Best Tek Screw Length and Gauge for Timber-to-Steel Fixings
The right Tek screw length must cover the full timber thickness plus enough of the steel below to allow the drill point to exit and the threads to engage. The right gauge depends on the steel thickness and the load the fixing needs to carry. Always check the screw's stated maximum drilling capacity before ordering.
Why Screw Size Matters for Timber-to-Steel Fixing
A Tek screw that is too short will not reach the steel properly. A screw that is too long can waste driving time and may not grip correctly.
Getting the size wrong can also cause a problem called jacking. This is where the screw thread grips the timber before the drill point reaches the steel. When that happens, the timber lifts away from the steel surface instead of pulling tight against it. The joint is then weak.
The gauge of the screw matters too. A screw that is too narrow may not have enough strength for the load. A screw that is too wide may split the timber or put too much stress on the fixing.
This guide helps you choose correctly so the fixing does its job from day one.
How to Calculate the Correct Screw Length
The screw must be long enough to pass through the timber and drill into the steel below. There are two things to account for.
The Two Key Measurements
Add the timber thickness to the steel thickness. Then check that the screw you are considering is rated to drill through the steel portion. The thread engagement in the steel is what provides the holding strength of the fixing.
Timber thickness plus steel thickness equals the minimum total length you need. Round up to the next available screw length. Never round down.
For hex head Tek screws, length is measured from the tip of the drill point to the underside of the head. For countersunk head screws, it is measured to the top of the head. Check which measurement applies to the product you are buying.
If you are fixing through more than one layer of timber or there is a gap between the timber and steel, measure all layers combined. The screw must still fully penetrate the steel after passing through everything above it.
Understanding Drill Point Numbers
Self-drilling Tek screws are rated by drill point number. This number tells you the maximum steel thickness the screw can drill through without a pre-drilled pilot hole.
A higher number means a longer drill point. A longer drill point can penetrate thicker steel. The drill point must fully exit the far side of the steel before the threads begin to engage. If the point is too short for the steel, the threads will bind before the point exits. This can cause the screw to fail during installation.
| Drill Point | Approx. Max Steel Thickness | Typical Use |
|---|---|---|
| No. 1 | Up to 0.7 mm | Very thin sheet metal and light cladding panels |
| No. 2 | Up to 1.5 mm | Light gauge steel sheet and thin framing sections |
| No. 3 | Up to 3 mm | Light steel framing, thin steel purlins, composite panel to steel |
| No. 4 | Up to 4 mm to 6 mm | Medium steel sections and structural steel framing |
| No. 5 | Up to 12 mm | Heavy section steel, structural steel beams, thick plate |
The figures above are general guidance only. Exact drilling capacities vary between manufacturers and product ranges. Always check the stated maximum drilling thickness for the specific screw you plan to use. Never assume a screw will drill through steel thicker than its rated capacity.
For timber-to-steel fixing, the drill point must drill through the steel after passing through the timber. A No. 3 or No. 5 point is most commonly used for this type of work, depending on how thick the steel section is.
Choosing the Right Gauge
Gauge refers to the diameter of the screw. A wider screw has more thread area in contact with the steel. This can improve holding strength. But a wider screw also creates more stress in the timber around the fixing.
Common gauge sizes used for timber-to-steel applications include 4.8 mm and 5.5 mm. These are widely used in UK construction for fixing timber to steel framing and structural sections.
| Gauge | Common Application | Notes |
|---|---|---|
| 4.2 mm | Light fixings, thin gauge steel, lighter timber sections | Lower clamping force. Suited to thinner steel within its rated capacity. |
| 4.8 mm | General timber-to-steel fixing in light to medium applications | Widely used. Check drill point number for your steel thickness. |
| 5.5 mm | Structural timber-to-steel fixing, heavier sections | Greater holding strength. Suitable for heavier loads and thicker steel with a No. 5 point. |
A wider gauge screw creates a larger hole in the timber. In softer or thinner timber close to a board edge, this can increase splitting risk. Pre-drilling may help in these cases. Read our guide on pre-drilling Tek screws in wood for more detail.
Light Steel vs Heavy Steel Sections
The thickness of the steel section changes which screw you need. Light steel framing and heavy structural steel are very different materials, and the drill point must match the job.
JALFT stocks TIMCO A2 stainless steel bi-metal drill screws in 5.5 mm gauge for heavy section steel up to 12 mm. These are available with and without EPDM washers for weatherproof applications.
How Timber Thickness Affects Your Choice
The timber layer is what the screw must pass through before it reaches the steel. The screw needs to clear this layer without gripping it too early.
For thicker timber, you need a longer screw. You also need a screw designed so the thread does not engage the timber prematurely. Specialist timber-to-steel Tek screws often use wings near the drill point to solve this problem. The wings widen the hole in the timber. They then break away when they reach the steel so the thread can grip the metal below.
If the timber is particularly thick, check that the screw you have chosen is long enough after accounting for the drill point length. The drill point itself takes up part of the total screw length and does not contribute to thread engagement in the steel.
Not all Tek screws have wings. Screws with wings are designed specifically for timber-to-steel applications. Do not assume every self-drilling screw will handle the clearance hole in timber automatically. Check the product description before ordering.
Step-by-Step Sizing Process
Follow these steps before ordering your screws. It takes a few minutes and avoids a costly mistake on site.
Use a tape measure or calipers to find the exact thickness of the timber you are fixing through. Write it down in millimetres.
Measure the exact thickness of the steel section the screw will drill into. If you do not have the steel in front of you, check the manufacturer's specification for the framing or structural product you are using.
Timber thickness plus steel thickness gives you a starting minimum. Always round up to the next available screw length. Never round down.
Select a drill point number that is rated to drill through your steel thickness. Check the product data sheet, not just the name or packaging. The drill point must be capable of penetrating the full steel depth.
Select a gauge suited to the load and the steel section. For most timber-to-steel work in construction, 4.8 mm or 5.5 mm is appropriate. Follow any guidance from your structural designer or engineer where a specification exists.
If the timber is more than around 25 mm thick, or the timber is dense, check whether a winged screw is available. Wings prevent the thread from engaging the timber prematurely and help seat the timber tightly against the steel.
For outdoor or exposed fixing, make sure the screw has an appropriate corrosion-resistant finish. Stainless steel or bi-metal screws with A2 grade material are suited to outdoor and damp environments. Check compatibility with any treated timber preservative.
Always test on an offcut of the same timber and steel before installing across a full job. This lets you confirm the screw size is correct and that the drill point can handle the steel without binding.
Common Sizing Mistakes to Avoid
Most problems with Tek screw fixings in timber and steel come down to getting the size wrong. Here are the most common errors.
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Choosing a screw that is too short If the screw does not fully penetrate the steel, the threads cannot engage the metal. The fixing will be weak or may pull out under load.
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Choosing a drill point number too low for the steel If the drill point is too short for the steel thickness, the point will stop before it exits the far side. The threads will then bind in the steel and the screw may break during driving.
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Not accounting for jacking in thick timber If the thread grips the timber before the drill point reaches the steel, the timber will lift away from the steel. Always check whether a winged screw is needed for your timber thickness.
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Assuming all Tek screws have the same drilling capacity Drill point numbers and maximum steel thicknesses vary between brands and products. Always read the product data sheet for the exact screw you plan to use.
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Using a screw with the wrong finish outdoors A zinc-plated screw is not suitable for permanent outdoor exposure. For outdoor timber-to-steel fixing, use a screw with a suitable exterior coating or an A2 stainless steel bi-metal option.
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Using a Tek screw where a woodscrew is the better choice If both pieces are timber with no metal substrate below, a dedicated woodscrew will almost always provide better thread engagement in the wood. Choose the fastener for the application, not just the appearance.
If both parts of your fixing are timber rather than timber and steel, a woodscrew is usually the more appropriate choice. Woodscrews are designed to grip wood fibres correctly and provide reliable holding strength in timber joints.
Frequently Asked Questions
Related JALFT Guides
Find the Right Tek Screw for Your Project
Length, gauge, drill point number, and finish all need to work together. JALFT stocks a range of self-drilling and woodscrews for UK trade and DIY, with next-day delivery available on orders placed before 2 pm.
