A Beginner's Guide to Wood Screws Types, Sizes, and When to Use Them

Wood screws are one of the most basic items in any toolkit  but walk into a builders' merchant and you'll find dozens of variations. Different heads, different threads, different coatings, different lengths. Knowing what the differences actually mean makes it much easier to pick the right one without guessing.

This guide covers everything a beginner (or anyone who just wants to stop guessing) needs to know about wood screws.

What Makes a Wood Screw Different

A wood screw is designed specifically to hold two pieces of timber together, or to fix timber to another material. The key features are the thread (which cuts into and grips the wood fibres) and the head shape (which determines how the screw sits flush with or below the surface).

Wood screws are different from machine screws (which thread into metal nuts) and self-tapping screws (which cut threads into metal). Using the wrong type for the material often causes the fixing to fail.

Thread Types

Fully threaded screws have thread running from near the head all the way to the point. These are good for pulling two pieces of timber tightly together or fixing into a single piece of material.

Partially threaded screws have a smooth shank near the head and thread only on the lower half. This design is better when you're joining two pieces of timber face-to-face. The smooth shank in the top piece means the screw doesn't grip there  so as you tighten, it pulls the two pieces tightly together. A fully threaded screw gripping both pieces can actually prevent them from closing up.

For decking and structural timber, partially threaded screws are often the better choice for this reason.

Head Types

Countersunk head  the most common for general woodworking. The angled head sits flush with or below the timber surface when driven home. Leaves a clean, flat finish. These are the standard for most applications.

Bugle head  similar to countersunk but with a curved underside that reduces the chance of splitting thin material. Used extensively for plasterboard and decking.

Pan head  a flat, wide head that sits above the surface. Used where you don't want the screw to sink into the material  fixing through metal plates, brackets, or where you need to be able to remove the screw easily.

Round head  less common in modern use but occasionally seen in older joinery and for decorative applications.

Wafer head  a very flat, wide head. Used for fixing sheet materials and plywood where the screw needs to stay flush without sinking deep.

Drive Types

Pozi drive  the most common drive in UK wood screws for general use. The cross-shaped recess with additional diagonal slots. Pozi is not the same as Phillips (a common confusion)  a Phillips bit will work but strips Pozi heads quickly. Use a proper Pozi bit.

Torx / star drive  increasingly popular because it resists cam-out (the bit slipping out of the recess) far better than Pozi. Most quality decking and structural screws now use Torx. Worth using for anything that needs to be driven with power tools or at high torque.

Hex head  driven with a socket or hex key. Used for coach screws and structural fixings where high torque is needed.

Slotted  the old straight slot. Largely obsolete for construction use. Difficult to use with power tools without slipping. You'll mostly see it on older furniture and decorative fittings.

Coatings and Materials

Bright zinc plated (BZP)  standard silver-coloured screws. Fine for indoor, dry applications. Will corrode if used outside or in damp conditions.

Yellow passivated zinc  slightly better corrosion resistance than BZP. Still indoor only.

Stainless steel (A2)  excellent corrosion resistance. Suitable for outdoor use and anywhere moisture is present. More expensive but a big step up in durability.

Stainless steel (A4/marine grade)  for coastal environments or use near water.

Green or brown coated  a protective organic coating. Better than zinc plating for outdoor use but not as long-lasting as stainless steel. Common for treated timber and decking.

Yellow dichromate / gold  often used for drywall and plasterboard screws. Not suitable for outdoor use.

Screw Sizes Explained

Wood screws are measured by two things: diameter (gauge) and length (in mm).

The gauge refers to the diameter of the shank. Common sizes for UK construction and woodworking:

• No. 6 (3.5mm)  light joinery, cabinet work, thin material
• No. 8 (4.0mm)  general purpose, the most common for typical timber work
• No. 10 (4.8mm)  heavier joinery, structural fixing into thicker timber
• No. 12 (5.5mm and above)  heavy structural and external applications

For length: as a general rule, the screw should be long enough that at least two thirds of its length goes into the base material (the piece you're fixing into), not the top piece.

So if you're fixing a 20mm batten onto a wall, you want the screw to go at least 40mm into the timber or masonry behind it  meaning a minimum 60mm screw.

Pre-Drilling  When and Why

Many modern wood screws are designed to go in without a pilot hole, especially in softwood. But there are cases where pre-drilling is important:

• Near the end of a board  within 50mm or so, the wood is at risk of splitting. A pilot hole releases the pressure.
• Hardwood  dense timber needs a pilot hole to allow the screw to drive without snapping or splitting the wood.
• Near the edge of a board  same principle as near the end.
• Thin material  plywood, MDF, and thin boards are prone to splitting without a pilot hole.

The pilot hole should be slightly smaller than the screw's core diameter (the diameter at the root of the threads). This lets the thread bite without splitting the wood.

Quick Reference

Wood screws might seem basic, but using the right one for each job makes work easier, gives a better finish, and means your projects hold together the way they should. Take a minute to match the screw to the material, the load, and the environment  it makes a real difference.