Bad pratice? grooving a joist to pass a cable through.

[Ltnman]

I’m thinking of getting an electrician in to fit some down lighting in the kitchen. The problem is crossing the joists with cable without wrecking the ceiling as there is no access from the floor above.

If I took out small sections of plasterboard using an 80mm hole cutter from underneath the joists and then grooved the joists to a depth of half an inch the electrician could pass a cable through the groove then I could carefully nail or screw back the plasterboard avioding the cable from the plastboard in the cutter and then easily smooth over the cut.

Would this method be considered to be bad practice and would you carry out this job if you were given this job by a householder with the grooves already in place? Is there a better way to do it?

There will be a total of 9 x 10w downlights with firehoods crossing maybe 3 or 4 joists in 3 rows in a 10ft x 10ft kitchen. The lights produce the equivalent of 45W each with a lumen of 470.

 

[sparkwright]

What floor is in the room above, and is this a house or flat?

Are you certain it's not a false lowered ceiling, as there could be a gap above the joists.

Some people just notch the plasterboard directly below each joist, leaving the joist intact. The cable is clipped to the joist bottom, then filled over. This is a bad job though, as the cable could easily get nailed through in the future, particularly if the ceiling is reboarded over.

The best way would be to neatly cut out a 200 mm strip of plasterboard, drill through the joists in the centre of this slot you have made, then re-fix the plasterboard.

Make sure this 'slot' is away from the downlighters as it will make installing them a bit easier.

 

[sparkwright]

You may be able to take an educated guess where the existing cables run. You could then drill some large holes between the joists, then use the existing holes in the joists for your new cable. Then repair the large holes.

 

[Ltnman]

What floor is in the room above, and is this a house or flat?

1990 built house with a bathroom above with a tiled floor

 

[sparkwright]

If you have coving you may be able to remove one length so all the cables where they need to cross the joists can do so here. Clip them tidily to the wall tight to the ceiling, then replace the coving. All cables going along the joists should hopefully be fished through without any trouble.

 

[Ltnman]

Thanks for the comments sparkwright. No coving and I don't want to repair large holes thus the idea of grooving the joists.

 

[JohnW2]

If I took out small sections of plasterboard using an 80mm hole cutter from underneath the joists and then grooved the joists to a depth of half an inch the electrician could pass a cable through the groove then I could carefully nail or screw back the plasterboard avioding the cable from the plastboard in the cutter and then easily smooth over the cut.

As sparkright has said, some people just 'notch' the plastboard where it passes over joists. Although that's not a very desirable idea, because of the risk of damage by nails/screws (or light-fitters hole cutters!), I don't actually think it wiuld be much worse than your proposal, since your half-inch notch in the joint would not be enough to avoid the risk of screws/nails etc.

Kind Regards, John.

 

[RMS]

If you have to notch the joists then you can use these,

These will prevent the penetration of nails and screws. They are called "safe plates".

 

[EFLImpudence]

There are no examples of, nor measurement given for, notches in the bottom of joists therefore I would think it is not allowed.

Drilled holes should be in specified areas only.

 

[seco services]

Building regs do state joists are not to be notched/drilled at the bottom.
All holes must be in the centre line or notches from above and within the specified areas.

 

[Ltnman]

Found this about notching http://www.eastdevon.gov.uk/bc-guidancenote7-notchingofjoists.pdf and seco services is correct .Seems I am not allowed to do it as a bottom notch weakens the joist.

 

[PrenticeBoyofDerry]

You can't notch on the underside of joists.
You can on top side but safe plates need to be installed or if you are my Building C.O. a length of metal trunking lid or 3mm steel plate.
What you a proposing to do is not going to be easy if no access from above, so either accept damage to the ceiling is going to be caused or ditch the downlights!

 

[JohnW2]

Found this about notching http://www.eastdevon.gov.uk/bc-guidancenote7-notchingofjoists.pdf and seco services is correct .Seems I am not allowed to do it as a bottom notch weakens the joist.

What you have found, and what other people are citing, is only a 'deemed-to-satisfy' statement, indicating what can be done without the need for structural calculations. As the Guidance you found says:

The guidance in this leaflet is only for use in domestic properties to determine the maximum sizes of notches and holes and the permitted location where they may be cut into timbers without structural calculations to justify their adequacy.

If you live in an older property with joists which are of generous size compared with current 'skin of the teeth' sizes (some in my house are 3"x9" or larger), then it's quite possible that it would still be structurally acceptable to notch the bottoms - after all, a half-inch notch is nothing like as 'bad' as having a whole joist which is half an inch less deep - so if your joists were two inches deeper than the current norm, there should be no problem. However, whether it would be worth the hassle and expense (unless you could do it yourself) of undertaking the structural calculations is an entrirely different matter!!

Kind Regards, John

 

[DetlefSchmitz]

John, That was also my first thought, as I have used that principle myself quite often for top notching. But on the underside there is a distinct danger of stress concentration caused by a notch, whereas on top the notch tends to get pushed together, so I wonder if there isn't a good reason to avoid notching underneath even if the rest of the joist size on its own would be sufficient. No idea really, but maybe worth considering.

 

[bernardgreen]

Notching the bottom of a joist can result in the joist splitting along the grain when the joist is bent downwards under loading from the floor it is supporting. A notch in the lower surface has far more potential for weakening or failure of the joist than the same size notch in the upper surface.