Flat roof joists in the way for ducting to pass through

[DerMitchie]

Our Victorian property has a flat roof side extension for the kitchen. The kitchen ceiling is in part under the original property and in part under the flat roof. A long RSJ props up the original wall of the house.

We are going to fit a ceiling extractor above a new kitchen island.

The challenge is to find the optimal exit point for the ducting. The fan housing is 300 x 300 x 200. so fits snugly with a gap of aprox. 60mm around two sides. The housing can be rotated to move the extraction output.

It's diameter is 150mm and I plan to reduce to 125mm.

Luckily there is a void above the ceiling that's under the original building and there is also a gap of approx. 40mm below the steel due to there being two layers of plasterboard, only one of which will be required.

My thinking is to reduce to 225*39 flat channel ducting and squeeze this underneath the RSJ into the void. Then go as high as I can with 125mm ducting and then connect back into a flat channel that ends up in an air brick.

My concern is the limited space around the fan housing.

My question therefore is whether I can move one of the joists? Here my concern is that the roof lantern adds weight to the flat roof and I am now hanging a fan off that.

Having said that, the joist is only approx. 400mm long.

The other question is whether i can/should remove one of the joists and connect like this

So - any advice here would be much appreciated! Fan dimensions below.

Read more: https://www.diynot.com/diy/threads/...-extractor-installation.562665/#ixzz6kefa8NGb

 

[IT Minion]

Why not go straight up?

 

[Lower]

The outlet of the fan at 150mm dia gives you a cross sectional duct area of 0.0177m2. If you transform that down to a rectangular section at 225mmx39mm (assuming those are the internal dimensions which they're not but would be even smaller) that gives you a cross sectional duct area of 0.0088m2. That's half the size of the fan outlet. You'll have significantly compromised the flow rate by doing that.

 

[sxturbo]

https://www.extractorfanworld.co.uk/through-roof-ducting-kit---flat-roofs-2575-p.asp

 

[DerMitchie]

As for Thanks everyone and sorry for not replying - I didn't get any alerts. As for ducting up and out, sadly there just isn't enough space above, so I'm running 204 x 60 flat ducting in the false ceiling and then out of the side of the garage.