Boarding Loft.

We want to board the loft and at the same time increase the amount of insulation up there.

Roof trusses run front to back at 600mm to centres, I am planning to add an additional layer of floor joists running at 90 degrees to the roof trusses and with 400mm to centres, which will allow me a lay an extra 150mm depth, 370mm wide, insulation over laying the old stuff also at 90 degrees.

Then I plan to lay 2400 x 600 x 18mm floor boards on top of the new joists.

Any issues with this or is it the perfect plan?

Thanks

V.

Sounds great - although the extra layer of joists and flooring will reduce the headroom by about 4.5".

Got to be careful of ventilation though, there's a risk of damp forming.

OP-That's exactly what I did (though I used smaller boards), though your 400mm centres seem a bit excessive - 600mm would be better I think. With too many joists you have less space for insulation (is the u value higher for wood than insulation?), and you are adding A LOT of extra weight to the existing joists. Better to use lsighting thicker boards if needed (though I wouldn't worry in a loft. Be careful to get your cross joints totally flat and level (set a couple of datums), and measure your spacings from the first batten (not the last one you did) so any errors don't add up (compound tolerancing). My exisiting joist were all over the place, so I had a nightmare routing notches into the cross battens to get the tops of them flush so I could board. If the existing battens aren't flush you run the risk of stressing the ceiling underneath when you screw the boards down...

Try useing kingspan between the floor joists (better, although more expensive, than ordinary loft insulation) then put down insulative floor boards (i think these also may be by kingspan).
As other have said, be wary of insulating roof joists

I went in one the other day where some underfloor heating 'specialists' had been in and they had notched the bottom member of the roof trusses about 30mm deep in various places......

it's going to be a warm job, I don't envy you

Yes, potentially.

Trussed rafter roofs work to some extent like triangulated space frames in racing cars; the idea is that the ceiling tie members and bracing struts are all in pure compression or tension, not under bending loads.

Also, and importantly, they're joined together with devices called 'gang nails', which are basically plates with teeth stamped out of them. These gang nail plates are directional... their strength is dependent upon the angle of the load in relation to the gang-nail (and the way the gang nail is loaded in relation to the grain of the wood). If you load the ceiling in such a way that the direction of the loads in the truss are changed, the stiffness of the whole roof structure can be dramatically reduced.

If its a Fink truss (W-braced) trussed rafter roof, it spans from wallplate to wallplate. The internal walls may be blockwork for a different reason (buttressing the external walls, noise reduction), but they're not there to support the ceiling ties on the trusses and, indeed, the trusses should not bear significantly upon them.

Now, the building industry being what it is, we tend to use fairly high factors of safety (it's a house, not a Lotus racing car, after all), so you'll probably get away with it, but it's really not a good idea.