Drilling an oversized joist

[Adrian80]

Hi guys,
(Not sure if this is best in building or elecs)

was wondering if people had any thoughts.

I have some fully suported timber joists with a short maximum span of 1.8m.

Trada tables say the recomended size is 39x120.

I have 44x180mm timber n place.

My feeling is that the top 60mm of the joist is redundant.
i want to chop some notches for a 40mm waste, therefore do i treat the new centre of the joist as 60mm from the base for drilling holes, or do i go for the original 90mm for the base.

What do people think?

A

 

[Scoby_Beasley]

On the centre line every time. Max .25 dia of joist depth (45mm dia for your 180mm). between .25 and .4 of joist span from supporting wall.

Those are for guidance for electricians. Plumbers seem to make their own rules up .

EDIT Speeling mistooks!!

 

[bernardgreen]

My feeling is that the top 60mm of the joist is redundant.

The MIDDLE of the joist is ""redundant.""

When the joist is trying to bend under the weight it is supporting the top is being compressed ( along its length ) and the bottom is being stretched.

 

[Adrian80]

Hi i understand the principle,

But i could cut 60mm of the top of the joist, it would still be strong enough, the new middle of that timber would be different to the previous.

I am still inclined to drill on the centre line of the original however.

Cheers
A

 

[bernardgreen]

With a gap in the top or bottom the comression or tension can cause the timber to delaminate leaving the top to the depth of the notch separating from the rest of the joist. I have seen photos of delaminated joists but never in real life.

 

[ban-all-sheds]

Plumbers seem to make their own rules up

Be fair - it's a lot harder to thread copper pipe through holes in joists than it is cable.

 

[chapeau]

My feeling is that the top 60mm of the joist is redundant.

(figures removed)

What do people think?

You can notch (without providing calcs) 0.125 of the depth of the joist anyway (towards the end). So if the joist was 120 deep you could notch 15mm of that too.

So you have 60 + 15 = 75 mm 'excess' timber to play with. So from a structural point of view I'd just do it and don't get too bogged down in the minutiae which you are bound to find on these sites.

 

[Scoby_Beasley]

You can notch (without providing calcs) 0.125 of the depth of the joist anyway (towards the end). So if the joist was 120 deep you could notch 15mm of that too.

So you have 60 + 15 = 75 mm 'excess' timber to play with. So from a structural point of view I'd just do it and don't get too bogged down in the minutiae which you are bound to find on these sites.

But that would then be a notch in excess of 40%, not the 12.5% (0.125) stated in regs?

I'd be happy with the fact that my joists were "over-engineered" by the latest standards. Not trying to whittle them down to the minimum I could get away with.

But that's just me.

 

[JohnW2]

Hi i understand the principle, But i could cut 60mm of the top of the joist, it would still be strong enough, the new middle of that timber would be different to the previous. I am still inclined to drill on the centre line of the original however.

I applaud, and sympathise with, your logic - but the realistic answer is that I suspect few inspectors would be flexible or clever enough to think like that. They would (IMO) therefore be very likely to think in terms of the centre of the joist 'as is', regardless of the fact that a less deep joist would have still been acceptable.

Kind Regards, John

 

[Adrian80]

I think your now seeing my dileema!

If i drill in the centre of the joist, @ 85mm down, and i was unlukcy enough for it to delaminate, the hole would form a notch in the top of the minimally sized timber.

I think i will speak to the bco next he comes round, and see what hes got to say on the matter.

worse comes to the worse, ill just run the waste under the ceiling, and box it in. It is only in the garage afteralll.

cheers
A

 

[Adrian80]

thanks for all your input by the way...

 

[JohnW2]

I applaud, and sympathise with, your logic - but the realistic answer is that I suspect few inspectors would be flexible or clever enough to think like that. They would (IMO) therefore be very likely to think in terms of the centre of the joist 'as is', regardless of the fact that a less deep joist would have still been acceptable.

I should perhaps have added that, if your concern is compliance with regulatuions and 'satisfying inspectors' (eitehr now in the future), it would seem almost impossible that anyone could ever claim that drilling the the middle of the joist 'as is' was not compliant with regulations (as written) -but if you adopted the 'more thoughtful' approach, there would always be scope for discussion, argument or, at worst, even an opinion/claim that it was non-compliant (with the word of the regs).

Kind Regards, John

 

[DetlefSchmitz]

OK the big question is where the centre holes are. Are they adjacent to the cut-out or e.g. a metre away. If adjacent drill through the middle of what is left. If a metre away drill through the actual centre.

If fact try not to drill adjacent to the cut-out anyway if you can help it.

 

[PrenticeBoyofDerry]

Link to the requirements
//www.diynot.com/wiki/electrics:installation_techniques:route

 

[ban-all-sheds]

A number of people here seem to be assuming that a joist larger than the minimum can have holes drilled or notches cut in it and only their incursion into the minimum "zone" taken to be of structural significance.

I am not a structural engineer, and I suspect neither are they, but I very much doubt that they are right.