Moving or notching flat roof beam for extractor installation

22 Jan 2021
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United Kingdom
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.

Extraction 2.jpg

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.

Extraction 1.jpg

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.

Extraction 3.jpg

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.

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

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