Mersey
Roundtuit wrote:
"No consideration is ever given to personnel access to ceiling voids by designers and architects."
Often true, but exactly the sort of issue that should be addressed when CDM applies to a new build or renovation.
But you are faced with what exists, so the first Q is how often people have to go into the crawl space, for how long, where, and what to do?
From a lighting perspective that should influence putting in permanent lighting, using portable lighting (e.g. helmet mounted) or a mix of both.
Whether rotted boards are going to collapse is going to be dependent on how rotten, thickness and span between joists (assuming the joists are not also about to fail).
In terms of what to do, you also need to consider how easy it is to put in replacement boards, lightweights and other protections.
To put this into perspective, at home I have five roof space areas. Three are easy to access via vertical hatches, with a degree of boarding over the joists where someone has put in MDF boards (quite possibly recycled from a kitchen), but most of the space is simply joists with glass fibre insulation between the joists supported by the ceilings below. But, in general there is no need to access such areas.
The other two roof spaces are via hatches in the ceilings, one above the top stair of a staircase and one above a box bedroom. I have never dared venture into either (at least in part influenced by having investigated a fatal accident where someone was attempting to access a roofspace from a hatch at the top of a stairwell)! I think there is a redundant header tank above the bedroom ceiling but it would be necessary to remove the ceiling to get it out, so it's best left well alone.
So, perhaps time to sit down with those who need to go in there to find out why they need to access and then bring in a structural engineer or building surveyor for a view on the safety of what exists.
Might even find that some elements were overengineered if it's an old building. So as example a rotten board might have been unnecessarily thick and could be treated rather than replaced.
Many years ago my upstairs neighbours (before we moved up in the world - from basement to top) had dry rot in an 1880 building and wanted us to contribute to remedial works including replacing a wooden lintel with a steel beam. After we had threatened an interdict as no engineering assessment had been made, we got in a rot specialist who advised that the 12 inch by 8 inch lintel was way more than needed, so it was injected [by the top name in the business] to deal with the rot and not replaced. Along with some other changes to specification the price tag dropped from £25,000 to £6,000.
Of course, if the joists are sound, and you don't have access issues, it might be easier to replace some boards rather than evaluate how sound they are.