cut into joists

[dundonald]

I'm laying flooring [1] in my loft. The wiring is simply thrown over the joists rather than a hole drilled centrally through the joist and the wiring passed through the hole. Is it good practice to cut a little nick out of the joist, enough to lay the wiring for the flooring to flush with the joist over the top? Is this detrimental to the structure?

I tried searching the archive of this group for the answer, but after a couple attempts trying to guess what others might call what I call a 'nick' [2] I gave up.

[1] chipboard variant [2] crescent type cut out

[Ian Stirling]

Yes. Cutting out a 10% section of beam at the top reduces strength by at best

20%. What about routing a groove in the chipboard?

[Mark Warren]

Or possibly nail thin strips of wood the same width of the joists on top of the joists. These only need to be deep enough to raise the floor above the wires. My loft was the same and this worked a treat. Doesn't add much weight and doesn't decrease strength of joists.

Mark

[Dave Plowman]

You get away with it on most floor joists because they're more than adequate. But those in roofs are often not - they're not meant to take much weight.

[Steve Wilson]

Is it good practice to cut a little nick out of

I've cut out the odd small notch in my joists for this purpose, but always felt that screwing T&G chipboard across the joists would more than make up for any weakness created by the notch. Am I dangerously wrong? Steve

[PoP]

I can't say from a technical perspective, but one thing I never do when adding loft boarding is cut the joists in any way. If that means that I have to lift the cables so that they run over the top of the boarding then that's the way I do it.

An insurance company would use a joist which had been tampered with as a way out of paying for a claim. And it could be very expensive!

PoP

[Ian Stirling]

Probably wrong, probably not very dangerously.

For the tounge and groove to add to the strength, it'd need to be bonded to the wood beneath. For roof joists, the top is in compression, and the bottom is in tension. If you cut out a notch at the top, that relaxes the beam at the top, and means that for a bit the very top of the beam does not contribute to the structure.

You can only make this up with the T&G if you jack the beam up to its original condition (so the gap where the notch is cut out is spread back out to its original size) sand the beam top flat, clamp and glue the T&G on. Simply screwing/nailing won't do much, or gluing the T&G without jacking.

If you fix it back, with the beam notched but without stretching, then the additional material is not stressed, so it's not contributing anything to the structure.

Plus, the T&G will be weaker as the grain will be going the wrong way.

[Elessar]

Which way does the grain run in chipboard? ;-)

[Pete C]

Hi,

How wide apart are the joists? It's probably not that hard to get a small drill in there and rethread the wire through the holes. If you have an angle grinder a cheap wood bit could be cut down if necessary.

If that's not possible and the wires are parallel to the joists in some places, another alternative would be to rout a diagonal slot in the top of the joist.

Anyone hazard a guess as to how much a diagonal 5*7.5mm slot in a

50x75 joist would reduce it's strength?

cheers, Pete.

[Ian Stirling]

Well, some of it will be running the wrong way... Argh.

[Michael Mcneil]

I have no idea on the legal and such aspects raised by the other posters here. If you are boarding out why not do a rewire? The nicks are not likely to detract much. Especially so if you do the dirty over a wall. Also you can add a strip to the side of the joist to increase the strength.

Speaking of which, what are you doing to increase the load carrying capacity of the timbers?

[Coherers]

IEE Wiring Regs 16th Edn:

"Where cables are installed through timber joists in the ceiling or floor, they shall not be less than 50mm from the top or bottom of the joist, unless mechanical protection that will stop the cable being penetrated by nails or screws etc. is provided. "

Sometime, I have to get around to drilling the joists and re-running all the lighting cables above a flat of mine for similar reasons. You only need diddly holes if they are lighting cables.......

[Kalico]

It really depends how thick the joists are. Those in lofts are often much less deep than those used under other floors.

Is re-routing the cable not an option. And I don't mean trhough holes drilled in the joists, since that could be difficult if they are thin.

Why not run a new length of cable up and along the underside of the rafters? You would then avoid any problems with cable overheating caused by loft insulation too.

Just a thought.

Rob

[Lobster]

It also has the further advantage that the cables are likely to be out of range of screws or nails inserted from below into the ceiling, or from above into the floor, by some unsuspecting person at some point in the future.

David

[dundonald]

Not quite sure how that would work.

[dundonald]

ah a notch, that's probably what I could have searched for instead of a 'nick'. :) Anyway, what did you cut the notch out with?

[dundonald]

I forget how wide the joists are off the top of my head but a board runs across two gaps, i.e. uses three joists. To thread the wire through the joists with a drilled hole, which is what I should do, would take a rewire and installing junction boxes. I may have to take the hit and do so properly. I was wondering if there was a quicker alternative without being detrimental to the structure.

Is this much different than a 'notch' in the joist?

[dundonald]

Nothing. It's only going to be used for storeage and as far as I understand (I may be wrong) roof joists should be adequate for this.

[dundonald]

And not a bad one. Only possible issue I can think of this is how the cable is routed down from the joist, through the flooring e.g. for ceiling lights below.

[Dave Plowman]

I'd be inclined to fix strips of wood across the joists with gaps for the cable, as suggested by Mark. Put steel straps across those gaps to prevent nailing into the cable. Then lay the flooring on top. Something like roofing battens should be ok and cheap.

Most ceiling joists are distinctly undersized for any sort of load.