Is this bad practice?
- Thread starter xdrive
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As irrelevent as it is to this topic, you CAN notch the underside of JOISTS in the prescribed zones.
It doesn't
Yes - but the last sentence of the quote:
states that they may only be used where they are not needed.
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It does appear to - but I confess that I had assumed that it had been badly written, and that what it actually intended to say was ...
".... SRCDs are only intended to provide supplementary protection downstream of the SRCD. SRCDs are intended for use in circuits where the fault protection and additional protection are already assured for parts of the circuit upstream of the SRCD."
In other words, I thought they were intending the stress the (correct) point that (in contrast with over-current devices) any residual current device only ever provides residual current protection downstream of itself - so that, if such protection is required upstream of the device, it has to be provided by something else.
If that's not what they intended then, as you say, it makes no sense, since it would be saying that SRCDs are only intended to be used when they were not needed (unless one wanted redundancy or 'active' RCD functionality, which most SRCDs provide)! However, maybe my assumption was wrong and they did 'write what they intended' (which I suppose would explain the absence of BS 7288 from the list of acceptable residual current devices in BS 7671) - in which case I really don't understand the thinking/sense!
This thread (and others) on the IET forum (click here) discusses this issue in much the same (partially incredulous) manner as we are discussing here.
However, I cannot deny that what you linked to is a perfect reproduction of what BS 7288 says, since the one page of it (other than the Table of Contents) one can see in the 'free preview' on the BSI website says:
It would obviously be good to be able to read subsequent pages, since they might well clarify what they really are saying.
That one page also provides some 'justification' for why BS 7288 has been created, in addition to BS EN 61008 and BS EN 61009, about which it says:
Kind Regards, John
That's contrary to what most people believe. To my surprise, Approved Doc A is silent on this matter (refering one to TRADA tables or various Standards for details concerning all structural woodwork) but many LABCs produce Technical Guidance Notes which indicate that only the upper side of joists (and in certain places and sizes) may be notched. For example, this from the Hertfordshire LABC (click here) ...
Kind Regards, John
Thanks for that.
Rightly or wrongly, there are a lot of diagrams on the internet showing notches cut in the underside of joists, supposedly allowed at the same intervals as notches at the top - although they do stipulate you must not have notches in the same spot both top and bottom.
Rightly or wrongly, there are a lot of diagrams on the internet showing notches cut in the underside of joists, supposedly allowed at the same intervals as notches at the top - although they do stipulate you must not have notches in the same spot both top and bottom.
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Does it depend where you live? Different councils; different rules?
https://www.eden.gov.uk/planning-an...ing-control-guidance-notes/notches-and-holes/
https://www.eden.gov.uk/planning-an...ing-control-guidance-notes/notches-and-holes/
I looked at several, and get the impression that they are all quoting from the same source (although none say what that is), since they all say very much the same thing.
That seems consistent with (not different from) the Hertfordshire one I posted, given that it says:
Kind Regards, John
Maybe, but the fact is that a notch of a given size and (lateral) position on the bottom of a top-loaded beam (such as a floor joist) has a much greater impact on the structural behaviour than does notch of the same size and position on the top of the timber.
It's also worth remembering that this thread is about ceilings. If it is a top-floor room, the 'ceiling joist' may not even be deep enough to be designed to take any appreciable loads (like people walking on it in a roofspace), even when intact, so any notching in it (top or bottom) would probably be highly undesirable.
Of course, these rules/guidances must make some assumptions about the size of the joist, probably that it corresponds to 'current practices'. In my house, some of the (mixed Victorian and Georgian) floor joists are, quite unnecessarily, about 12" x 3" - and there is, in engineering terms, no good reason why I shouldn't create a very sizeable notch in the underside of one of those
Kind Regards, John
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