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If there's no way for air in the void to get out, you'd need a pretty strong pump in the hood to push the air in to it. So yeah, it's just not going to work, unless you vent the roof void externally somehow.
Flat roofs are bastards to keep working so I'd avoid putting a hole in that.
Best bet might be just to replace the whole lot with a recirculating hood type.
Howard
It would appear not! In fact, now it's all connected it really doesn't extract anything at all! Sitting here in smoke after frying a breakfast pork patty!
The hob instructions say a 150mm exhaust is required, but you can go down to 120mm. I didn't measure at the time, but I can't imagine the pvc duct hose is more than 100mm. The ceiling also has a glass lantern in it and I imagine the construction of the flat roof means it's only venting into a very small void - the flow must be awful now!
I think my immediate plan has to be to get a 150mm aluminium flexible duct and route it to a small side window when in use.
Longer term I need to decide how best to fit a proper 150mm duct and whether I try to fit it myself or get someone in. Looks like I have three options as it's a flat roof - go straight up and out through the top, go through the roof void to the back of the house, or install an exposed duct inside the kitchen (recently saw something like that in the kitchen thread).
Going through the roof void looks more complicated as there will be two or three joists to go through and the external pvc fascia. Going through the roof seems more diy friendly, but I don't want to create any leaks in the roof. Internal routing seems doable, but I'd need a core drill for the wall and to cut a 150mm hole in the side of the stainless shroud - also not sure if the wife would like the look!