I recently bought a two-story condo unit. I decided to replace the bathroom exhaust fans upstairs, but when I removed the old fans, I discovered the exhaust duct was not connected to the roof vent: both ducts traveled all the way to the vent and joined at a tee, but there was a six-inch gap between tee and roof vent. Like this:

exhaust duct meets concrete ceiling

As you can see, it's a flat concrete roof (I think it's hollow-core slab), and the duct is 4" galvanized. An opening was cut into the concrete for the duct and vent. The duct tape on the tee makes me think it was attached to something before, but when I get my phone in to look straight up, I see this:

vent seen from below

The roof vent is flush with the concrete, but off-center, so it is obstructed (only by about half an inch) on one side. It's unclear how the tee ever could have connected, and there's no sign of any missing pieces.

The question is: How do I attach a new exhaust duct to this vent?

I am accessing the space from a 7" by 7" hole in the bathroom wall, which is tiled:

shower and fan location

All I can see from the hole is what you see in the first picture; the vent can't be seen directly.

I could use tools to chip away the concrete, but I'd need a tool that can be used one-handed in a confined space without any visibility. (I tried a drill with a masonry bit in the hopes of knocking some chunks off, but it's simply too big and I can't see what I'm doing.) I've thought of using flexible duct taped from the inside, which could work around the obstruction, but would the tape eventually fail from the heat and humidity? I could make a better access hole, perhaps directly underneath, but then I'd have to retile my bathroom ceiling.

Ideally I'd like a solution that doesn't involve roofing work, since then the condo association has to get involved to approve roof work, making it harder to get contractors interested in the job.

  • I can not help you here, my bathroom vents go horizontally to the side walls, not to the roof. However, I would caution you chipping off the roof concrete
    – Traveler
    Jul 9, 2022 at 22:36
  • Is the existing tee opening the same size as the duct going up to the roof?
    – JACK
    Jul 9, 2022 at 23:11
  • The exterior vent is 4" and the tee is 4". There are reducers on the ducts, since the old exhaust fans had 3" exhausts. I plan to replace the ductwork with 4", since my new fans are 4". Jul 9, 2022 at 23:21
  • Would you be able to drop a 3.5" duct inside the 4" duct from the roof into the tee?
    – JACK
    Jul 9, 2022 at 23:33
  • Possibly. It would likely fit, but would it need to be sealed somehow? Jul 10, 2022 at 0:04

2 Answers 2


I'd get some duct that is 1/2" diameter smaller than the duct going through the concrete. Run the smaller duct inside the larger one up to the roof then connect the bottom of the new duct to the tee with some mastic and foil tape and squirt some expanding foam between the old and new ducts.


Only proper way now is to make an access above the shower, so you can get at the whole lot from inside the shower cubicle. Yes, it'll involve cutting some tiles, but for the future, it'll be easier to access for any problems. Then you'll be able to open up the concrete at its lower level, and be able, using both access shafts, to put it all together far better than the cobbled up arrangement we see. Something like a small loft hatch, tiled the same as the top of the cubicle would look good.

Or, hire a small camera on a flexi, and chip away like a surgeon doing a keyhole operation.

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