Placing a stud against a concrete wall using screws?

[Kentaclinta78]

Is it technically correct to always pre-drill a larger hole in the wood/beam/board than the diameter of the self-drilling/concrete screw, through the material like on a beam or board that is to be fastened tightly against hard concrete on a wall and must withstand high weights?

Refers to
Concrete screw FFS, 7.5 x 82 mm, 50 pcs.

For what reason and do you always do so or use a smaller hole?

AI Answer:

Here is the explanation why a narrower hole in the beam can create problems:

Why 8–8.5 mm in the beam is better than 7.5 mm or less:

Avoid threading in the wood: If you use a narrow hole in the wood (the same size as the screw or smaller), the screw threads will grip both the wood and the concrete at the same time. This creates a situation where the screw pulls the wood towards itself instead of pulling the entire beam against the concrete wall.

• Ensure correct clamping force: A larger hole in the beam (8-8.5 mm) acts as a "free hole" (through hole). It allows the screw to rotate freely through the wood until it reaches the concrete. All the tightening force is then exclusively used to press the beam's surface against the concrete wall.
  
• Risk of the screw breaking or pulling out: If the screw threads in the wood, you can get a "catching" effect. When you try to tighten the screw properly against the wall, you might either:
  
  • Fail to pull it tight so the beam does not sit tight.
    
  • Overload the screw and risk it breaking right at the transition between wood and concrete.
    
  • Tear the threads in the concrete because the clamping force is not transferred correctly.
    

Summary

• The hole in the concrete (6 mm): Must be exactly the right dimension for the screw to cut its threads and achieve maximum strength.
  
• The hole in the beam (8–8.5 mm): Is a "slack hole" that ensures the screw can tighten the beam properly against the concrete without resistance in the wood.

By using the larger hole in the beam, you ensure a correct and strong fastening where the beam is tight and secure against the wall.

 

[Mats-S]

Avoid threading in the wood: If you use a narrow hole in the wood (the same size as the screw or smaller), the screw's threads will grip both the wood and the concrete simultaneously. This creates a situation where the screw pulls the wood towards itself instead of pulling the entire rule towards the concrete wall.

That's the key to determining the dimension of the pre-drilled hole in the rule. It should be exactly on the verge of the screw's threads gripping the rule, but NOT larger.
And you can only determine the hole's dimension by testing until it is correct; there is no absolute rule that it has to be a few millimeters larger; the appearance of the screw decides.

If the screw is smooth throughout the rule, you can reduce the hole somewhat, as the smooth part cannot lift the rule from the surface.

 

[Kentaclinta78]

That is the key to what dimension you should have for the pre-drilled hole in the rule. It should be exactly so that you are on the edge where the screw threads grip the rule, but NOT larger.
And the dimension of the hole can only be determined by testing until it is correct, there is no absolute rule that it should be a few millimeters larger, the appearance of the screw determines it.

If the screw is smooth all the way through the rule, you can reduce the hole slightly because the smooth part cannot lift the rule from the surface.

Thanks for the answer. That the threads sticking out on the screw cut into the wood (rule) is basically the same method as drilling a smaller hole of about 6 mm for an 8 mm diameter -concrete- screw (2 mm). Sounds completely logical when you put it that way.

Screw model https://www.biltema.se/bygg/fastelement/byggskruvar/betongskruv-ffs-75-x-82-mm-50-st-2000057374