m. Don't buy short posts to save money and bury them shallower, it's the single fastest way to end up with a leaning fence after two years of weather.

Concrete post types

Slotted concrete posts come in three configurations. Intermediate posts have two grooves on opposite faces and sit between panels. End posts have one groove and anchor the last panel in a run. Corner posts have two grooves at 90 degrees and sit where the boundary turns. A standard domestic fence run uses one end post, intermediates every 1.83m, and corner posts as needed. Getting the post types in the right order at the planning stage avoids the frustration of a corner post arriving at an intermediate position.

How to work with fence posts

Post depth: the 1/3 rule

The rule every fencing supplier quotes is 1/3 of the total post length in the ground. For a 1.83m fence using a 2.4m post, that's 0.6m (2ft) buried, 1.83m visible. This is the absolute minimum for standard conditions and it works for most UK gardens.

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Four situations require deeper burial:

• Sandy, coastal or waterlogged soil. Add 150mm to the hole depth. Sand provides less lateral resistance so you need more depth to stop the post tilting.
• Exposed sites, tall fences (over 1.83m), or anywhere catching prevailing wind. 900mm (3ft) for a 2m fence in an exposed location is standard, and some installers go to 1m for coastal sites.
• Gate posts. A gate swings, twists, and slams. Gate posts need 150-200mm more depth than line posts, and they should be set in proper C20 mixed concrete rather than Postcrete. A gate opening also needs a post each side, both set deeper than line posts. Budget for it.
• Corner posts. Every corner is a stress point because two panels pull on the post from different angles. Go 150mm deeper than the line posts in the same run.

600mm is an absolute floor regardless of what the 1/3 rule says for a short fence. Below that, any frost heave or sustained wind will start rocking the post loose.

Hole width

The rule: three times the post width. A 100x100mm post wants a 300mm-diameter hole. Too narrow and there isn't enough Postcrete around the post to resist lateral load. Too wide and you're buying twice the Postcrete you need for no structural benefit.

A post-hole auger (clamshell digger, hand auger, or hired petrol auger) produces a clean cylindrical hole at the right diameter. A spade and crowbar works but makes a messy hole that's hard to line out. National Tool Hire charges around £26 per day for a manual post hole digger, £95 per day for a petrol borer. For a boundary run of ten or more posts, the petrol borer pays for itself in time saved and in your back.

Postcrete vs traditional mix

Postcrete (Blue Circle's rapid-set post mix, also sold as Instacrete and generic "post mix") is the default for domestic fencing. It's a pre-blended aggregate-cement mix designed to set in the hole using water added after. For a standard 100x100mm post in a 300mm hole, you need 1.5 to 2 bags per post. Buy whole bags, round up. Leftover Postcrete keeps indefinitely in the unopened bag.

Method: fill the hole one-third with water, pour in the dry Postcrete, mix in the hole with a timber offcut until it's uniform, repeat in layers, finish with a domed concrete collar at the top that sheds water away from the post. It stiffens in 10–15 minutes. Panels can be eased in after 30 minutes. Do not load the fence for 24 hours.

Traditional mixed concrete (1:2:4 cement:sand:ballast, or C20 ready-mix) costs less per hole and sets stronger, but takes 48 to 72 hours to cure. For gate posts and corner posts taking heavy load, traditional mix is the right choice. For line posts across a domestic garden, Postcrete is faster and fine.

Gravel drainage layer at the base

Every post needs 75-100mm of compacted gravel or pea shingle in the base of the hole before the post goes in. This is the step most homeowners skip and it's the step that decides whether the post rots in ten years or twenty. The gravel lets water drain away from the post base rather than pooling against the end grain, which is where rot starts.

Water wicks up timber fastest through cut end-grain. Every fence post has a cut end-grain surface at the top (where you cut to length) and the bottom (where the log was sawn). UC4 treatment penetrates the sapwood thoroughly and about 5mm into the heartwood. The core of the post is less well treated. Cut end-grain exposes that core to water. The gravel drainage layer keeps the bottom end-grain out of standing water.

End-grain preservative: the step everyone skips

Every time you cut a UC4 post, you expose untreated heartwood. The treatment process penetrates through the sides of the post, not along the length. Every saw cut or drill hole into the post is a direct path for water into the unprotected core, which rots in two or three years while the rest of the post looks fine.

Every cut end must be treated with an end-grain preservative. Cuprinol End Grain Preserver and Ronseal End Grain Preserver are widely available at around £12 for 750ml. Brush it liberally onto every cut surface. For the bottom of the post (the factory-sawn end that's going into the ground), give it two coats and let it dry before setting. For drill holes for panel fixings, run a screw covered in preservative into the hole before driving the final screw. A bottle of preservative lasts for a ten-post fence and costs less than replacing one rotten post.

Some merchants sell UC4 posts with a pre-applied bituminous coating on the below-ground section, sometimes branded as Postsaver Pro-Wrap. These cost £5-10 more per post and come with a 20-year rot guarantee. On a boundary you never want to replace, the premium is worth paying.

Setting technique

Drop the post into the hole, resting on the compacted gravel. Hold it plumb using a spirit level on two adjacent faces (not opposite faces, you need to check in both axes). Brace with two timber offcuts nailed loosely to the post and the ground, to keep it upright while you concrete. Pour Postcrete in thirds, checking plumb between each layer. Dome the concrete at the top, sloping water away from the post. Wait 30 minutes before attaching panels, 24 hours before any significant load.

Which post type for which situation

A simple decision:

• Standard garden boundary, 1.83m fence, average UK wind exposure, you plan to live in the house long-term. UC4 timber, 100x100x2400mm, Postcrete-set, 600mm deep. Screw panels to the post through pilot holes in the panel frame. This is the right answer for 70% of UK gardens.

• Long boundary (10+ panels), you want to set it and forget it, or you're re-fencing after a construction project and don't want to do it again. Concrete slotted intermediate posts with concrete gravel boards and timber panels. 25-50 year system. Expensive and heavy (each concrete post weighs 30kg+) but genuinely one-and-done. Rub candle wax into the grooves so panels slide in smoothly.

• Exposed coastal or hilltop site with sustained high winds. UC4 timber with screw-fixed panels. Concrete slotted posts rattle and panels can blow out of the grooves in a gale. Timber posts with screwed panels transfer wind load through fixings rather than gravity and survive far better in storms. Go 900mm deep, use mixed concrete not Postcrete.

• Lightweight temporary screen, trellis, or short decorative fence under 1.2m in a sheltered spot. Metal wedge-grip spike. Quick, cheap, and appropriate for the load. Don't use spikes for anything over 1.2m or anywhere wind-exposed.

• Re-posting a rotten post where the existing concrete base is sound. A concrete repair spur (£29 at Wickes for a 75x100x1200mm spur) bolts to the side of a new short post and uses the original concrete footing. Saves excavating the old concrete lump, which is the hardest part of the job. Fencemate steel repair spurs work similarly.

How much do you need

Fence posts go at the ends and intersections of every panel. For a straight run of fence with no corners or gates:

Number of posts = (run length ÷ 1.83m) + 1

So a 20m run needs (20 ÷ 1.83) + 1 = 12 posts. For each gate opening, add 2 posts (the gate has a post each side). For each corner, the corner post is a shared post between two runs, so count it once. For the total post count across a garden with multiple runs, add up each straight section plus gates plus corners.

Postcrete: 1.5 to 2 bags (20kg) per standard 100x100mm post in a 300mm hole at 600mm depth. Gate posts and corner posts typically take 3 bags. For a 10-post boundary, order 20 bags and keep a few in reserve.

Gravel for drainage layers: roughly 3kg (about one small trowel) per hole. A 25kg bag of pea shingle does eight posts easily.

Other consumables: end-grain preservative (750ml per 10 posts), galvanised screws for panel fixing (4 per panel side, so 8 per panel), panel U-clips if using them.

Always order an extra post. You will mark one wrongly, split one driving a screw into an unpredrilled hole, or discover mid-install that you miscounted. A spare post costs £20. Another merchant delivery costs more.

Cost and where to buy

UC4 timber posts vary dramatically by merchant. The same 100x100x2400mm incised post is £27 at Wickes, £19.90 at Elliotts, and £9.98 at Bradfords. Travis Perkins is in the £17-25 bracket depending on branch and trade status. The national DIY chains (Wickes, B&Q) charge a convenience premium. Independent builders' merchants undercut them consistently.

Concrete slotted intermediate posts are more uniform in price. Wickes stocks a 2440mm (8ft) slotted intermediary at £35 and a 1830mm (6ft) version at £34. MyJobQuote's 2026 cost guide puts them in the £30-35 range for 8ft and £25-35 for 6ft at national coverage. Corner posts cost about 20% more than intermediaries. Slot-matched concrete gravel boards (150mm or 305mm tall) run £20-50 each.

Metal wedge-grip spikes are £12 each at Wickes for the standard 750mm version fitting 100x100mm posts, or £8.40 each buying ten. B&Q stocks a 600mm industrial spike with driving tool at £24.99 for a two-pack. Bolt-grip spikes (the heavy-duty version with twin bolts clamping the post) are £13.50 at Wickes, or £10.50 each in a six-pack.

Postcrete is £6.75 per 20kg bag at Wickes. Bulk deals bring this below £6.50 for 30-bag orders. Jewson and Travis Perkins trade accounts achieve similar pricing.

Alternatives to the standard options

Composite fence posts are available from a handful of specialist suppliers. Rot-proof, insect-proof, and cosmetically similar to timber. Around 3-4 times the price of UC4 timber. If budget is no object and you want a very long-lasting boundary with timber aesthetics, composite is an option. Expensive enough that concrete posts with timber cladding usually wins on value.

Steel powder-coated posts (DuraPost and similar) are roughly 80% lighter than concrete, installable by one person, and come with 25-year powder-coat warranties. Panels bolt to the post and can't be lifted out, which is a genuine security advantage over slotted concrete. Premium price. Suited to modern-aesthetic gardens and commercial boundaries.

Concrete posts clad in timber is the hybrid approach that gives you the longevity of concrete with the look of timber. Timber cladding boards screw to the face of the concrete post after installation. Double the cost of either material alone, but for a long-frontage boundary on a period property where aesthetics matter, the compromise works well.

Concrete repair spurs deserve a second mention. If you have a 15-year-old timber fence where two or three posts have rotted at the groundline but the rest of the posts are sound, spurs are the right fix. Dig a trench alongside the failed post, break out the old post stub (leaving the original concrete base in place if it's solid), bolt a concrete spur to the side of a new short timber post, and set the spur in fresh concrete against the original base. Saves the worst part of the excavation. Wickes and Toolstation both stock them at £25-30.

Where you'll need this

Fence posts appear whenever a boundary is disturbed or replaced during construction:

• Patio and paving - when a new patio runs to the boundary line and existing fencing needs re-posting to align with the hard landscaping
• Drainage works - soakaways or rainwater runs often cross boundary lines and require fence posts to be lifted and reset
• Garden reinstatement after construction - fencing damaged by scaffolding, skip delivery, or plant access gets replaced at practical completion

Fence posts appear in external works and landscaping phases of any extension, new build, or garden project. They're also the single most common reason for boundary disputes with neighbours, so always confirm the exact boundary line from deeds before setting new posts. An inch on the wrong side is a legal problem.

Common mistakes

Using UC3 timber in ground contact. The biggest and most expensive error. UC3 looks identical to UC4, costs about the same, and rots at the groundline in three to five years. Always confirm UC4 in writing at point of sale. Green tint (CCB preservative) or brown tint (oil-based preservative) both indicate treatment, but neither colour alone confirms UC4 penetration. Ask for the specification sheet.

Skipping the gravel drainage layer. Saves five minutes per hole. Costs you the post fifteen years early. Every hole gets 75-100mm of compacted gravel before the post goes in.

Cutting posts to length without sealing the cut end. Every cut exposes untreated heartwood. The £12 bottle of end-grain preservative is the cheapest insurance in the entire project. Always brush both cut ends before setting.

Setting posts less than 600mm deep. The 1/3 rule gives you 600mm for a 1.83m fence. Anything shallower is asking for a leaning fence after two years. Measure before you dig, not after.

Loading fresh Postcrete within 24 hours. The concrete stiffens in ten minutes but hasn't reached working strength until 24 hours. Fitting the fence panel immediately applies wind load before the post is anchored. The post rocks in the hole, loosens the concrete, and you're repouring within a fortnight.

Metal spikes for tall or heavy fences. Spikes work for 1.2m lightweight screens in stable soil. Anything over that height or weight, or on soft or sandy ground, will fail within five years. Frost heave displaces the spike, water pools in the socket, and the post rots from the base up. Use concrete for anything permanent.

Buying at Wickes without a price check. A ten-post boundary bought at Wickes retail costs £270 in timber posts. The same posts from a local builders' merchant are £100-190. Two phone calls before ordering saves £80-170. Independent merchants also deliver for trade rates and often include Postcrete in the same load.