Holes in joists

[geraldthehamster]

I want to drill some holes in some 50 x 170mm joists, that I am installing in a first floor, for wiring to go through. The joists are

3.6m long and the holes will be not far off the middle of the span.

Can anyone tell me what is the biggest hole I am allowed/advised to drill, and how far down the joist it should be?

Thanks Richard

[ac1951]

I believe that the cables must be a minimum of 50mm both from the finished floor level and the finished ceiling level. So that should give you the area within which the hole can be made.

Regarding the max hole dia. that depends on loading factors and you would need to consult a contruction engineer for an acurate figure. That said if you're talking typically house wiring then I would drill the size needed to allow the cable to pass without chaffing and certainly no single hole larger than 25mm.

[Lobster]

But the holes should still ideally be made on the centre line of the joist, which minimises weakening of its strength.

David

[ac1951]

and also in line with the centre of a floor board to minimise the risk of someone driving a 3" nail through to the cables.

[Ray]

Typically a 22mm hole is sufficient for house rewires.

[Vass]

"geraldthehamster" wrote in

Rather than go big, drill 2 or 3 smaller

[dom]

Approved Document A (old 1992 edition)

"holes should be no greater diameter than 0.25 times the depth of the joist; should be drilled at the neutral axis; should be not less than

3 diameters (centre to centre) apart; and should be located between 0.25 and 0.4 times the span from the support"

i.e. not at mid span!

[Andy Wade]

The literal answer (at centre of span) is zero.

The rules are:

- between 7% and 25% of the span, measured from either end, you can notch the top edge, up to one-eighth of the depth of the joist;

- between 25% and 40% of the span you can drill on the centre line with a max. hole diameter of one-quarter of the depth;

- multiple holes should be spaced 3 diameters apart.

These apply without any need for structural design input.

NB: notches are for plumbing & heating pipework, they're too shallow to use for running electric cables.

[geraldthehamster]

Thanks all

Regards Richard

[dicegeorge]

in my new old house there are big notches with wires and pipes going through them, more than in your building regualtions, what shall i do?

cover them up and forget them? strengthen them somehow?

[g]

[<me9]

And in the middle half of the joist, ie not near the ends. If nrar the ends, notches would be required in the upper/lower edge of the joists, and protection of the cable from penetration.

[mail]

Dear diceg If there is room, I suggest you use pinned, sliding wedges in the gap above the services to strenthen up the compression zone and enable the couple (with the tension zone) to be more effective. In practical terms - measure the distance you can "fill" the notch without affecting the services - get some boarding that thickness - make a two angled timbers out of a rectangle by cutting at an appropriate angle say of 5 or 10 degrees - put in some glue and hammer the two together in the gap until it is tight and pin it (carefully) . you will need longer bits than the gap to do this practically Chris G

[geraldthehamster]

It's an interesting question. I want to do mine "properly" because the BCO will be up there some time soon on another matter, and I don't want to risk problems if he twigs the floor's new. However, being done to regulations and being safe aren't necessarily the same thing. I wonder how far you can really mutilate joists before a floor actually becomes dangerous?

[John]

Tip: I am eternally grateful to my builder for marking the floor where there are pipes or cables below. It helped me to understand how things were routed (as well as making sure I avoid nailing)

[Lobster]

I always mark positions of services on the floor myself when I've had the boards up for some reason or another - but have to say I'd never rely on such markings done by somebody else.

Apart from anything else, I've noticed that plumbers in particular seem to have a bizarre propensity for putting floorboards back in the "wrong" place (and then wonder why they are left at the end with a hole in the floor which can't be filled with any combination of the bits of board remaining...)

I'm also slowly building up a digital photo-library of the house in different states of repair, both for interest's sake plus to help with future project planning / problem diagnosis etc).

David