How do I fit a coach screw properly?

A coach screw is a heavy square or hexagon-headed screw with a coarse thread and a plain shank, used for structural timber connections — joist hangers, pergola posts, gate hinges, heavy brackets. It is driven by a spanner, and it will happily strip out the timber it is meant to hold.

What you'll need

• coach screws and washers
• socket set or spanner
• drill and bits
• soap or wax (optional)

Step by step

1. Check the length is right.The threaded part must be entirely in the receiving timber, with the plain shank passing through the piece being fixed. A coach screw whose thread grips both pieces cannot pull them together.
2. Drill the clearance hole first.Through the piece being fixed, at the shank diameter, so the screw slides freely through it.
3. Drill the pilot into the receiving timber.At roughly seventy per cent of the thread's outside diameter in softwood, and a little larger in hardwood. Too small and it will split or strip; too large and it will not grip.
4. Make the pilot deep enough.At least the full length of the threaded portion. A screw bottoming out will feel tight when it is actually just jammed, and will then strip the thread as you keep turning.
5. Always fit a washer.The head is small relative to the load and will crush straight into softwood without one. A round washer is the minimum; a square plate washer is better in structural work.
6. Lubricate the thread.Wax, soap or candle on the thread markedly reduces driving torque, which reduces the chance of stripping and of snapping smaller sizes.
7. Drive it with a socket or spanner.A ratchet gives control and feel. An impact wrench drives them fast but strips the timber easily and offers no warning before it does.
8. Stop as soon as the washer bites.Firm, not maximum. The moment the head starts turning without getting tighter, the thread has stripped in the timber and it will not recover.
9. Check the fixing after a season outdoors.Timber shrinks as it dries, and structural coach screws in green or wet timber commonly need one further nip a few months in.

Tips

• Wax on the thread is the difference between a coach screw that goes in smoothly and one that fights you the whole way.
• A square plate washer spreads the load far better than a round one in soft structural timber.
• If a coach screw suddenly turns easily, stop immediately. Continuing only enlarges the stripped hole.

Common mistakes

• No pilot hole — A coach screw's thread is coarse and its shank is thick. Driven into solid timber without a pilot it splits the piece, and in hardwood it will simply shear off.
• Driving with an impact wrench to the limit — There is no feel and no warning. It strips the thread in the timber a fraction of a second after it becomes tight, and a stripped hole means starting again in a new position.

If it doesn't work

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