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I'm interested in installing a bathroom extractor fan where there was a previously small regular fan, but there isn't an attic above the bathroom.

The switch is already wired from the previous fan which saves me from having to snake it, but I have questions about how to run the ducting from the fan, and about installing the fan itself.

The duct is 4", and I plan on running it through the roof to a proper exhaust vent right next to the bathroom. I'm not entirely sure how to know where a good position to make the 5" hole with the hole saw would be (since I can't see from the inside or put a marker like a nail for marking a place to drill), or how to get the ducting to a place where I could reach it and attach it to the vent.

What tactics should I use to locate a good place to drill using the hole saw?

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  • I assume you can't reuse the ducting from the previous fan (why not?) – dabi Dec 23 '18 at 0:35
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You say "no attic". That can mean one of two possibilities:

  • Bathroom ceiling is very close to the roof

In this case, you should cut the hole in the ceiling for mounting the new fan and simply uses a reasonably long drill bit like this 12" drill bit to drill a pilot hole, then go on the roof to drill a larger hole around it. In this case, there won't be much ductwork needed and the installation should be very easy.

  • Bathroom ceiling is far from the roof

You could easily have a few feet between ceiling and roof without having a functional (and accessible) attic. In which case you need a really long drill bit like this 72" (6 feet) one to drill a pilot hole, then go on the roof to drill a larger hole around it. The problem then will be the ductwork. With a little luck, and a bit of work, you may be able to get rigid ductwork lined up just right and drop in from the top and get it over the fan location. But in that situation it may be a lot easier to use flexible ductwork.

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    I had made similar observations after quantifying the problem exactly and taking approximate measurements of things. The attic is approximatle 1/3m (1 foot roughly) away from it. Wow! I didn't even know that crazy long drill bits like that were even made! – Sarah Szabo Dec 23 '18 at 4:08
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    @SarahSzabo I remember the first time my electrician brought in his really long drill bit and said "I'm here to make your house holey" – manassehkatz-Moving 2 Codidact Dec 23 '18 at 5:14

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