Screws may seem simple, but using the wrong type can turn a quick fix into a frustrating ordeal. Ever found yourself needing a screw, only to realise the one you have is either too short, too long, too thick, or too thin? Or worse, when you finally find the right size, you don’t have the correct screwdriver or a rawl plug?
This guide breaks down everything you need to know about screws, from sizing and head shapes to thread types and tips for avoiding common DIY mishaps. Please note, this does not cover Allen keys (Hex) or other types of fasteners or fixings.
Understanding Screw Sizes
A wood screw is classified by two key measurements:
- Length – How far the screw penetrates the material.
- Gauge – The diameter of the shank (the solid part of the screw before the thread starts).
Gauge Number System (Thickness)
Screw thickness is measured using a gauge number, with 6, 8, and 10 being the most common sizes.
- The higher the gauge number, the thicker the screw.
- For example, a No.10 screw is thicker than a No.6 screw.
Understanding Screw Length
Here’s a key detail most people overlook:
- A 1-inch countersunk screw is shorter than a 1-inch round head screw.
Why? Because the length refers to how much of the screw is buried into the material, not the total screw length.
Countersunk screws – The length is measured from the top of the screw to the tip.
Round head screws – The length is measured from the bottom of the head to the tip.
Choosing the Right Head Shape
Screw heads are designed for different applications and influence how the screw sits on the surface.
| Head Shape | Description | Common Uses |
|---|---|---|
| Countersunk | Sits flush with the surface when fully screwed in. | Woodwork, door hinges, furniture assembly. |
| Raised | Has a slightly domed head; partially countersunk. | Door furniture, decorative fittings. |
| Round | Sits above the surface; used where screws are not countersunk. | Fixing metal brackets, exposed screws for aesthetic reasons. |
Understanding Screw Slot Types
Choosing the right slot type prevents camming out (when the screwdriver slips from the screw head) and ensures a secure grip.
| Slot Type | Description | Best Used With |
|---|---|---|
| Slotted | Traditional single-slot head. Can slip easily if not carefully aligned. | Standard flathead screwdrivers. |
| Phillips | Cross-head design to reduce slipping. | Phillips screwdrivers. |
| Pozidriv/Supadriv | Similar to Phillips but with additional cross markings for better grip. | Power screwdrivers, modern construction. |
| Robertson (Square Drive) | Canadian invention with a square-shaped slot for a secure fit. | Ideal for overhead work as prevents the screwdriver slipping. |
Tip: If using a power screwdriver, opt for Supadriv or Pozidriv screws, as the driver locks in securely, making jobs faster and easier. While Phillips are the most common types of screw, they can strip easily.
Choosing the Right Thread Type
Different screw threads are designed for specific materials and applications.
| Thread Type | Description | Best For |
|---|---|---|
| Twin-start thread | Sharp, double-threaded for faster driving. | Power screwdrivers, quick assembly. |
| Standard thread | Traditional single-thread design. | Manual screwing, general woodwork. |
Avoid using standard-thread screws in power screwdrivers, as they take longer to drive in and are more prone to stripping.
Handy Screw Tips & Tricks
Getting the right screw diameter:
Use this simple formula to calculate screw shank diameter (in inches) for your fastener:
Multiply the screw gauge number by 0.013 and add 0.060
Example:
- No.8 screw: (8 × 0.013) + 0.060 = 0.164 inches
- No.10 screw: (10 × 0.013) + 0.060 = 0.190 inches
Convert to millimetres: Multiply the result by 25.4
Example:
- No.8 screw: 0.164 × 25.4 = 4.166 mm
- No.6 screw: 0.138 × 25.4 = 3.505 mm
Always measure carefully – just a fraction too long can poke through a surface, while one too short may not hold securely.
Practical DIY Screw Tips
Prevent Screws from Sticking
Rub a little grease or Vaseline on the threads before inserting. Avoid soap, as it can attract moisture and lead to rusting.
Prevent Wood Splitting
Drill small pilot holes before inserting screws—especially in hardwood or near the edge of a board.
Avoid Stripping Screw Heads
Use the correct screwdriver size and apply even pressure while driving screws in.
Avoid Wood Splitting Along the Grain
Stagger screws instead of lining them up along the wood grain to reduce splitting.
Screw Materials & Best Use Cases
Choosing the Right Screw Material
The material of a screw affects its strength, corrosion resistance, and ideal usage. Selecting the wrong material could lead to rust, breakage, or improper fastening.
Comparison of Common Screw Materials
| Material | Properties | Best Used For | Pros | Cons |
|---|---|---|---|---|
| Stainless Steel | High corrosion resistance, non-magnetic. | Outdoor use, marine environments, kitchens, bathrooms. | Rust-proof, strong in damp conditions. | Can be softer than hardened steel. |
| Brass | Non-corrosive, decorative. | Furniture, electrical fittings, decorative projects. | Aesthetic appeal, does not rust. | Softer than steel, prone to bending. |
| Zinc-plated Steel | Steel coated in zinc for corrosion resistance. | Indoor general-purpose projects. | Affordable, decent rust protection. | Not suitable for wet conditions. |
| Galvanised Steel | Steel coated in thick zinc layer for enhanced rust resistance. | Outdoor structures, fencing, decking. | Very durable, corrosion-resistant. | Can be brittle, expensive. |
| Black Phosphate | Steel coated in phosphate for smooth driving. | Drywall installations. | Strong grip, holds paint well. | Rusts quickly if exposed to moisture. |
| Silicon Bronze | High strength, excellent corrosion resistance. | Marine and boating applications. | Highly durable, resistant to seawater. | Expensive, not widely available. |
Quick Tip: For outdoor and wet environments, stainless steel or galvanised screws are the best choice. Avoid brass and black phosphate screws, as they are prone to corrosion.
Best Screws for Specific DIY Jobs
Choosing the right screw for the right material ensures strong, secure fastening and prevents damage.
Best Screws for Drywall
Use: Drywall screws
- Designed with a bugle-shaped head to prevent tearing the paper surface.
- Coarse-thread for wood studs, fine-thread for metal studs.
- Often black phosphate-coated for better grip and easy driving.
Avoid using wood screws in drywall, as they lack the appropriate grip and may loosen over time.
Best Screws for Masonry (Brick, Concrete, Stone)
Use: Masonry screws (Concrete Screws)
- Require pilot holes drilled with a masonry drill bit.
- Have hardened steel bodies and deep threads for strong grip in stone.
- Blue-coated or galvanised for rust resistance.
Avoid using standard wood screws in brick or concrete—they will strip and loosen over time.
Best Screws for Outdoor Projects (Fences, Decking, Garden Structures)
Use: Exterior screws (Decking Screws, Galvanised Screws, Stainless Steel Screws)
- Coated or stainless steel to prevent rusting in rain and moisture.
- Some deck screws have self-drilling tips to make installation easier.
- Designed to withstand high tension and load-bearing stress.
Avoid zinc-plated screws outdoors, as they will rust quickly in rain.
Final DIY Screw Selection Guide
| Project Type | Best Screw Type | Key Features |
|---|---|---|
| Hanging Drywall | Drywall Screws | Fine/coarse thread, black phosphate coating. |
| Fixing Wood to Brick | Masonry Screws | Requires pilot holes, hardened steel. |
| Building a Deck/Fence | Exterior Screws | Stainless steel or galvanised, rust-proof. |
| Furniture Assembly | Brass or Wood Screws | Decorative, soft metal, traditional woodwork. |
| Bathroom/Kitchen Fixtures | Stainless Steel Screws | Non-corrosive, resistant to moisture. |
| General Indoor Use | Zinc-Plated Screws | Affordable, moderate corrosion resistance. |
Final Thoughts: Choosing the Right Screw for the Job
- Consider the size, head shape, thread type, and slot type before starting a job.
- Use Supadriv or Pozidriv screws for power tools to prevent stripping.
- Pre-drill holes for a clean, professional finish—especially in delicate materials.
- Use corrosion-resistant screws (e.g., stainless steel) in damp environments.
Where to Buy Screws in the UK
If you’re in the UK, you can find a variety of screws at:
- B&Q – General-purpose screws for home projects.
- Screwfix – A broad selection of screws for construction and trade professionals.
- Toolstation – Affordable options for DIYers.
- Local hardware stores – Great for small purchases and specialist advice.
