Why do my screws keep snapping when I drive them in?
Torque, material and metallurgy — the four causes of sheared screws, including why brass and plasterboard screws snap where a normal screw survives.
- Difficulty
- beginner
- Time
- 12 min
- Read
- 3 min
- Safety
- caution
Short answer
A screw snaps when the torque needed to turn it exceeds the torque the shank can carry. Reduce what it needs — pilot properly, use wax, back off the impact driver — or increase what it can take by using a bigger gauge or a screw that is not brittle. Brass, hardened plasterboard screws and cheap zinc screws all fail early for different reasons.
Cordless drivers now produce far more torque than a hand screwdriver ever did, and screws have not all kept up. A screw is a small twisted bar of steel: it has a torsional limit, and modern tools reach it easily in dense material. The fix is almost always to lower the resistance rather than to buy a stronger screw.
Safety
Step by step
- Pilot properly in anything hard.The single biggest reduction in driving torque. Drill at the screw's core diameter — the solid part between the threads — full depth. In oak, beech, iroko and dense exterior hardwoods this is not optional.
- Add a clearance hole through the top piece.If the thread is cutting through both pieces, it is doing twice the work for no benefit. Clearing the top piece halves the resistance at a stroke.
- Wax the thread.A candle end, beeswax or a proprietary screw lubricant rubbed on the thread makes a startling difference in hardwood and in long screws. Do not use soap — it holds moisture and corrodes the screw.
- Turn the driver's clutch down and stop using impact for the last turn.An impact driver delivers torque in hammer blows that a screw's shank does not resist as well as smooth torque. Seat the last few turns with a drill/driver on a low clutch setting, or by hand.
- Go up a gauge rather than down.Torsional strength rises steeply with diameter. A 5mm screw is far stronger in torsion than a 4mm one, not slightly. If 4mm screws keep failing in the same job, 5mm usually ends it.
- Recognise the brittle types.Hardened plasterboard screws and cheap heat-treated screws are hard, which means they do not bend before they break. In dense timber they shear with no warning at all. Use a proper wood or construction screw instead.
- Treat brass as a special case.Brass screws are soft and shear at a fraction of a steel screw's torque. Cut the thread first with a steel screw of the same size, remove it, then wind the brass screw into the finished thread with a hand driver — never a power tool.
- Check you are not screwing into something you cannot see.A screw that snaps suddenly at a consistent depth is often hitting a nail, a plate, an old fixing or a knot. Move and try again rather than pushing on.
Tips
- Screws that come out visibly twisted are telling you the job is close to the limit even where none have failed yet. Change something before it does.
- In treated softwood, long screws often snap because the timber is wet and swollen tight on the shank. Pilot the full depth, not just the first half.
- Reusing screws is generally fine for anything light, but not for a screw that has been driven hard, has a damaged recess, or shows any twist along the shank.
Common mistakes
- Driving brass screws with a cordless driver — Brass has perhaps a third of the shear strength of steel. It will twist off flush with the surface, in a hole you now cannot use.
- Using plasterboard screws for structural fixing — They are made hard so the recess survives high-speed collating tools. Hard means brittle, and in shear or torsion they snap where a mild construction screw simply bends.