If there’s one thing people consistently guess when building anything out of timber, it’s screw length. They’ll carefully measure the boards, check the levels twice, maybe even debate post spacing… and then grab a random box of screws because “that looks about right.” Sometimes that works. Quite often it doesn’t.
Choosing the correct screw length isn’t complicated, but it does matter. Too short and the joint won’t hold properly. Too long and you risk splitting timber or wasting money on unnecessary length. Use the wrong coating outdoors and you’ll be looking at rust streaks running down your fencing within a year. Fixings are one of the smallest costs in most timber projects, but they are often the difference between something that lasts ten years and something that loosens up after one winter.
So let’s simplify it properly.
The Golden Rule (And Why It Works)
When fixing timber to timber, your screw should generally be two to two and a half times the thickness of the top piece of timber. That’s the rule we use in the yard, and it works across fencing, decking, framing and landscaping projects.
The reason is straightforward. The top board needs to be fully secured, but the bottom board needs enough penetration for real holding strength. If you only just catch the bottom timber, the joint can move. Timber expands, contracts, dries and swells depending on the weather. If the screw doesn’t have enough bite, movement starts. Movement leads to squeaks, loose rails, wobbly sleeper stacks and eventually failure.
As an example, if you’re fixing a 22mm featheredge board to a rail, multiply 22mm by 2.5 and you get 55mm. That means you’re looking for something in the 50–60mm range. If you’re fixing a 45mm fence rail into a post, 45mm × 2.5 gives you 112mm. That puts you in the 100–120mm screw range. It’s not guesswork — it’s simple maths.
Try It Yourself – Quick Screw Length Selector
To make this even easier, here’s a quick selector you can use directly on this page:
Screw Length Selector
Enter the thickness (mm) of the top piece of timber:
Screw Length Selector
Enter the thickness (mm) of the top piece of timber:
Rule used: roughly 2× to 2.5× the top timber thickness.
This keeps things simple and saves you standing in the aisle squinting at packaging.
What Size Screws for Sleepers?
Sleepers are where people most often undersize their fixings. Standard landscaping sleepers are typically 100mm thick. When you’re fixing sleeper to sleeper, especially for raised beds or retaining edges, you are dealing with heavy timber that will move over time.
Using the rule, 100mm × 2 gives you 200mm for full structural strength, although 150mm can be a workable minimum in lighter-duty situations. In practice, 150–200mm structural landscaping screws are the right choice. Anything shorter is usually a compromise. If you’re building raised beds using treated sleepers, proper structural screws will make the difference between something that stays square and something that slowly shifts out of line after a wet winter.
If you’re browsing landscaping sleepers, always think about your fixings at the same time. Timber and screws work together — one without the other is half the job.
What Size Screws for Fence Rails and Featheredge?
Fence rails are commonly 38–45mm thick. That puts you in the 80–120mm screw range depending on the rail size. Featheredge boards, typically around 22mm thick, usually need 50–60mm exterior screws when fixing to rails.
This is also where coating matters. Treated fencing timber contains preservatives that can accelerate corrosion. Bright zinc screws outdoors are a false economy. Exterior-coated, galvanised or suitable structural screws are the right approach for fencing projects. It prevents staining, premature failure and callbacks later.
What Size Screws for Decking?
Decking boards are generally between 19mm and 28mm thick. In most cases, 50–70mm exterior decking screws are appropriate. The aim is enough penetration into the joist to prevent uplift and movement, but not so much length that you’re punching unnecessarily deep into the subframe.
Decking screws are specifically designed for treated outdoor timber. Using interior screws outside is one of the most common mistakes we see, and it rarely ends well long term.
Fixing Timber to Masonry Is Different
When fixing timber to brick or block, the 2.5× rule no longer applies. Instead, you calculate screw length by adding the timber thickness to the wall plug length. For example, if you’re fixing a 25mm batten using a 50mm plug, you need at least a 75mm screw. Always check plug manufacturer guidance to make sure your screw engages fully.
Can Screws Be Too Long?
Yes. Excessively long screws can split timber, especially near edges or in hardwood. They can also reduce holding strength if they pass too far beyond the joint area. Longer is not automatically better. Correct length is better.
Do I Need to Pre-Drill?
For hardwood, thick structural screws, or when fixing near board edges, pre-drilling reduces splitting and improves finish. It’s often worth the extra few minutes.
Exterior vs Interior Screws
If your project lives outdoors — fencing, decking, sleepers, garden structures — you need exterior-rated screws. Treated timber is corrosive. The wrong screw will rust and stain. Galvanised, coated or stainless options are designed to handle that environment.
Final Word
Screw selection doesn’t need to be complicated, but it shouldn’t be guesswork either. Multiply the top timber thickness by two to two and a half, choose the correct coating for the environment, and don’t undersize structural joints. It’s a small decision at the start of a project that makes a big difference over time.
If you’re building fencing, installing decking or working with landscaping sleepers, make sure your fixings match the job. Timber does the visible work — screws do the structural work. Both matter.
