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After having our roof re-configured and larger overhangs added, I noticed that the contractor placed the bathroom exhaust vent that used to go out the side wall just under the eaves just inside the overhang of the roof. I think they meant for it to vent out through the vent holes in the aluminum soffit material, but I am concerned that instead, moisture will accumulate on the underside of the roof sheathing.

I know I should have the contractor come and fix it, but they have been slow to respond and I would just like to take care of this.

I would like to address this, and I am aware the most proper way would be to go out the roof. But I am not comfortable cutting the hole there and trying to manipulate the shingles to fit a new vent pass-through piece, so instead I would like to set it up like shown in the related question. The vent pipe already terminates underneath the soffit I believe, so I would just need to make a round hole in the soffit material, pull the flexible ducting through, attach the vent piece, and mount it with screws.

I am just not sure what the best way to cut a round hole into the soffit material that is already installed, without taking it down. I am thinking I would mark a template, then drill a small-ish hole in the middle and use tin snips to try to make a round cutout. Alternatively, maybe I could use a Dremel with a cutting wheel. Is there a better way? I am not opposed to buying tools if that's necessary.

  • 1
    What diameter? A hole cutter might be best...
    – Solar Mike
    Commented Oct 25, 2019 at 16:13
  • I'll have to check to be sure, but I assume it would be either 3 or 4 inch ducting. The hole would need to be slightly larger so the clamp that holds the duct fits through. Would a regular wood hole saw work OK for aluminum? My concern is that those tend to get "hung up" sometimes, and I could see that kinking the material pretty easily. Commented Oct 25, 2019 at 16:35
  • How firm is the material if you push on it? I ask because they make adjustable circle-cutters that chuck into a drill, but if the material flexes I don't think it would work well. Commented Oct 25, 2019 at 18:07
  • If I remember right, it's quite flimsy. Commented Oct 25, 2019 at 18:46

3 Answers 3


You would want to use a bi metal hole saw. You are right to not want to use a wood only hole saw, the teeth tend to grab, not cut.

Try something like this:

enter image description here

Good luck.


This old house has an excellent video on why the soffit location is bad, and how to vent through a common shingle roof.


How to Vent a Bath Fan Through the Roof, August 2014. The homeowner installed the same way your contractor did, and in just one season there was mold under the roof.

Your contractor should pay for this: the soffit installation was incorrect and hazardous to your health. You certainly don't want the vent inside the soffit, nor do you want it under the soffit.

  • Thanks for the video. In my case, I do not have access from above, the fan is located in a suspended ceiling in the bathroom, and the ductwork currently is routed through the suspended ceiling and goes out the wall of the house (formerly exited below the soffit, now inside the soffit). Commented Oct 25, 2019 at 18:08
  • My plan was to install the new vent in between the vented parts of the soffit, and use the exhaust shown in the linked question, which would direct most of the air away from the underside of the soffit (hopefully).Before the roof re-do, the soffits weren't vented at all, so this was not an issue then. Commented Oct 25, 2019 at 18:47
  • Your question says "I am not comfortable cutting the hole [in the roof] and trying to manipulate the shingles to fit a new vent pass-through piece". If you're not comfortable doing it right, then you should really get your roofer back out to do it for you. Depending on wind and weather, your soffit solution will have the exact problems you said you were worried about.
    – Bryce
    Commented Oct 26, 2019 at 6:07

I would try one of two ways: a) use a small drill bit 1/4" perhaps, and drill holes just inside the edge of the desired circle. The use tin snips or a dremel tool. Cutting the entire thing with Dremel isn't going to work unless you have a LOT of cutting wheels at your disposal.


b) just drill one 3/8-1/2" bit at the edge. The use a jigsaw with metal cutting blade to make the cut out. a) might be more time consuming, but also may be easier than trying to hold a jigsaw upside-down to make the cut- depending on the diameter, the height of the soffit, etc.

Of course Jack's suggestion is good too, but may be too costly to justify for a one-time use. (assuming, of course you already have a jigsaw).

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