What timber for adding height to overboarded insulation?

[redhexagonal]

i have 170mm joists, and I want to board an area 1.2m x 2.4m . i was thinking of using C24 untreated timber which is 90mm high, so I’d get 260mm of fibreglass insulation plus the OSB boards on top.

Is it ok to use untreated timber for this?
Is it ok to not have an air gap above the insulation for an area this small? Theres very little head height.
Is it OK to screw the timber at 90 degrees to the joists?
And what spacing should I use for 1220mm OSB? The pack says 300mm which seems a bit cautious, will 400mm be OK?

 

[HERTS P&D]

Use 'loft legs'.

Andy

 

[redhexagonal]

Use 'loft legs'.

Andy

Loft legs are too big. I already have 17cm joists and little headroom

 

[Tigercubrider]

I used strips of OSB which had 2x1 screwed onto the sides to make a sort of I beam.

 

[redhexagonal]

I used strips of OSB which had 2x1 screwed onto the sides to make a sort of I beam.

So that would raise the boards…about 1.5 inches? Not sure i’m with you

 

[Tigercubrider]

Take a sheet of 8x4 rip it into a number of equal strips 8 feet by 6” (example)
then fit a 2x1 timber to each long edge, so that it’s wider at the top and bottom, waisted in the middle.
it is a lightweight truss/ girder
it works really well.

I did it in my loft to hold “standard” sized loft boarding
My ceiling joists are spaced too close together for the modern boards, so running new ones at right angles allows me to adjust the gaps to suit modern boards and have more insulation