My elderly parents had their roof insulated recently under some govt grant or other; I haven't seen the result but apparently the installers have laid a thick layer of Rockwool-type stuff at right angles to the joists.
I recall as a teenager when I lived there myself, back in the 70s, laying the original 3" insulation between the joists - bugger of a job as it's an old house with lath and plaster ceilings, the spacing of the joists is very variable, and the joists themselves are in poor shape, very irregular in cross-section and many certainly wouldn't take a man's weight if you were to step across them without using crawling boards.
So now, I gather from Mum (and can picture), the whole roof space is just a homogenous sea of glassfibre; its impossible to tell where any of the joists are, let alone the decent ones you can stand on. I'm sure this scenario happens a lot, and probably doesn't matter too much provided the roof space doesn't need to be accessed; but the main problem with my parent's house is that there's a somewhat bizarre arrangement with the chimney whereby the flue from the solid-fuel fire it has a horizontal run of some 5 feet (IIRC) within the roof space. This means there's a flue box up there (ie, a cast iron door in the side of the flue) which the chimney sweep needs to access to clean the chimney. Mum is convinced (with some justification IMO) that next time the guy comes to clean the chimney , he's going to wind up falling through on to the spare room bed...
Is there any obvious solution to this? Must be a reasonably common problem. Short of lifting all the new Rockwool, all I can think of is to just lay new, long, crawling boards between the loft hatch and the flue box (quite a long way) and assume that there will be adequate support along the way and at the ends...
David