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I need to route lighting and socket cables in a small single story extension with a pitched ceiling. The rafters are 150mm and we'll be putting 100mm of insulation between the rafters. The cables need to run through the rafters. I believe that the holes for the cable are supposed to be drilled centrally in the rafters to avoid weakening the structure, but this would mean that the cables would be surrounded by insulation which apparently is a bad idea.
I can't go through the floor as it's a concrete slab, so the rafters are the only option. Any advice on how to compromise between weakening the rafters or having overheating cables? There will be a 50mm cavity between the insulation and the roof membrane. Can I shift my rafter holes up a bit to allow the cable to sit on top of the insulation? Or is that a bad idea?

Do you have a 50mm void above the rafters, if so you can avoid holing them for cable routes.

If routing cable through thermal insulation, then the cable must have the correct derating applied for reference method used of installing it.
For most lighting circuits that are protected by a 6A device using 1.00mm2 T&E you should be fine. But any power circuits, it will mean the cable size of the circuit must be increased to safely carry the current demand of the circuit.

http://www.nhbc.co.uk/NHBCPublications/ ... 553,en.pdf