Well here’s the official heads-up, another long and boring post (but then I’m gonna tell you not to trust the rules ...)
Da rules man:
Electrical cables should come not less than 50mm closer to the top of the joist.
Holes and slots (for water and electric cables) should only go through the joists between 0.25 ~ 0.4 of the span length from load bearing point.
Notches are allowed on top of joist between 0.1 ~ 0.2 of the span length from load bearing point.
75mm from edge of joist to walls are reserved for services.
Real Life:
Now I’m going to tell you to ignore the above, because 1. You cant guarantee the chippies, sparkies and plumbers don’t disobey the above (I’ve had to personally mark up keep-out areas to stop them), 2. Sparkies don’t P clip the cables down properly at the base of the notches, or the P clips ping out when they are stretched, 3. Plastic plumbing bows up when pressurised and they are laid in the holes and over notches unpressurised!
So … just believe that immediately under your floor there are electrical cables and plastic plumbing.
Now What???:
So what can you do? … you have to select a screw that is exactly the length of the thickness of the material you are about to screw down plus the thickness of the floorboards already in place! (then you know you wont pierce any pipes or cables)
So how do you know what the thickness of your floor is? Well you can always drill a pilot hole and measure it (hole thro floor – small wire with L shape hook, poke through hole and measure the thickness), or if you aren’t allowed to do that, then look at the following for guidance...
Help mum – I’m still scared of drilling a test hole:
If you really really really don’t want to even drill a pilot hole and measure via that (and I think you should) then measure your joist pitch (do this by looking at the rows of joist screws from above) , then identify your floor material, then check out the boring chart below (and assume this is the screw length you need to add to the thickness you are adding) ….
Joist Centres … 400mm / 450mm / 600mm
Material of floor and the thickness it should be : -
T & G softwood boarding … 16 / 16 / 19mm
Chipboard … 18 / 18 / 22mm
Plywood … 12 / 12 / 16mm
Da bottom line:
… so in summary, your screw length should be … the total thickness of the materials you are adding + the thickness of the floor you have now (and I would pilot hole, make a mickey-mouse depth gauge as described to find that out, don’t trust the trades have kept to the rules, and put a pilot hole through the stuff you are adding so the torque is taken from the floor and jacks the new covering down tight – get advice from supplier ref your floor material ref screw type – I would simply go for Spax screw, cos they’re the mutt-nuts. As can be seen elsewhere on the thread, safe screws can be quite short – so buy really nice ones!)
As Alan sez, if you do ID a good joist with no cable or pipe then whack a longer screw through. (again look at the current joist screws) … pitch for screws defer to expert tilers here, I think you are looking at 150mm pitch! … big box of screws and fully charged driver so you don’t get lazy and skimp, cos its not worth it if you want to kill bounce.
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…. And your freebee part B which you didn’t ask but someone will ….
What about walls?
… I’d check out all of the relevant parts above, but also consider the following:-
Electrical cables without special protection (and even if they had steel conduit I can whack through that with an SDS hammer or long screw no probs!) are allowed vertical and horizontal of any switch or socket position, plus 150mm from the corner of the rooms – so they are your keep-out zones ref screws even though the cables should again be 50mm below the surface.
You say chipboard though :yikes: are you overboarding it ?
