I've installed some new ceiling register boots to use with flexible ducting and realized after install that they have a smooth connection with no lip. I'm assuming they are made for metal ducting, as I think flexible requires a lip for a tight connection. How can I securely attach flexible ducting?

One idea I had was putting five or six screws around the connection to act as a lip. I also thought maybe there is a tool that I could make some dimples that would do the same thing. Is a lip even necessary for a solid connection?


UPDATE: Since I don't really know the proper names for things, I think pictures might help.

Here is pretty close to what I have:

enter image description here

Here is the lip I'm talking about that is on the current register boxes:

enter image description here

Here is one with no lip:

enter image description here

On my current ducts, there are two zip-ties, one on the inner core and one on the outer insulation. The inner core zip tie is behind the "lip". I assumed this was necessary for a solid connection (Note: I'm using mastic, not tape. Whether one is better than the other is another topic). Here's a picture of the inner core I was referring to that I want to put the zip-tie on.

So I've now changed my question to: Is that lip needed and if so, any way to fake it if not there?

enter image description here


4 Answers 4


New answer thanks to updated question.

Yes, the lip around the rim of the opening I'd call it a "ridge" but I dunno if that's right) on the boot is there to help ensure a firm connection. It's to keep the zip tie on the inner core from slipping free. This is necessary (the keeping the duct snug, not nessarily the tie) if your ducting is going to be pulling away from the boot due to gravitational stresses - such as when the boot is above the duct and the duct can be pulled down, such as along a wall or below a ceiling or floor. If your boot points down and the duct sits on a horizontal surface, you're good - no extra tension should happen.

Important note - neither mastic nor tape are intended to be structural - they don't serve to hold the ductwork in place, they serve to seal the ductwork so that there are no air leaks.

So the question really is - how do I properly secure my flexible ducting to the boot when there's no lip on the collar? Answer: Self-tapping screws. enter image description here

Get yourself a box of those (the hardware aisle or the ductwork aisle). When you go to mount your ducting, pull the outer insulation back up away from the core, pull the core over the collar, and use TWO screws on opposite sides of the duct (180 degrees from each other - noon and 6'o clock (or 1 and 7, 3 and 9, you get the idea) to ensure that neither side of the duct can pull free.

Use the inner to snug up the connection (don't over tighten!), seal with mastic (I'd recommend tape though cause of the insulation layer...), slide the insulation up as far as it will go and snug it up with the outer zip.

  • Perfect! That's exactly what I was looking for. Thanks. Nov 12, 2011 at 17:09

If I understand your question, you have a boot like this:

enter image description here

And you want to put a crimp like this on it:

enter image description here

So that you can slide some flex duct to it.

If that's the case, then there is a crimp tool to do that:

enter image description here

It's also possible to do using a pair of long needle nose pliers (which saves you the expensive of buying/renting a speciality tool for a one-off job). Just grab on, twist 30 degrees or so, move the plies over a quarter inch, and repeat. It takes a minute or so, and probably doesn't produce as nice results as a professional crimp, but when you put the duct on and seal it with foil tape (as you should be doing anyway), it works perfectly fine.

enter image description here

  • Wow I totally didn't get crimped vs. non crimped from that but its a great thought! Nov 11, 2011 at 17:27
  • My fault. I don't know the proper terms for things. I've updated the question to hopefully make more sense. Nov 12, 2011 at 6:06

I believe that the boot you installed is designed for insertion of the crimped end of a metal duct pipe into it, not the flexible ducting you are using. If your space allows and you wish to do things to code, use a short coupling pipe with crimping on one end and a ridge on the other. Now you can connect the flexible pipe correctly. Use three sheet metal screws to attach the coupling to boot. By the way, never puncture the inner lining of flexible ducting with sheet metal screws or anything else. It is designed to be clamped on with drawbands or large hose clamps. If the coupling pipe solution is not feasible, then attach your flexible ducting directly to the boot with a large hose clamp which can be tightened sufficiently so that these parts will never come apart. In all cases, seal all the joints with mastic.


They make a specific collar made for just this called a splice collar. Used for linking two lengsth of flex duct at the middle. I used them as the link between boots and flex duct all the timeenter image description here

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