I want to board a section of my loft. Up there my rafters (?) are all 19.2″ spacing.

Standard cheap loft boards are 1.22m long. These don’t fit (they span 2 and a half gaps). I could cut them all short, but that would be *a lot* of wastage (~20%).

Proper floorboards, even cheapo ones, are 8ft. So they would fit (across 5 gaps). But my access is very narrow, so width wise I need 300mm (like the boards the wrong length) not the standard 600mm.

This spacing must be a standard of some kind. It’s even marked by a diamond on the tape measure (TIL). But I can only find boards the right length, or a good width, but not both.

Can I get super long skinny loft boards anywhere? Or do I need to get 600m ones cut lengthways? (Or, admittedly) is this spacing even an indication I shouldn’t be boarding this part of my house at all?

It’s not for living space. Just for light storage, and to let me scoot across to access the exterior wall now and then.

Can you add some noggins to support the gap where the boards finish between joists?

Could you use old school pine floorboards rather than chipboard? These are usually some odd measurement like 144 mm (all sorts of other options available). You could then lay them in 8 foot lengths? If you need to shorten to get them in you could cut some to 3 widths and some to 2 widths and stagger the joints when installing.
Link below to examples from Wickes (other timber merchants will have very similar stock).
https://www.wickes.co.uk/Products/Building-Materials/Timber/Timber-Floorboards/c/1000206

Add timber battens at 90degrees to joists to the area you want to the centres required. You then have more depth for insulation under the boards.

I could @wooksterbo – indeed that was my plan B. It becomes a question of which is more work/more money between adding battens and cutting boards to fit.
Thanks @dc1988 I could do but I think I’m more inclined (and capable) to add battens.
What I really really want, @slowol, is something that size but cheaper and more cheerful. So chipboard. £/m^2 seems to double if I use proper boards like that, OK not quite as bad if I compare to cutting and wasting but still (can I still use this word?) spendy.
This place is so helpful. Thank you.

If it were much cheaper (probably not) you could probably get away with 12 mm tongue and groove in a loft floor or get OSB cut into 300 mm widths by the woodshop (even B&Q will do this for you). The 300 mm wide floorboards only seem to come in short lengths. It would be good if they did the 2.4 m by 300mm as shifting the 600 wide one on your own is a killer. They weigh a tonne. OSB and ply are a fair bit lighter.

I had never seen that 19.2″ mark before but apparently it’s intended to allow 8ft boards to exactly span 5 gaps, as you say.

I had the opposite problem, an 8ft OSB board was too long (and heavy) to get into the loft, but I’d already bought them. So I cut them with a circular saw with the blade at 45º, so that there was some support, and positioned the cut so that the boards were supported under the lower side of the cut. If you used battens as suggested you could do similar, but lengthways.

Stagger your boards. The T&G end seam can sit between rafters so long as its supported either side by boards that make the full span – thats the point of them being T&G. This is especially case with boards are only 300mm wide. All the seams running lengthways are making a bigger span than that without support anyway – just using the T&G connection between them to share loads.. 1200mm x 300mm OSB boards (sometimes sold as ‘Stirling Board’ or ‘Smart Ply’) is lighter and stronger than chipboard if the access is difficult.