What is the difference between a self-tapping and a self-drilling screw?
One cuts a thread in a hole you made; the other drills the hole as well. How to tell them apart, and what each will and will not go through.
- Difficulty
- beginner
- Time
- 10 min
- Read
- 3 min
Short answer
A self-tapping screw cuts its own thread but needs a pilot hole drilled first. A self-drilling screw has a small drill point on the end that makes the hole as it goes, so it needs no pilot — but only up to the thickness of metal that point can clear. Look at the tip: a plain sharp point is self-tapping, a tiny twist-drill flute is self-drilling.
These two terms are used almost interchangeably in shops and they mean different things. The distinction matters the moment you are fixing into metal, because getting it wrong means either a screw skating around on a shiny surface or a screw that stalls halfway and shears.
Step by step
- Look at the tip to identify what you have.A self-drilling screw — often called a tek screw — has a recognisable miniature drill bit formed on the end, with flutes. A self-tapping screw ends in a plain sharp point or a blunt taper.
- Use a self-tapping screw where you can drill a pilot.Drill the pilot at the screw's core diameter, not its outer diameter. The screw then forms the thread as it goes. This is how screws go into plastic, thin sheet, and pre-punched brackets.
- Use a self-drilling screw on sheet metal you cannot easily pilot.Steel studwork, roofing sheet, ducting, purlins, brackets on box section. The drill point makes the hole and the thread follows immediately behind it.
- Check the drill point is long enough for the metal.The point must fully clear the material before the thread reaches it, or the thread starts cutting while the point is still drilling and the screw stalls. Points are sold in numbered lengths for exactly this reason.
- Run slowly with firm steady pressure into metal.Speed generates heat and blunts the point. Steady pressure keeps the point cutting rather than rubbing, which is what work-hardens stainless and makes a hole impossible to finish.
- Use the right head for the material below.A flange or washer head with a bonded EPDM washer for anything outdoors or weathertight; a countersunk head only where you have a recess for it; a wafer head where the material is thin and could be pulled through.
- Do not use either in solid masonry.Neither type is hardened for brick or concrete. That job wants a concrete screw or a plug.
Tips
- Self-drilling screws come in specific point sizes for metal thickness. If a screw spins forever without progressing, the point is too short for the steel and no amount of pressure will fix it.
- In stainless steel, a plain carbon self-drilling screw will fail. Use a screw specified for stainless, run slow, and use cutting fluid.
- Fine threads suit metal, coarse threads suit plastic and timber, regardless of which of the two types you use.
Common mistakes
- Piloting a self-tapping screw at its outer diameter — The thread then has nothing to bite. Pilot at the core diameter, which is the solid part between the threads.
- Using a high-speed drill setting on metal — The drill point overheats, blunts and starts rubbing instead of cutting, which hardens the steel under it and makes the hole unfinishable.
Questions people ask
Can I use a self-drilling screw in wood?
You can, and it will work, but the drill point removes material where a thread would otherwise grip, so it holds less than a plain wood screw of the same size.
What is a tek screw?
A common trade name for a self-drilling screw, usually with a hex washer head and often with a bonded sealing washer for roofing and cladding.