Coach Bolts: The Right Fixing for Structural Timber Connections

A heavy beam bracket fixed with coach bolts but no washers. The nuts have pulled 3mm into the softwood over twelve months. The bracket is loose. The beam has dropped. That's a structural failure caused by a 4p washer that wasn't fitted.

Coach bolts are one of the simplest fixings in construction, and one of the most commonly mis-installed. The bolt itself does its job. The problems are always around it: wrong hole size, missing washer, wrong material for outdoor use, over-tightening that crushes the timber. Get those details right and a coach bolt connection will outlast the building it's in.

What a coach bolt is and how it works

A coach bolt (also called a carriage bolt or cup head bolt) is a through-bolt designed for timber connections. It passes all the way through two pieces of timber (or timber and steel), and is secured on the far side with a nut and washer. You can't tighten the head side. That's the point.

The bolt has four distinct features. A smooth, domed head that sits flush against the timber surface. Below the head, a short square shank (the "neck") that bites into the timber when you tap it in, locking the bolt so it can't spin. Below that, a cylindrical shaft. And the threaded section at the end, where the nut goes.

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The square shank is the clever bit. When you drill a hole the same diameter as the bolt shaft and tap the bolt through, the square corners of the neck dig into the timber. This stops the bolt from rotating. You tighten (or loosen) entirely from the nut side with a spanner or socket. No need for two people, no need to hold the head. On the head side, the smooth dome means there's nothing to grip, nothing to snag, and nothing that protrudes.

This one-sided tightening is why coach bolts exist. In a floor where you're bolting a trimmer beam to a trimmed joist, you often can't reach the head side once the bolt is in. You don't need to. Tap it in, fit the washer and nut from the accessible side, tighten up.

Types, sizes, and why mild steel is right

Coach bolts are manufactured to DIN 603 (the European dimensional standard). The sizes that matter for domestic construction are M8, M10, and M12. "M" is the metric designation for the shaft diameter in millimetres.

Lengths range from 50mm to 300mm. The length you need is the total thickness of the timbers being joined, plus about 25-30mm for the washer and nut to engage fully. Two 47mm timbers need a bolt at least 120mm long. Two 72mm timbers need 170mm or more.

M10 is the workhorse. If you're bolting structural timbers in a domestic extension (floor trimmers, beam connections, post brackets), M10 covers 80% of applications. M12 is for heavy-duty work: triple-ply beams, large post connections, or anywhere the engineer's specification calls for it. M8 handles lighter connections like fence post brackets and garden structures.

Why mild steel, not high tensile

Here's something that catches people out. Coach bolts are grade 4.6 mild steel. That's deliberate. They're softer than the grade 8.8 high-tensile bolts you'd use in steelwork, and that softness is a feature.

Timber moves. It shrinks as it dries, swells when it gets damp, and flexes under load. A mild steel bolt bends and accommodates that movement without snapping. A high-tensile bolt resists the movement, builds up stress at the connection point, and eventually fails by fracturing. Brittle failure in a structural connection, with no warning. That's the worst kind.

Don't let anyone tell you that "stronger" bolts are better for timber work. Grade 4.6 is what you want. The strength of the connection is governed by the timber's bearing capacity, not the bolt's tensile strength. An M12 grade 4.6 bolt can handle 28 kN in shear through the shaft. The C16 softwood it's sitting in will fail at around 2.7 kN. The timber is always the weak link, so upgrading the bolt achieves nothing except adding brittleness.

Finishes

Four options, and the choice is straightforward.

Bright zinc plated (BZP) is the standard. Silver appearance, adequate corrosion resistance for interior use and sheltered exterior applications. This is what you'll find in every Screwfix and Toolstation pack.

Hot-dip galvanised provides heavier corrosion protection for fully exposed outdoor use. The coating is thicker and rougher than BZP. Use these for anything in contact with preservative-treated timber, which accelerates corrosion on lighter coatings.

A2 stainless steel (304 grade) won't rust at all. The right choice for coastal locations, permanently damp environments, or where you simply never want to think about corrosion. Costs 3-5 times more than zinc plated.

A4 stainless steel (316 grade) is marine-grade. Only relevant if you're building a jetty.

For a standard domestic extension, BZP is fine for internal structural connections. Galvanised for anything external. Stainless if you're near the coast.

How to install a coach bolt properly

This is a four-step process. Do it right and the connection is solid. Skip a step and you'll be back in six months.

1. Drill the clearance hole

   Drill a hole through both timbers that matches the bolt shaft diameter exactly. 8mm for M8, 10mm for M10, 12mm for M12. Not undersized, not oversized. Use a flat-bit or auger bit for clean holes in timber. Clamp the timbers together before drilling so the holes align perfectly.

2. Tap the bolt through

   Push the bolt through from the head side. If it doesn't slide in easily, tap the domed head with a hammer until the square shank bites into the timber. You should feel it grip and stop rotating. If the bolt spins freely, the hole is too large and the square neck can't bite. You'll need to move the bolt position or use a timber connector plate behind the head.

3. Fit the washer and nut

   On the exit side, slide a penny washer (also called a repair washer) over the threaded end, then thread on the hex nut by hand. The washer must go between the nut and the timber. This is not optional.

4. Tighten with a socket or spanner

   Use a ratchet with the correct socket (13mm for M8, 17mm for M10, 19mm for M12) or a ring spanner. Tighten until the washer beds firmly against the timber and the joint is pulled snug. Stop there. Do not keep going until the washer starts to embed into the wood. Over-tightening crushes the timber fibres beneath the washer, weakens the connection, and can cause splitting.

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Washer selection matters more than you think

The washer's job is to spread the clamping force across a wider area of timber. Without a washer, the nut sits on a contact area the size of the nut's face, roughly 17mm across for an M10. Under load, that small area crushes straight into softwood. The nut sinks, the connection loosens, and the joint fails.

A penny washer (or repair washer) gives you a bearing surface 25-30mm across for M10, spreading the force over three to four times the area. For structural connections in softwood, this is the minimum.

The engineering guidance from Eurocode 5 is specific: the washer diameter should be at least three times the bolt diameter, with a thickness of 0.25 times the bolt diameter. For an M10 bolt, that's a 30mm washer at 2.5mm thick. For M12, it's 36mm at 3mm thick.

Standard form A washers (the small, thin ones that come bundled with some bolt packs) are not large enough for structural timber work. They'll pull through C16 softwood under sustained load. Buy penny washers separately.

Bolt spacing rules

If you're fitting multiple bolts in a row (common on beam-to-beam connections), keep bolts at least five times the bolt diameter apart. That's 50mm for M10, 60mm for M12. Closer than that and the timber between the bolt holes becomes a weak point that can split under load.

Keep bolts at least 2.5 times the bolt diameter from the edge of the timber. For M10, that's 25mm minimum from the timber edge. Closer and you risk splitting the timber outward from the bolt hole.

How many do you need

The number of bolts per connection depends on the load and the timber size, but for domestic structural work, these rules of thumb cover most situations.

A simple timber-to-timber lap joint (two timbers side by side): two M10 bolts minimum, staggered vertically.

A beam bracket or post connection: follow the bracket manufacturer's specification. Most joist hanger and beam bracket fixings specify two or four coach bolts per connection.

A triple-ply trimmer beam (three timbers bolted together): pairs of M12 bolts at 300-400mm centres along the beam length. A 2.4m trimmer needs six to eight bolts.

If an engineer has specified the connection, use exactly what's on the drawing. Don't add more and don't use fewer.

What coach bolts cost

Zinc plated coach bolts are cheap. Stainless steel is not. Prices below are per bolt at typical retail pack sizes, including VAT, as of early 2026.

An M8 x 100mm in zinc plated runs TBC per bolt. Move up to M12 x 200mm and you're looking at about TBC each. Stainless A2 costs three to five times more: an M8 x 100mm in stainless is around TBC, and M10 x 130mm reaches TBC.

Bulk packs bring the per-bolt cost down noticeably. Screwfix sells M8 x 100mm zinc plated in packs of 100 for TBC, which works out to about 18p each. Toolstation sells the same size in 10-packs for TBC, around 20p each. If you need more than 20 bolts of the same size, the bulk pack is the better buy.

Don't forget to budget for nuts and washers separately. Not every coach bolt pack includes them. A bag of M10 hex nuts costs a couple of pounds. A bag of M10 penny washers is similar. Small money, but you need them.

Alternatives to coach bolts

Coach bolts aren't the only option for timber connections. Two alternatives cover different situations.

Wood screws and coach screws

For connections that don't need to be adjustable or retightened, coach screws (DIN 571) are faster to install. A coach screw has a hex head and a wood-thread tip. You drive it with a socket or spanner, and it pulls itself into the timber without needing a nut on the other side. No through-hole needed, no access to the back face required.

The trade-off: you can't retighten a screw connection as timber dries and shrinks. Once it's in, it's in. If the timber moves, the connection loosens and there's nothing you can do except remove and re-drive, which rarely works cleanly in the same hole.

For non-structural connections (fence panels, garden benches, pergola rafters), coach screws are perfectly fine and much faster to fit.

Structural screws

Products like GRK, Spax, and TIMco In-Dex structural screws are a modern alternative that's gaining ground. They're engineered to provide bolt-equivalent holding power without through-bolting. Faster to install, no nut or washer needed, no drilling a clearance hole.

For many applications that traditionally used coach bolts (rim board to post connections, beam hangers, rafter ties), structural screws are now accepted under modern building codes. They're a legitimate choice where speed matters and through-access is limited.

But for the most critical connections, through-bolting with coach bolts remains the proven default. The ability to retighten the nut as timber seasons is something no screw can match. If your structural engineer has specified coach bolts, don't substitute structural screws without checking with them first.

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Where you'll need coach bolts on a build

Structural timber connections are where coach bolts earn their keep.

Floor construction. Triple-ply trimmer beams around stairwell openings and chimney breasts are bolted together with M12 coach bolts at regular centres. You can't nail or screw a structural trimmer together with enough strength. Bolting is the standard method.

Post-to-beam connections. Where a vertical post supports a horizontal beam, coach bolts through a steel bracket connect the two. The bolt passes through bracket and timber, tightened from the accessible side.

Fence and gate posts. Heavy-duty gate hinges and post brackets use M8 or M10 coach bolts. The smooth domed head on the public side looks clean and offers nothing to grip or unscrew from outside.

Roof timbers. Purlin-to-rafter connections, ridge beam splice joints, bolted truss repairs. Less common in modern trussed roof construction (which uses nailplates), but regular in cut-roof carpentry.

Garden structures. Pergola posts, heavy raised bed frames, playground equipment. Anywhere you're connecting large section timber and need a lasting joint.

Common mistakes

These are the errors that cause coach bolt connections to fail. Every one of them is avoidable.

No washer. The single most common mistake. The nut pulls into softwood under sustained load, the connection loosens, and the joint opens up. Always use a penny washer on the nut side. On the head side, the domed head is wide enough to spread the load in most cases, but if you're bolting into very soft timber (like fresh-sawn green oak or low-grade softwood), add a washer under the head too.

Wrong hole size. The clearance hole must match the bolt shaft exactly. An 8mm hole for M8, a 10mm hole for M10. Drill it 1mm undersized and you risk splitting the timber when hammering the bolt through. Drill it 1mm oversized and the square shank can't grip, so the bolt spins when you try to tighten the nut. You end up holding the domed head with pliers while someone else holds the spanner. A two-person job that should be a one-person job.

Over-tightening. The nut should be snug, with the washer bedded flat against the timber. Keep going and the washer crushes into the wood, compressing the fibres. The connection feels tight at first, then loosens as the crushed timber relaxes. Worse, over-tightening near the end of a timber can cause it to split along the grain. Snug means snug. If the washer is starting to dish into the surface, you've gone too far.

Wrong finish for outdoor use. Zinc plated bolts in exposed outdoor locations will start to rust within 12-18 months. The zinc layer is thin and wears through quickly. For anything outdoors, use galvanised or stainless. For bolts in contact with tanalised (preservative-treated) timber, galvanised is the minimum, because the copper compounds in the treatment accelerate corrosion of plain zinc coatings.

Wrong length. The threaded end must protrude far enough past the timber for the washer and full nut engagement. At least 20mm of thread beyond the back face of the timber. A bolt that's only just long enough will have the nut barely started on the thread, and under load it can strip or pop off entirely. Too long is better than too short. You can always add a second nut as a locknut on excess thread.

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Not retightening after timber dries. If you're bolting unseasoned or recently installed timber, the wood will shrink as it dries over the following months. That shrinkage opens up the joint slightly, loosening the nut. Come back after six months and retighten every bolted connection. This is the whole reason coach bolts exist instead of screws: you can retighten them. Take advantage of it.

The bottom line

Coach bolts are simple, cheap, and proven. An M10 x 130mm zinc plated bolt costs about 40p. Add a penny washer and nut and you're under 50p for a connection that will hold a structural timber joint for decades.

Match the hole to the shaft diameter. Fit a penny washer. Tighten snug but not brutal. Use the right finish for the environment. Come back and retighten after the timber has dried. That's it.