Drilling Holes in joists

[Smudgey]

Is there a tool for drilling holes in joists as a normal drill wouldn't fit other than making the hole slanted, is there a set size for a hole?

 

[didthathurt]

I started off with an attachment that fitted a normal drill and allowed me to drill at 90 deg to the plane of the drill.

I now have a drill which does the same thing itself like this one:

http://www.toolbank.com/product.cfm...=4E1A1B61160CF1D5484EF32C32958388a2lfkfa2lfjf

Building regs do lay down rules for holes as follows:

1. can only be drilled in a region that lies between 0.25 and 0.4 of the span of the joist from each end.

2. Should be no bigger in diameter than 0.25 of the depth of the joist "ie 100mm joist = max hole 25mm diameter).

3. Adjacent holes to be no closer than 3 diameters centre to centre

4. Should be drilled so that the hole centre lies on the joist neutral axis (normally the centre of the joist)

5. If the hole is less tham 50mm from the top or bottom of the joist, the cable should have mechanical protection as it goes through the hole.

Is that what you are looking for?

 

[ebee]

Normally, if you lay the drill on top of the joists (upside down) and use a flat bit or auger, then although slight angle, it is almost at 90 degrees and the difference is hardly noticable.

Angled drills are OK but decent ones or decent attachments for standard drills cost a bit because of the use of bevel gears

 

[Smudgey]

Thanks for the replys

Didthathurt That is just what I wanted to know, was going to ask those questions another time but don't need to now.

Can you explain this point please.
4. Should be drilled so that the hole centre lies on the joist neutral axis (normally the centre of the joist)


The whole reason I ask is I am moving a light switch in the living room over to the door where you come in and not near the back of the room.

I have looked under the floor upstairs, and the house has had a rewire sometime. Old black cables have been left through the joists but not connected to anything. The rewire has used a different route than the old black wires. I plan to use the holes from the black wires to put the wire for the light switch, I will need to drill 3 more joists to complete the route for my light switch.

The rewire doesn't seem that old, but I still have a very old metal CU which when the cover is off, the live contacts on the main swith could be touched. One day I will get it replaced. The upstairs has 3 sockets, 1 in each of two bedrooms and 1 on the landing. The third bedroom has no sockets. The 2 sockets in the living room seem to be spurs from the upstairs ring. Sometime I would like to extend the ring upstairs and add a few more sockets in each of the rooms.

My next question back to the joists is, could the cable for the ring go in the same holes in the joists as the cable for the light switch.

What are the rules for routing cables through holes in joists and when you go along a joist does the cable need to be clipped in place. At the moment the lighting circuit seems to be resting on the plasterboard.

Many thanks

 

[didthathurt]

First, Neutral axis. If you imagine a joist 10m long then place a wight in the middle of that joist, the joist will deflect downwards, slightly, in an inverted curve. If you then measured the length of the top surface it would be a little under 10m as the top surface would be slightly compressed. Likewise, the bottom surface would be a little over 10m under a slight tension. In theory, therefore, there is somewhere on the joist depth that is still 10m end to end and is, therefore, neither compressed or stretched and is, therefore, not absorbing any of the load placed on the joist. That's the neutral axis (and before I get shot down, yes this is a qualitative explanation to explain the situation ignoring the implications of Newtons 3rd Law and the resulting couples of inertia a load on a beam creates).
The result of that is that if you drill on that axis, you are reducing the load bearing capacity by a minimum amount, which, for practical purposes, means if the centre of your hole is exactly half way between the top and bottom surfaces of the joist you've hit the neutral axis. If you can use the existing holes, use them - you are maintaing the structural integrity to the best level you can.

I still have a very old metal CU which when the cover is off, the live contacts on the main swith could be touched.

Then don't take it off!!!

Yes, you can route cables through the same hole. If it's more than 2 (max 4 in the tables) the load carrying capacity is reduced by a couple of amps as long as the cables aren't surrounded by insulation.. This, on average, isn't significant.

I wlways, where possible, clip cables to joists. I get this from my father, an ex-naval artificer electrician who started domestic work upon his demob in 1947. His view was that neatness and precision go hand in hand. I reckon clipping, where possible, minimises the possibility of damage to the cable at some future point, though some may disagree. The only actual "rules" are based on supporting the cable by this method that prescribes clips to be a max of 250mm apart for other than the larger cables.

 

[plugwash]

the disadvantage to clipping is it makes it much harder/more disruptive to change stuff later. Especially if floorboards must be raised to gain access.