If you want to drill joists the holes should be centred and no more than 0.25 the depth of the joist in diameter. They should be positioned in the zone between 0.25 to 0.4 of the span from either end. (Regulation 522-12-03 of BS 7671)
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"Hello pushrod"
Without going to look at the British Standard or the Regulation that You have quoted I am fairly sure that what You quoted applies only to Floor Joists.
Even then Joists of less than 200mm [8"] should NOT be Drilled or Notched without additional reinforcement being applied to the Joists - for example 18 / 20 mm [3/4"] Ply being Laminated onto the sides of the Joists.
An example of NOT Drilling or Notching Joists of less than 200mm [8"] Deep is that Ground Floor Joists which are usually only about 100mm [4"] - as You know these should NEVER be Notched or Drilled - Even though these Joists will be supported on Wooden Plates and Brick Piers at regular intervals.
So although these 100mm / 4" Joists are well supported they should NOT be Drilled or Notched - It follows that a similar size Ceiling Joist which is only supported on the Room Walls [If Then] should NEVER be Drilled or Notched.
Ceiling `Joists` are usually part of the Roof Truss construction as was mentioned in previous Posts - So to ensure that the Roof Structure is NOT compromised under NO Circumstances should they be Drilled for Cables or Pipes.
Apart from weakening them in relation to the Roof Structure they would also be weakened with regard to walking across them when People have to access the Loft.
I have actually seen Ceiling Joists with Drill Holes in them for cables - the Homeowner had told the Electrician that they were going to Board Out the Loft Space - So the Electrician Drilled 25mm Holes right across the Loft - In the MIDDLE of the Span between the outside wall and the internal walls - to be close to the Room Lights in the Centre of the Rooms !
Imagine that on Ceiling Joists that were only 95mm X 47 mm [3 3/4" X 1 7/8"] !
Luckily I saw this as I am a fairly Big Man - I might well have been heavy enough to have these Drilled Ceiling Joists Break under My Weight !
Chris
P.S. - I am NOT `being funny` about noting the Metric and Imperial Measurements - the Imperial are what the Joists actually Measured - the Sizes of Wood that were used at the Time of Construction of the Houses.
