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I'm installing a ceiling-mounted extractor fan above a kitchen island. The problem is with the placement of ventilation ducting in the ceiling: the joists are running in the opposite direction to the only viable wall where the ducting needs to travel towards.

However; in the ceiling there is about a 120mm (4.5 inch) void above the joists before the the floor above. This gap is easily big enough to fit flat channel ducting so that I can rest the ducting on the joists in a perpendicular fashion. (see diagram below)

My question is: Is this OK to do? I haven't seen this done elsewhere.

diagram of ceiling and proposed extractor fan / ducting Thank you!

Addition: Heres a quick photo:

photo

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I'm confused/surprised by the existence of the void, but if you've got the space, then go for it.

The only bit of mathematics to be concerned with is matching the cross section of the various sections of pipe.

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    My thoughts exactly. I'd love to know what's holding the floor up. – isherwood Feb 3 '17 at 18:41
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    Thanks Aloysius. Ah, I've added a photo - there are additional planks not shown on the diagram going in the the same direction as the proposed duct where the 120mm void is - i.e. they intersect the joists shown at 90 degrees, connecting them to the ceiling almost like a grid. It's part of a block of flats built around 2004. – jlo Feb 3 '17 at 19:33
  • Aaah, the additional information makes it all make sense. Thanks for clarifying. – Aloysius Defenestrate Feb 3 '17 at 22:48

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