The span rules we use are just a quick ready reckoner sort of thing really, if you follow them then you should, in practice, avoid all the pitfalls.
You can, if you wish and have sufficient knowledge, calculate sheer force and bending moments to prove acceptable but if building control has any oversight on the installation then they will want evidence of your calculations in order to check their validity.
I once got involved in a situation whereas I first fixed and took particular care to put the holes and cables exactly within our span limits then had to draw off for a few days whilst a Gove Grant Central Heating System was fitted - They hacked off the joists so close to my holes that , well as a builder put it, "They have butchered the joists", I was doing the rewire via Care and Repair UK and they involved the Local Authority Building Inspector, it was a right hooh har. The heating team`s supervisor came in, examined them and declared them as "perfectly safe", the Building Inspector told the firm he wanted to see their Sheer Force and Bending Moments calculations.
The case dragged on and on and I think the owner got fed up with the hassle of becoming the piggy in the middle and had the flooring relayed.
The saving grace was, as I stated, that the joists were short spans even though this butcher had occurred throughout and I felt there that this would not be a problem, this week, this month or this year but eventually could become a problem a few years down the line.
So, how long are these joists, how thick are they and have the relevant calculations been done to prove/disprove suitability?
