The coach screw is one of the workhorses of timber construction — a heavy-duty hex-head screw that bites into wood with serious holding power. Here's what a coach screw actually is, the sizes we stock and what each is typically used for, how to drive them properly (including whether you need a pilot hole), and when a coach bolt is the better choice.
What is a coach screw?
A coach screw is a large, heavy-duty wood screw with a hexagon head (driven with a spanner or socket, not a screwdriver bit) and a coarse, deep thread designed to grip timber hard. The hex head lets you apply far more torque than a conventional woodscrew, which is what gives coach screws their pulling power for structural joints, gate hinges, decking frames and landscaping work.
They're also sometimes called lag screws or lag bolts — same fixing, different name.
Coach screw sizes: what we stock
Our coach screw range runs M6 to M12 in thread diameter, with lengths from 25mm up to 200mm, in two finishes:
- Zinc plated — the everyday choice for indoor and sheltered outdoor use.
- A2 stainless steel — for outdoor work exposed to the weather, and anywhere corrosion resistance matters: gates, coastal fencing, treated timber.
A quick guide to where each diameter typically fits:
| Size | Typical use |
|---|---|
| M6 | Lighter structural work, small brackets, joinery |
| M8 | Fencing, gate furniture, general timber framing |
| M10 | Decking subframes, heavy gates, structural timbers |
| M12 | The heavy end — large structural connections, landscaping timbers |
Pick a length that gives good thread embedment into the timber you're fixing to — as a rule of thumb, at least half the screw's length should end up in the base material. Going into hardwood, or fixing near end grain? Treat pilot holes as non-negotiable.
Do coach screws need a pilot hole?
Usually, yes — especially in the larger diameters and in hardwood. A coach screw driven without a pilot hole can split the timber, and the extra torque needed to drive it can shear the screw or round off the hex head.
A typical published approach is to drill a pilot hole sized to the shank (the unthreaded core) — roughly 50–70% of the thread diameter in softwoods, closer to the full core size in hardwoods — plus a clearance hole through any piece you're fixing through. Exact sizes vary by timber species, so treat these as starting points and test on offcuts where the joint matters.
The exception: some modern coach screws are designed to drive without pre-drilling in most softwoods. Our Timco Advanced coach screws are exactly that — see Timco Advanced coach screws: no pre-drilling needed for the shortcut.
How to fit a coach screw
- Mark up and clamp the joint — coach screws pull components tight, so get alignment right before driving.
- Drill the pilot hole to the depth of thread embedment you want.
- Start the screw square to the surface — a hex head driven at an angle loses grip fast.
- Drive with a socket or spanner, not an impact driver at full violence; steady pressure threads the timber cleanly.
- Stop when tight. Over-torquing strips the thread in the timber or snaps the screw. If the head is burying into soft timber, add a washer to spread the load.
Coach screws vs coach bolts — which do you need?
The two get confused constantly because the names overlap, but they're different fixings for different jobs: a coach screw taps its own thread into timber, while a coach bolt passes right through and is clamped by a nut on the other side. We've covered the full decision — plus where each wins — in Coach Bolts vs Coach Screws: which one do you need?
Short version: bolt-through joints, or anything you might want to take apart later → coach bolt. Everything screwed into timber from one side → coach screw.
Ordering coach screws from Fastco
The full range — M6–M12, 25mm–200mm, zinc plated and A2 stainless — is on our coach screws category page, with fast UK delivery and no account needed to order. For sizing advice on a specific job, speak to an expert on 01270 252 326 (Monday–Friday, 8:00am–4:30pm) or email [email protected] — we've been supplying trades for over 25 years.
FAQ
What sizes do coach screws come in?
Our range runs M6, M8, M10 and M12 in lengths from 25mm to 200mm.
What's the difference between a coach screw and a coach bolt?
A coach screw threads into timber from one side; a coach bolt passes through and is secured with a nut. See our full comparison guide.
Do I need to pre-drill for coach screws?
For standard coach screws, yes — a pilot hole around the core diameter prevents splitting and snap-offs. Timco Advanced coach screws are designed to drive without pre-drilling in softwoods.
Which finish — zinc plated or stainless?
Zinc plated suits indoor and sheltered use; A2 stainless is the pick for outdoor, damp or coastal environments.
