Which screw do I need for the job?

Reading the size code, matching head and thread to the material, and the coatings that survive outdoors.

Difficulty: beginner
Time: 20 min
Read: 3 min

Short answer

Screws are sized as gauge or diameter by length — a 4.0 x 50mm is 4mm thick and 50mm long. Match the head to the finish, the thread to the material, and use stainless or properly coated screws outdoors.

The screw aisle looks overwhelming and reduces to four decisions: size, head, thread and coating.

Step by step

Read the size correctly.Modern screws are diameter by length in millimetres. Older gauges run 6, 8, 10 and so on. A 4.0 x 50 is 4mm diameter and 50mm long including the head on a countersunk screw.
Choose the length by the grip rule.Roughly two thirds of the screw should be in the receiving material. Fixing 18mm board to a stud wants around 50mm.
Pick the head for the finish.Countersunk to sit flush, pan or round head for washers and thin materials, flange for a wide bearing surface, and bugle head for plasterboard so it does not tear the paper.
Match the thread to the material.Coarse threads for softwood, plasterboard and chipboard. Fine threads for hardwood and metal. Twin-thread for fast driving in softwood.
Look for a partial thread on joins.A shank with no thread near the head lets the top piece be pulled tight against the bottom one. Fully threaded screws hold the two apart.
Choose the coating for the environment.Zinc for dry interiors, organic-coated or hot dip galvanised for outdoors, and A4 stainless near the coast or in contact with treated timber.
Use stainless with treated timber.Modern preservative treatments corrode ordinary and even galvanised fixings within a few years. Decking and fence screws should be stainless or specifically rated for it.
Prefer Torx or Pozidriv drives.Torx grips far better and effectively does not cam out. Pozidriv is the UK default — use Pozidriv bits, not Phillips.
Match the screw to the plug.Wall plugs specify a screw gauge and drill size. Using a smaller screw means the plug never expands and the fixing pulls out.

Tips

Two thirds of the screw should be in the receiving material.
Partial-thread screws pull two pieces together; fully threaded ones hold them apart.
Treated timber corrodes ordinary screws. Use stainless outdoors.

Common mistakes

Using galvanised screws in modern treated timber — Current preservatives are corrosive to zinc coatings, so fixings fail within a few years and the joint loosens or stains. Stainless or purpose-rated screws are needed.
Using a fully threaded screw to join two boards — The thread grips the top board as well as the bottom, so the screw jacks the pieces apart instead of pulling them together. Use a partially threaded screw.

If it doesn't work

Boards not pulling tight together

Cause: Fully threaded screw — Fix: Use a partial-thread screw, or drill a clearance hole through the top piece.

Screw spinning without gripping

Cause: Hole too large or thread stripped — Fix: Move slightly, use a larger gauge, or plug the hole and refix.

Rust streaks on outdoor timber

Cause: Wrong coating — Fix: Replace with stainless. The staining is permanent in the timber.

Timber splitting

Cause: No pilot hole, or screw too thick — Fix: Pilot near ends and edges, and use a finer gauge in hardwood.

Questions people ask

What do the two numbers on a screw box mean?

Diameter and length in millimetres — a 4.0 x 50 is 4mm thick and 50mm long. Older imperial gauges such as 8 or 10 describe diameter only.

Can I use indoor screws outside?

Not for long. Zinc-plated screws rust quickly outdoors and stain timber. Use organic-coated, hot dip galvanised or stainless depending on exposure.

Are Torx screws better?

For driving, clearly — the star recess does not cam out, so heads rarely strip. They cost slightly more and need Torx bits.