Bolts, coach screws and structural fixings

Learn to choose, drill for and tighten coach screws and bolts for strong, safe structural fixings.

BC-01Learn the difference between a screw, a coach screw and a boltA normal wood screw has a thin shaft, a full thread and a slot or Pozi head for a screwdriver. A coach screw is much thicker, has a hexagonal head like a bolt, and needs a spanner or a socket on a ratchet to turn it - never a screwdriver. A bolt has a smooth unthreaded shaft and passes right through both pieces of material, with a washer and nut fitted on the far side to lock it in place. Beginners often try to force a coach screw round with a screwdriver and round off the head, or assume a coach screw will grip in brick the way it grips in wood - it won't, brick needs a separate masonry anchor.

BC-02Match the fixing to the material you're fixing intoUse a coach screw when you're fixing something heavy into solid timber, such as a joist, a stud, or a fence post, and you can only get at one side. Use a nut and bolt when you can reach both sides of the material, such as bolting two timbers together or through a thin bracket. For brick, block or concrete, a coach screw on its own will just spin in the hole - you need a purpose-made masonry anchor rated for the load, fitted into a hole drilled with a masonry bit. Picking the wrong fixing for the material is the single biggest mistake at this stage.

BC-03Mark your hole and check what's behind it before you drillMark the exact spot with a pencil. Before drilling into any wall, floor or ceiling, run a cable and pipe detector over the area - but treat this as a warning system, not a guarantee. Detectors can miss cables that run deep, sit behind metal-backed plasterboard, or run outside the usual safe zones, and older or amateur wiring often ignores those safe zones completely. If there's any doubt at all, isolate the circuit at the consumer unit before you drill, and turn the water off at the stopcock before cutting into anything that could carry a pipe. Always expect something to be there and drill carefully to prove otherwise, rather than assuming the detector's all-clear means it's safe.

BC-04Drill a pilot hole sized to the fixingFor a coach screw in timber, drill a pilot hole using a bit slightly narrower than the solid core of the screw, not its outer thread width - this stops the wood splitting while still giving the thread something to bite into. For a masonry anchor, drill to the exact diameter stated on the anchor's packaging, no wider. Drill straight and steady, keeping the drill square to the surface, and clear dust from the hole as you go.

BC-05Drive the coach screw with a socket or spannerSlide a washer onto the coach screw first - this spreads the load and stops the head pulling into the wood. Fit a socket or ring spanner over the hexagonal head and turn clockwise with steady, even pressure, keeping the screw square to the surface. It should feel like it gets progressively stiffer to turn as it seats; if it suddenly turns easily or spins freely, the wood has split and the screw needs moving to a new spot. Never use an impact driver on a coach screw without a proper pilot hole - it can crack timber, especially fence posts, in one go.

4 each · Coach screws

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4 each · Washers

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BC-06Fit and tighten a through-bolt with two spannersPush the bolt through the aligned holes, then fit a washer and nut onto the far end. Hold the bolt head still with one spanner while turning the nut with a second spanner - this stops the whole bolt twisting round uselessly. Tighten until the washer sits flat and snug against the surface with no gap, but stop as soon as it's firm; over-tightening crushes wood fibres and can crack timber or strip the thread.

4 each · Bolts (with matching nuts)

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8 each · Washers

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BC-07Check the fixing has actually done its jobGrip the fixed item firmly and try to rock or wobble it - a good fixing should not move at all. Look closely at the wood around the fixing for any fresh cracks or splits. On a coach screw, the washer should sit flush against the surface, not sunk in or still proud. On a bolt, one or two threads should show just past the nut - if none show, the bolt is too short for the job and needs replacing with a longer one.

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