Kitchen Extractor Ducting and Rafters

[nemesis3]

We are currently in the process of installing our kitchen and have bought a chimney style extractor, so the ducting (ideally 150mm) would normally go up the chimney into the ceiling\floor void and run to the external wall.

The problem that we are faced with is that the void between the two rafters that the chimney goes into has a soil pipe coming down through it just by the wall. Since taking the ceiling down we can now also see that the rafters from the adjacent room run perpendicular to the kitchen rafters and overhang into this same void.

Here are some pictures:

In this one you can see the soil stack at the end of the rafters and also the overhanging rafters from the adjacent room:

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• nemesis3
• 18 Jan 2011
• 1

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• nemesis3
• 18 Jan 2011
• 1

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• nemesis3
• 18 Jan 2011
• 1

Normally I would never consider cutting a rafter however I was wondering if we could pack out the gap between the rafter and overhanging rafters, and then screw\bolt them together. Something like this:

Before:

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• nemesis3
• 18 Jan 2011
• 1

After:

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• nemesis3
• 18 Jan 2011
• 1

In the picture I have shown the section of joist taken complete out this would not be completely necessary, but a significant hole would need to be made into it.

Does anyone have any thoughts? My alternative plan is to have a fake ceiling that is 100mm lower and extends about 450mm away from the wall so that the ducting can run under the joist and into the adjacent void, however this won't look as neat.

 

[Deluks]

So this extractor is above an island unit? Otherwise you can just take it through the wall.
Otherwise, taking large chunks out of joists isn't wise, you can get flat channel ducting which is about 55mm deep if you want to batten out the ceiling with 2x2, then you can take a few mm off the bottom of the joist.

Alternatively you can remove part of the joist but it will need reinforcing with a metal plate either side.

You could also run the ducting between existing joists to the side wall, then drop it down and run along the wall, either boxed in, or hidden by wall cabinets.

 

[nemesis3]

The extractor is against the wall, however our lounge is the other side so going through it is not really an option.

The equivalent flat channel ducting for 150mm ducting is 90mm deep, so would need a bigger drop in the ceiling. Is it acceptable to put notches in the bottom of a joist? I thought you could only do that in the top?

Are there metal plates designed to do this with?

 

[Deluks]

You can get adaptors to convert down to a smaller size, I can't see why you can't fit some flexi ducting through the gap between the joist and the wall. The soil pipe can fit in there and that's gotta be 110mm?

If not, then looking at your 'after' picture, you could continue the joists from the other side (the ones that sit on the wall) to fix into the full length uncut joist, and run your ends of the cut joist into these.

These new trimmers should be the same size as existing, and be at least 500mm alongside the existing joist. Each one fixed with about 6 chunky nails or screws, or a 10mm bolt at each end of the overlapping section.

I've gotta go out now, so if any other questions, someone else will likely chip in soon.

 

[nemesis3]

I could drop to a smaller size however I've read that noise can be a problem with the more powerful extractors.

The overhanging joists leave a gap of approx 1inch between the end of them and the joist; so I would have to lop off the end of each of those overhangs. If I did this I would still have the problem of the soil pipe at then end of this channel.

If I understand correctly what you are saying with regard to continuing the joists to the uncut joist, then this will new piece will block the channel to the external wall, so i have the same problem just in a different location don't I?

Is there a particular issue screwing\bolting the rafter to the ends of the overhanging joists?

 

[ivixor]

If wall units are going back where the old ones were, you can go up from the chimney hood, through 1 or two of those overhanging rafters (cut them back) and then back down on top of the units, and out that wall duct. Cover the ducting with some plasterboard fitted on top of the units, flush to the ceiling, and get it all plastered when the ceiling is put back.