100mm Ceiling Joists

[Geko]

Running a the lighting circuit in my place. The joists are only 100mm deep. At first I was going to put a 15mm hole through the centre to enable me to run the cabling for the ceiling light. But... Now I am worried about structural strength and I'm probably going to just pass it over the top of the joist at a suitable location, such as close to the roof edge... I want to board it afterwards, which is the reason I intended to pass it through the joist originally.

[robbo0070]

my ceiling joist are about the same size as yours i boarded mine but i put 3x2 running the opposite way for extra strength then the electrician run the cable over the top of the old joist.

[DavidF]

If you must drill a hole then 1. Make it as small as poss - 15 sounds fine for 1.5mm T+E which is what you will use. Secondly Position the hole in center of the joist but about 400mm+ out from the edge of the joists.....but as a spark this was/is a rule of thumb. Im not a structural engineer. There is also technical guidance if you want to be exact - here

[wadgey]

Ceiling joists in the loft can be small because they are not designed to carry heavy loads, just the weight of the ceiling. You'll be fine drilling through.