If you are doing the job properly you would pay the timber merchant to "regularise" the joists, passing them all across a circular saw to skim them off to ensure they are all exactly the same height. But not many merchants have a saw or machinist these days.


Andy Betteridge

An electrician or a plumber who smashes the bridging or strutting out from between the joists to install fan ducting and waste pipes will potentially damage the floor structure far more than the guy who drilled the 16 mm hole in what is apparently the wrong place.


Andy B

Under load, the top of a joist will be in compression and the bottom will be in tension with a gradual transition of these forces vertically from top to bottom.  The middle of the joist (vertically) only experiences the relatively small vertical force and this would be the best place for a hole, anywhere along its length.    The worst place for material removal is along the top or bottom of the joist where the forces are highest.

Interesting comments above, none of which I would disagree with. I have not read the requirements but from my college days training as a mechanical engineer I would say that to minimise loss of strength in the joist any holes should be on the centreline (that is mid-way between the top and the bottom as opposed to half way between the ends) as the top and the bottom have all the tensile/compressive stresses. Similarly if there are any notches made  they should be at the ends of the joist rather than the middle as the greatest compressive stresses will be experienced at the mid-point, diminishing to zero at the ends. Also in order to minimise any loss of strength there must not be notches and holes at the same point along the joist.

In order to regularise this it is easy to put rules in place to say "notches are allowed along this section of the joist" and "holes are allowed along this section of the joist" and to make sure nobody puts a hole and a notch at the same point make sure they don't overlap. Regardless of the rules, holes are better than notches for minimising loss of strength, but are not always the practical solution so it has to be accepted that some notches will be cut. If you make a hole in the section where they are supposedly not allowed, the joist will retain more strength than if you put a notch, but can you be sure that a plumber will not come along at a later date and put notches in the same place?

Alasdair

You could calculate both sheer stress and bending moments about joists etc, I last did this at college and never used the maths in real life though. The regs give us the rules about holes and notches as an easy fix to stay well within the need for the calcs. So for a simple life goodho says I.


It`s not too difficult to lift T & G floorboards, easipeasi methinks chipboard, especially if nailed not screwd down is another matter though.Moving furniture is all part of the job but if you can avoid it then good.


You do see places where somebody has put a hole near a notch or vice versa though. Case in point my avatar is just such a thing. I carefully cut and removed said chipboard and drilled holes in correct places then threaded cables. I had to break off to allow a "Warm Front" team to do their bit. Guess what? the used the same parts of the span and put joists over/near the holes (they also soldered their pipework without using a solder mat and splattered solder all ove the cables too. Ruffnecks.


PS those notches they butchered in were massive compared to what was needed. They just did not give a 4X about it!

Cut brads are not that bad (usually) . 2 1/2" lost heads or ovals are better though ?


Don`t talk to me about layers of hardboard or ply though! I had gaspipe pierced and water pipe too both in more than one place and both under T & G boards I had not removed and the water was escaping thru the ceiling right under the board I had lifted. I would have been blammed for it it if it were not the fact the council electrician turned up as I was about to relift the board I had put back down.

He did confide in me it had happend before due to the carpet fitters tacking the plyboards down with slightly too long nails and the points resting on pipework. Future vibrations such as nailing adjacent or nearby  boards down sent them that extra push to pierce the pipes.

Most of the problems I see with joists have been caused by idiots installing down lights.


I have been into the loft of a recently built house and seen a section of the bottom chord of a roof truss that acts as the ceiling joists completely removed above the bathroom ceiling to allow a down light to be installed.


I was actually in another house undertaking electrical work in the kitchen for a kitchen company and an “electrician” the customer knew from down the pub was installing down lights, to get one in he chewed away five inches of a seven inch floor joist with an auger in an electric drill, as he started I said something about altering the layout, but he assured me that it would be okay as it was.


Both of those are actually structural damage to the customers homes caused by idiots, I have seen numerous other similar examples over the years. But, although I have seen some very badly placed holes and notches in joists that leave the cables at risk of being damaged I haven’t actually seen any that could really be described as structural damage.


Andy Betteridge

What did wind up the NHBC inspectors was electricians using big fat metre long wood augers so that they could drill a big angled holes through the joists with both feet on the floor to avoid having to use a hop up, rather than getting up between the joists to drill the small holes they actually needed horizontally through the joists.


Andy B.