I need to purchase sheet metal screws for two residential HVAC DIY projects. I'm assembling basic rigid round peices, reducers, elbows, register boxes etc.

I'm wondering what the HVAC guys reach for when buying a box/bucket of screws.

Do you go TEK or ZIP, #8 or #10, and what length is a standard "go to" fastener?

I understand the TEK is self-tapping where as the ZIP is a piercing screw. Are they personal preference or is there one preferred in the HVAC trade over the other?

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  • 2
    Watching this question as I am curious as well. I would think that anything over 1/2" is just overkill: we're talking about joining 2 pieces of sheet metal and longer screws can only mean more screw to catch debris and cleaning brushes on inside the duct later, right?
    – spuck
    Jan 17, 2023 at 17:35
  • There are pros/cons to each. The piercing screws will drill into the duct faster with less effort, but will more likely snag your clothing or cut your fingers if you are reaching into the ducts. The self tapping take more effort to drill but will not hurt as much if you drill into your hand.
    – Glen Yates
    Jan 17, 2023 at 19:54

3 Answers 3


In my region the self-piercing (sharp tip) screw in #7x1/2 size is the standard. They pop through 28 or 30 gauge sheet almost instantly and they work decently well in heavier 24 or 26 gauge sheet too. They'll easily pierce through the first ply of metal and continue on into another ply (or several - I sometimes put them through 5 plies when securing an S cleat!). Sometimes the inner ply of sheet metal is a little too far away or deflects a little too easily. We keep #7x1 size on hand for those times because they'll grab the inner sheet from a greater distance.

The self-drilling type don't grab multiple plies of metal as well so they're less-commonly used in ducting. But equipment cabinets (furnace, AC coil case) are made of heavier metal than the ducts are; I use the self-drilling screw when attaching to heavier gauge metal where a self-piercing screw won't pierce. Again, usually 1/2 length is plenty. I'm not actually sure whether we typically get these in #7 or #8, but definitely not #10.

  • hmm… your self-tappers also cut their own hole? I've never seen those, ever. That probably negates my answer, which presumes self-tappers must be pre-drilled. [My entire 'career' of self drillers was always through 1 - 2mm thick metals, which may make the difference; self-tappers wouldn't touch it.]
    – Tetsujin
    Jan 17, 2023 at 18:38
  • 1
    @Tetsujin Perhaps I was too bold in my claims about self drilling vs tapping. On further review there's quite a lack of consistency of terms in the industry, and also lots of variety. A machine thread screw that taps its own threads might or might not self-drill for instance, and more people than what I'd realized do use the term tapping for forming either machine or sheet metal type thread.. I'll just delete my paragraph on that subject. Thanks!
    – Greg Hill
    Jan 17, 2023 at 20:51
  • This is very interesting! I've never seen self-drilling screws (doesn't mean they don't exist or that they were there, just that I didn't see them). I've always used the self-tapping screws for any light-duty sheet metal work including duct work. They're a bit of a pain waiting for them to drill their hole (@Tetsujin - they will make their own hole. Eventually.) but they do the job. Now I have a reason to go buy some more boxes of screws!
    – FreeMan
    Jan 18, 2023 at 12:23
  • @FreeMan - they seem to be a well-kept secret. Ours were supplied by 'work' so I never had to source them myself. I recall on one job we were running short so we dropped into a nearby very well-equipped tool shop, to discover the owner had never heard of them. We brought one in for him to look at & his eyes lit up with possibilities. One thing you do need for them is a nice tight-fitting bit, or they have a tendency to fall over rather than drill through. Annoying ;)
    – Tetsujin
    Jan 18, 2023 at 12:27

This is difficult to answer without just throwing an opinion out, but I go with the piercing screws for one reason and I think it's objective enough to count as a non-opinion. Self-drilling screws have the potential to throw shavings which is undesirable when working in an assembled duct.

  • Chris that's exactly the type of info I was hoping to get, I completely understand at it's core it's just an opinion since both work fine, however, guys in the trade have that little extra bit of info such as the shavings! I didn't think of that and that's a great comment/answer. Thanks!
    – Richard
    Jan 17, 2023 at 17:56
  • @Richard you've been around long enough to know the proper way of saying "thanks"...
    – FreeMan
    Jan 17, 2023 at 18:07
  • @FreeMan I guess I haven’t been around as long as you think, I’m not sure I understand the “context” of your comment? If you’re suggesting I mark Chris’s answer as correct, yes, I get it then! Chris answered quickly with an interesting tip, I will come back and mark an answer a bit later, I didn’t want to shut down more info from coming in. Such a simple “question” but some great basic info - granted, boring to the guys in the trade, but to a DIY that wants to leave a job like a pro did it, it’s really helpful. ;)
    – Richard
    Jan 17, 2023 at 20:41
  • Clicking the up vote arrow, @Richard, on every answer that's helpful, then clicking the check mark on the one that's most helpful (usually after 48 hours or so to allow time for other answers). There were no votes on this answer at all, so that's how I knew you hadn't voted on it when I typed that.
    – FreeMan
    Jan 18, 2023 at 12:18
  • @FreeMan ah yes! Okay thank you for the additional context. I agree and have up voted. I hesitate as the poster to make sure I don’t “wave off” additional input too quickly, I picked up that habit from the early days on Stack Overflow!
    – Richard
    Jan 18, 2023 at 14:55

Neither thread type is good at pulling faces together, unless your first face has a hole larger than the thread, so if neither face is pre-drilled/punched, then you [may see note below] have to DIY drill at least one of them..

I don't know your terminology [TEK vs ZIP]. To me they're self-drillers or self-tappers. Self-drillers need no pre-drilled hole, of course, they make their own. Self-tappers must always be pre-drilled.

If the elements you are fixing together have one hole already, then self-drillers every time… on a drill-driver. You cannot do them by hand.
If you have no drill-driver, your choice is down to self-tappers only.

For self-tappers, if neither face is pre-drilled, that means you also need two drill-bits to make two hole sizes - one that doesn't tap followed by one that does. This is just a time-waster.
This puts it firmly back in self-driller territory, where you only have to drill the first face.

The swarf argument is not in the game. Both methods leave swarf.

Note: There is a trick with self-drillers [& I suppose self-tappers, though I've never tried it with them], that if you drill through the first face without hitting the second, then let the bolt spin a few seconds after it's hit the end of the thread, that will strip the thread in the first face & allow you to pull the second face in tight, so long as you don't over-tension as the second face pulls in. This takes a bit of practise to get the drill tension just right, but is easy enough once you have a rhythm.
If you cannot leave sufficient gap between faces once assembled, you can do this pre-fit then back them out again. You've damaged the first face sufficiently that it will not impede the tightening action as you pull up the second face.

I used to get through self-drillers by the kilo. I love them ;)

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