Replacing an ancient radiator with a newer onem in my bedroom that is slightly narrower means that I have to run the 8mm microbore pipe (yes, I know) through a single joist to get to the radiator.
Solid joist 175mm x 50mm and the hole would probably be 12 mm diameter. Problem is that for easy routing I would like to drill the hole 25mm from the edge of the joist instead of the 80 mm calculated by the regulations.
so:
1. drill the hole and not worry about it as it is small diameter and it is only one joist.
2. Do number 1 and glue & screw thick plywood around the hole to increase the strength
3. Cut a notch as this does meet the regs. Problem is I don't like notches and a hole seems "better". Don't really understand why a notch is acceptable close to the end whereas a hole isn't.
If it’s microbore, it’s such an insignificant size compared to the overall size of the joist, I’d not worry. If you’re worried about splitting the joist, then start with a pilot hole and work your way up to the diameter required.
I come across joisted sub-floors all the time - you wouldn’t want to know what kind of butchery gets done with joists, especially very old ones, over the years.
Don’t really understand why a notch is acceptable close to the end whereas a hole isn’t
There are two main stresses at work in a joist.
Bending generally in the middle and sheer at the ends.
Bending ina normal span puts tension along the bottom and compression along the top the tension. The bottom of the joist does the most (by alot) work at resisting bending so you don't touch the bottom of the beam. However a notch in the top which does pretty much nothing against bending is okay.
Sheer is most concentrated at the very ends of the beam and tapers down as you move away from the wall. Its well resisted by the rectangle of the beam of sufficient depth to resist the bending. However if you drill a hole in the zone of death you reduce the strength of the beam and run the beam. Additionaly run the risk of splitting the beam you chnage how it might behave.
If you draw where these <i>zones are most significant and add a buffer you realise along the top of the beam is pretty much all fair game.</i>
Ie. Notch isnt better it just has more available safe zone.
you wouldn’t want to know what kind of butchery gets done with joists, especially very old ones, over the years.
Oh I do know… when we had the house re plumbed in 2018 the plumber and builder were meticulously keeping to the regs with notches and holes in ‘safe’ zones in the joists. Then they lifted one section of floorboards and found what a cowboy had done many years ago ago before we bought the house. Let’s just say quite a lot of reinforcement was required
I have tried to dit that like three times and am failing hard.
Tldr.
Notches have more available safe zone because they are largely along the top of the beam except at the extreme ends.
So they aren't better you can just fit them in more locations.
Also sheer failure is the scary one because its hard the predict. Bending failure is pretty clearbecause it has to bend quite alot first.
Also before you go wild. Can you not jut fit longer tails?
Get some galvanised flat brackets (Ebay, screw fix) and brace above and below each side. It doesn't really need it but piece of mind and all that. Suggest 50mm x 200mm flat brackets would do it.
Thanks for the responses. In the end I followed the regs. Cut a small notch in the top in the allowed area. Used a drill to cut the notch by drilling through at the top of the joist (horizontally) so that everything is nice a smooth and round, no sharp corners for stress concentration. The notch was only about 14mm x 14mm.