Been away for a while as we've been busy moving house and planning kitchen and garage extensions which are due to commence in the next couple of months (Covid permitting). Rather happy to say that my good lady has given the green light for a layout in the loft which means I can go far bigger than my original 8x4 layout which I started a couple of years ago that stalled completely as a house move became imminent and there was little point in carrying on to have to strip it.
Main question here is what's needed to convert a loft into a suitable layout space. Not looking to do a full blown loft conversion just get it usable for the task. Very aware it's freezing in winter and like a sweatbox in the summer which will need to be addressed. Happy to add Velux type windows but will need to obtain planning permission.
The thought of having either a loop around the whole loft or an L shape board (potentially 4m x 12m) is to say the least very appealing .
The problem with the Velux is the local authority. In their minds a Velux indicates a living space, so the roof has to be converted from coldroof to warmroof standards and then the rafters doubled up to become joists, and before you know it, you are back with a full blown conversion. I don't think you need PP for putting in lightpipes so you might get away with two or three light pipe terminals but they won't give you the same ventilation. Otherwise put in some larger vents and put shutter plates on the inside for the cold weather.
I'm doing a similar round-the-rafters build. Basically I'm going between both Vs of the trusses with horizontal battens laid across the roof timbers to support the layout. The two runs are to be joined up by boards running across the gable ends of the house. Here you can do a more conventional construction with legs down to the floor and screws into the wall. For obvious reasons, never cut into any of the woodwork up there unless you like having your roof in the bedroom!
I laid a chipboard floor above the existing insulation. It's a bit hot up there in the middle of summer, but having south-facing solar panels some of the heat is taken away as electricity which probably helps, but you don't get that many of those days and you can just build kits downstairs instead! Anything to improve ventilation will help in the summer- fans just blow hot air around unless it's got somewhere to escape. Maybe a vent or two with an attached exhaust fan to push hot air out? Something like a bathroom extractor would be the obvious answer.
Lighting can be arranged from the existing upstairs light circuit- consult an electrician if unsure. As for mains power, I'd stick with a long cable thrown down the hatch to a socket downstairs- that avoids you leaving something switched on up there.
I went the whole hog 20 years ago, put in a heat converter, piped the hot air from the bathrooms below, and exchanged it with the cold air outside, to heat the loft space with a inline fan. Second fan box taking stale air from loft and pumping it into downstairs hall. Insulated the roof space with both polystyrene and lined the internal roof walls with insulated wall boards, the smooth painted white surface made so much difference to working up there, half the lighting [power needed, thus cooler in summer. to spread the wight, laid bathroom quality chip board, with polystyrene floated floor insulation and second chipboard floor in opposite direction floated over, seemed to work fine.
layout suspended on frame just 500mm from floor, bloody difficult to wire in retrospect, but gave more space for actual track, and operated by eith me on floor cushions, or legs dangling, sitting in ladder well. Double insulation raised floor a bit, and there was a wooden step where the loft ladder folded up. reason I left it down when operating the trains.
It wasn't cheap, but my home heating bills dropped big time in the winter, and I was able to heat the space with only a little fan electric heater in mid winter. Summer was ok, unless it got into the 40's then it started to become a bit stuffy. No windows, but two massive 150 mm vents drilled into the apex wall, which was also plastered up with insulation plaster board. added bonus, quiet, total escapism. Downside, had to do messy stuff outside, bugger to take dust out of environment.
I went down the route you are considering. I decided to do a looped layout but first had to build the deck to run it on in the roof space. I chose to fit battens across the diagonal members of the roof trusses at a suitable height and then cover the area in ply boarding finished with a cork covering. Doing the ply and cork gives you a good working surface and gives complete flexibility in what you want to do especially if like me you think it out as you go!
Thanks for the comments so far it certainly seems that this idea isn't out of reach. Unfortunately we are in a heritage area due to being a Garden City so although we don't need planning permission for sun pipes I do need to get consent from the Heritage Foundation. Electrics aren't a problem as I don't mind getting stuck in and have a tame electrician for a mate who's grandfather was a big model railway fan so he's quite interested when I share pics on a night out (rest of my friends cant fathom why I "play with toy trains" as they put it, but they also don't understand why I throw cars round racing circuits so given up trying to explain myself).
Was thinking of using chipboard for the vertical supports and thick ply for the track bed. Due to the chimney stack and loft hatch position the modelled area will be about 8m in length with the rest of the track simply looping the loft space. Did look at doing an L shape with a helix at each end but the helix's eat in to the modelling space and access could be iffy for derailments etc.
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