Cutting a stainless steel cooker hood for sloped roof & installation

Here is a much better option. If you have a wall unit either side of the extractor then you do a flyover between the two, fixed (but not quite yet) at the top with 4 furniture blocks, 2 either side. This is just a matching panel that will be the same depth off the wall as the wall units and its length will be the distance between the wall units. Next you accurately measure the distance between the top of the extractor (where the chimney sits on the extractor) to the top of your flyover and add 5mm let's say. Transfer that onto your chimney, use a black marker pen to draw a line all round, and with a steady hand and a pair of safety gloves cut along the line with a grinder cutting towards you all the time. Tip--- to start let the blade sink in 15mm. Clean off sharp edges with a file. Next cut the flyover very accurately to fit around it making sure that the rough cut is on top. I have done this procedure several times with a 100% success rate. Tip-- use a low wattage grinder. Good luck. Kevin.