Hi I am having to shift my kitchen. The waste pipe which I believe has to be
35mm is going to have to go under a joist (or through). If go under the joist, I lose the slope required to take away waste. If I go through it do I breach building regs ? The joist runs parallel to an interior wall (old tenement flat) about 10cm away. I'm in Edinburgh dont know if regs vary with location. Thanx in advance. Dave.
A 32 mm hole through the joist is OK if you can keep it nearer the top of the joist than in the middle. Just try to keep the hole size to the minimum needed to pass the pipe through.
Apologies for raising a concern, but surely it isn't acceptable to chop a 32mm hole in a joist? Wouldn't that seriously weaken the joist to the point of possible failure?
Please note that I am not basing my query on knowledge or fact, it just seems unlikely to take out that much material without there being an adverse effect.
PoP
Actually middle is better, but don't worry.
I'm not sure either, but I do know that the hole should be round; no corners, as they are a bad thing.
Now that's what I thought, but a joiner told me to try and keep a hole of that size nearer the top of the joist. I didn't ask him to elaborate to much on it at the time, I just did what he said because I tought he knew better. I might just go and ask him why now.
In article , BigWallop writes
Middle sounds better - if the joist flexes, there is wood at the top and bottom to take the stresses, remove one or other of these, and its a bit like the way you chop down a tree, isnt it?
Yep, also what I thought, but then, when the joiner told me to put the holes nearer the top of the joist, my thought was that maybe the floor board being screwed back down across the space would have something to do with giving the joist some reinforcement. But I have to admit that I to thought the holes were better in the middle of the joists.
I think square corners were the cause of the first jetliners mysteriously and randomly responding to gravity back in the 50's. Eventually they figured that cracked were raking out from the window corners, destroying the aircraft frame.
PoP
I have to admit to being quietly amazed by this discussion!
A joist might be what - 100mm in height? And you drill out 32mm, thus removing about 30% of the strength of the joist. That joist might have an inconvenient knot around about that area to further weaken the structure. And even if the joist was 200mm in height that would still represent a 15% loss of strength overall - which in my book sounds pretty significant.
Doesn't add up for me, however I am happy to bow to the combined expertise of others on this matter!
PoP
There have been nasty examples on ships, too. (seen in that wonderful book on structures mentioned a while back...)
Yes, it weakens it. Its not necessarily serious tho, because joists are rather crude atructural devices. They are engineered not to not break, but not to bend too much. As such they are well over strength for loading.
Putting a hole through them at one spotonly slightly reduces their stiffness. It DOES reduce strenght, particularly in teh center of a long span. but on a short span or at the edges, its not serious.
I was thinking of a simpler way to say a whole lot of bullshit maths, and you did it for me. Thanks.
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Hi guys
Building regs state:
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 dimeters (centre to centre) apart; and should be located between 0.25 and 0.4 times the span from the support.
HTH Gordon
BTW The neutral axis means the centre of the joist. Notches may only be
0.125 times the depth of the joist.
Fatigue and stress concentration. Relevant to a metal stressed skin aircraft - less so to a wooden beam.
Most issues with square holes are to do with introducing stress concentrations, but I don;t think its seriously significant in te current case. A couple of holes and a pad saw will make a decent slot for two pipes, but arguably two separate holes are better.
That point is valid, as e.g. a chpiboard floor will form a stressd skin and reduce flexure, however, one of teh chippies will always manage to nail through the pipe if you have em laying in the top.
Its something to do with the away magnetic descalers work. Pipes full of water attract nails. :-)
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yes. Liberty ships or summat.
Simple thinking always produces apparently simple results that are completely inaccurate.
"A 10x4 joist will break at 3 tons"
Ok, so if I get a 10x4 joist and put it across the Mersey, I can drive a mini over it right?
I wonder why all those engineers bother with all those thousands of tons of steel and concete, when a simple bit of wood would work just as well?
No, and it takes a HECK of a lot of maths to determine limits in any sort of accurate way.
However plumbers have been driving holes through joists ecver since they were invented, and hardly any grand pianos have crashed into the living room as a result.
Not really. The compresive and tensile stresses are disproportionally taken up at the top and bottom edges of the joist. The middle plays comparitively little part in providing the strength. Having the top and bottom further apart is what makes the joist strong, not the additional material in between. This is why steel joists often have many holes cast in the middle. It reduces weight without much effect on strength.
Notching is far more detrimental to joist strength, particularly if done in the middle. However, notching the top of the joist at the ends has little effect, because not much of the stress is expressed there.
Christian.
A simple rule for carpenters.
In practice, you can break it, but an understanding of structural theory is necessary :-)
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