in this Q and A video we're going to be answering the question where in a joist can I drill a hole
it is important to note here that our focus is specifically on the structural integrity of solid
timber joists that are often encountered in older installations rather than the engineering joists
found in 70% of new builds these days these will be addressed in another video well what do the
regs say about drilling through joists part A the building regulations specify that buildings
shall be constructed so that loads are sustained and transmitted by to the ground A safely and B
without causing such deflection or defamation of any part of the building or disproportionate
collapse furthermore regulation 522.8.14 BS7671 states that no wiring system should penetrate
an element of construction that is intended to be load bearing unless the load-bearing ability
of the element remains satisfactory after such penetration now these kinds of statements could
lead us to a potential debate between structural engineers watching this video as to what kind of
penetration would be allowable before load-bearing ability is affected words like neutral axis
compressive force tensile load geometric centers deflection flanges shear force and bending moments
may begin to fly around in the comments section however remember that most Sparks aren't trained
Structural Engineers and therefore must rely on guidelines such as those provided by the IET the
word in satisfactory is also somewhat ambiguous so let us seek further clarification from IET
sources such as the electrician's Guide to the building regulations and On-Site Guide as before
us explains exactly what the guidance is or what we can or can't do to the joist I've popped into
the workshop to point out that all the drill bits you see in this video are made by Mandrex to drill
those holes we use the flat speed bit with the spiral tip perfect for making quick individual
halls in joists and available singular or in sets where you've got a whole house of joists
to drill you might prefer the power Xbit for flow auger bits instead perfect for an effective waste
removal making drilling super quick anyways just before I get back to playing with these awesome
bits of kit I'll finish up watching what Ross has to say about making sure that I don't drill holes
in the wrong place so what are the guidelines well the IET offers some very specific guidance on wear
holes and notches can be made in joists notches for example are only allowed between 0.07 to 0.25
with a span from either support and no deeper than 0.125 times the depth of the joist however we will
investigate running cables in notch joists in a little more detail in another video according to
paragraph 10.1.1 of the Electrician's Guide to the Building Regulations holes however are permitted
to be drilled in solid joists providing that they are A NO greater diameter than 0.25 times the
depth of the joist B drilled at the neutral axis C not less than three diameters center to center
apart and finally D located between 0.25 and 0.4 times the span from the support so let's break
these down a little further let's take standard solid joist with a depth of 150 mil for example
if we do a quick calculation of 0.25 times 150 millimeters we will deduce that the largest
diameter hole we are permitted to drill is 37.5 millimeters do we go with that 38 millimeter
Mandrex ultimate speed Xbit flatwood bit here then or perhaps we stick with the 32 millimeters also
on offer to be on the safe side B drilled at the neutral axis as per figure 10.1.1 this tells us
that we drill our holes along the center line of the joist between the span allowed if we do
some more quick maths here by dividing our 150 millimeter depth by two we have 75 millimeters
clearance either side of our center line a 32 millimeter spade bit will give us a radius of
16 millimeters either side of that center line therefore 75 millimeters minus 16 millimeters
will leave us 59 millimeter clearance from the top and bottom of the joist this strategically avoids
activating BS7671 regulation 522.6.204 where we then have to provide some sort of mechanical
protection for our cables if we could not allow that 50 millimeters either side as stipulated by
regulation 5 to 2.6 0.201 C not less than three diameters center to center apart and not from
the edge or circumference of the first hole if we stick with the example of the 32 millimeter
spade bit and some more quick maths three times 32 millimeters and we would have to measure 96
millimeters from the center of our first hole to give us a center for our next permitted hole and
finally D located between 0.25 and 0.4 times the span from the support firstly it is important to
note that this measurement applies to either side of the joist and secondly as an electrician this
could mean that before we lift that board which has obviously been lifted in the past we might
want to get our tape measure out and do some quick measurements if we assume a span of four thousand
millimeters for example then 0.25 times 4 000 and 0.4 times 4000 equals one thousand and sixteen
hundred millimeters respectively effectively that leaves us with a zone of 600 millimeters from
exactly one meter on the left or the right to the support where we are allowed to drill our holes
you might want to think that the repercussions of not doing these quick measurements
the most drastic consequences is the failure collapse deflection or deformation of any part
of the building the more likely of consequences however is that building control officer who
pops in unannounced to inspect your notifiable works resulting in you having to pull out all
your hard work and then take any penalties on the chin for the repairs that they're deemed required
not to mention the loss in your reputation due to the delay and distrust no one handed the
clients if you don't want to see bright copper staring back at you after lifting that pre-cut
tongue and groove board perhaps snip down to the van grab your take measure and circular
saw and lift the board on the other side of the house for more information on the Building Regs
and how they impact on your electrical work please check out our free training package to
help you with your CPD by clicking the link in the description in the meantime happy fishing