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Welcome to the Bunnings Workshop community @alansyd. It's wonderful to have you join us, and many thanks for your question about fixing timber to metal.
It sounds like you are working with structural steel, and at around 10 mm thick, there really are no conventional screws that can reliably cut or hold a thread in material that heavy. Metal tek screws are designed for sheet steel, usually in the range of about 1 mm to 3 mm, sometimes up to 5 mm at the upper limit. Beyond that thickness, the cutting flutes simply cannot bite, the threads tend to shear, and even if you managed to drive one in, it would not have the strength or holding capacity you need for framing. Ten millimetres of steel is in bolt territory rather than screw territory.
In situations like yours, the appropriate method is almost always to bolt the timber to the steel. If you can access the back side of the steel, even partly, you can drill a clean hole through both materials and use galvanised structural bolts or coach bolts with washers. This gives you proper clamping pressure, predictable performance, and compliance with structural requirements. If the back of the steel is not accessible, it becomes a different discussion, but you would typically look at welding on a plate, adding threaded inserts designed for heavy steel, or using a different fixing strategy entirely. Are you able to access the rear of the steel section to run bolts through it, or is it enclosed or embedded in a way that makes that difficult?
If this connection is part of a structural alteration, it is best to have a qualified builder or structural engineer advise on the correct fixing method. They can confirm the right bolt size and layout so the connection remains safe and compliant.
Please let me know if you have any questions.
Mitchell
no access from the back. I know it can be done which lots of tradies do it all the time by predrilling a hole first then put a screw through the timber into the steel post. See the photos I attached below. Maybe it's only 5mm steel for the steel post
Hello @alansyd
If this structure you are building is based off a plan that an engineer has drawn up, there should be an indicator on the plan what kind of fixing needs to be used. I highly recommend speaking to your builder or structural engineer and asking them to advise you on the correct fixing method. They will be able to confirm the right bolt size and layout so the fixing will be safe and compliant. Using the incorrect fitting can cause structural failure and possibly injury.
If you need further assistance, please let us know.
Eric
"Maybe it's only 5mm steel for the steel post"
I'm doubting even this @alansyd - possibly 100 X 100 @ 2mm or 3mm thickness - especially based on the insertion photos above.
This didn't require pre-drilling - straight though 2 mm steel and 35 mm Radiata with a cheap impact driver.
For 3 mm - I would definitely pre-drill.
Are you able to share the purpose/plan/project you're on?
Cheers.
