Ahhh, now I understand. 
Ways of routing twin and earth across joists......
- Thread starter 3george
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Indeed, good post big-all, but what is the gap between what the regulations say you should do and something which becomes structurally dangerous?
What margin of error is allowed for?
When I was renovating our house I uncovered a 7inch upstairs joist which had been notched two-thirds of its thickness, from the bottom, at around 4 inches wide!
I remember wondering at the time how that joist stayed in one piece, bearing in mind only a third of its thickness remained above the notch. But it was like that for 50 years…
What margin of error is allowed for?
When I was renovating our house I uncovered a 7inch upstairs joist which had been notched two-thirds of its thickness, from the bottom, at around 4 inches wide!
I remember wondering at the time how that joist stayed in one piece, bearing in mind only a third of its thickness remained above the notch. But it was like that for 50 years…
It's called the safety margin.
Dunno. Enough so that if people use a Mk I eyeball instead of a measure, or push it a bit because they want the hole or notch somewhere else they won't bring the house down.
You could always acquire a good enough understanding of structural engineering to do your own stress and loading calculations and drill/notch in other places.
The OSG is a good analogy - the standard circuits are there for people who can't or won't do their own design from first principles...
50 years ago they didn't have computers to tell them that they only needed a 4" joist, so they over-engineered. If you'd been left with only 1.3" of joist you might not have been so lucky...
Points understood BAS. If we split the safety margin up on a scale between (1) and (5) as follows:-
(1) Regulations say you must do this…
(2) Common sense / practicalities.
(3) Mid-point of safety margin.
(4) Getting dodgy here.
(5) Dangerous.
Then in respect of this particular issue I regard the drilling of a small hole in the centre of my joists outside the recommended zones as somewhere around No (2).
So I won’t lose any sleep over it.
(1) Regulations say you must do this…
(2) Common sense / practicalities.
(3) Mid-point of safety margin.
(4) Getting dodgy here.
(5) Dangerous.
Then in respect of this particular issue I regard the drilling of a small hole in the centre of my joists outside the recommended zones as somewhere around No (2).
So I won’t lose any sleep over it.
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ok more elaboration
the stresses are not constant if you take 10 as maximum 0 then - 10
there is probably only a few inches that are on 10+ or- 10 loads or indeed zero every where else the load is going up or down across the length breadth or width the calculations just decide where the load are best handled
as for removing too much timber no "big deal" [read on]
if you remove 70 percent of a structure [joist ] you remove probably 80 percent strength
the floor then deflects in that area until the adjoining joist take the the extra weight they in turn may deflect and transfer load to the next joist
the problem comes when the joist either side or the original joist can not give enough support and the deflection [sagging in the floor] goes so far as to bend the weakened joist so it breaks this can lead to further joist failure across the floor
another point worth mentioning the joist in any floor will in general be constant size throught the each floor this means that the joists are designed for the largest span on that floor [main bedroom for example]
in general bathrooms have quite small spans assuming there is a load bearing wall corrisponding beneath the bathroom walls this means you have joist that are probably bigger than nessisery
part of this will be offset by the bath full of water
but any extra goes towards a bigger safety margin
the stresses are not constant if you take 10 as maximum 0 then - 10
there is probably only a few inches that are on 10+ or- 10 loads or indeed zero every where else the load is going up or down across the length breadth or width the calculations just decide where the load are best handled
as for removing too much timber no "big deal" [read on]
if you remove 70 percent of a structure [joist ] you remove probably 80 percent strength
the floor then deflects in that area until the adjoining joist take the the extra weight they in turn may deflect and transfer load to the next joist
the problem comes when the joist either side or the original joist can not give enough support and the deflection [sagging in the floor] goes so far as to bend the weakened joist so it breaks this can lead to further joist failure across the floor
another point worth mentioning the joist in any floor will in general be constant size throught the each floor this means that the joists are designed for the largest span on that floor [main bedroom for example]
in general bathrooms have quite small spans assuming there is a load bearing wall corrisponding beneath the bathroom walls this means you have joist that are probably bigger than nessisery
part of this will be offset by the bath full of water
but any extra goes towards a bigger safety margin
I wouldn't drill or notch joists outside the safe zones in a modern house. Reason: They are all (mostly) built to meet the minimum standard in the regs. No-one wastes money nowadays by building at higher standards than the law requires. Joists spans and thicknesses are classic cases of minimum acceptable standards in modern houses.
If a homeowner drills through a joist, at minimum standard, outside the safe zone and it later collapses, for whatever reason, the builder is not responsible and the insurance company might not pay out.
If a homeowner drills through a joist, at minimum standard, outside the safe zone and it later collapses, for whatever reason, the builder is not responsible and the insurance company might not pay out.
And don't get me started on those b****y trussed roofs....
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