how much can you notch into a timber joist from below?

Question

joist is 145x45mm. is there a specific guideline/rule of thumb - perhaps from the timber code?

Answer 1

Any holes or notches reduce the cross-sectional area. The cross-sectional area is what engineers use to size the beam to determine it's strength based on the required loading.
In your case the x-area is 145 x 45 = 6525 sq mm.
Typically, in all structural design, there is a built in margin of safety. This is to account for damages, minor rot or decay over the years, additional unforeseen loading, etc.
In my opinion you should be able to lose 10 to 15% cross section and still be quite safe. That would be about 975 sq mm. That would give you a hole size of about 18mm.
Therefore, I recommend not bigger than 18mm.
If you need to go bigger, consider reinforcing the beam.

Answer 2

Drill a hole, centre of joist, up to 25% of depth of joist (ie 35mm maximum diameter), near end of joist (for support) is best. Alternatively, you can notch out up to 25% of depth either at top or beneath. Guidelines provided by Building Regulations which set minimum standards for all aspects of building work. (I am a secretary within local government)

Answer 3

Do not notch the underside at all please.

Drill a hole through the centre or Notch the top.

Please try to keep the notches near to the supports.

Be very careful as these are load bearing members.

Answer 4

I don't think you should notch at all on the underside, unless its a very large joist and a very small notch, better to drill a hole, or notch the topside.

http://www.luton.gov.uk/Media%20Library/Pdf/Environment%20&%20regeneration/Planning/Building%20Control/%201%20Limits%20of%20drilling.pdf#search='notching%20of%20joists'

Answer 5

30% rings a bell, but I don't know where from.

Answer 6

one and a half to two inches.