Can I ply a 1.2m gap between 8"x3" roof joists?

[chrisdadd]

I'm looking to board our loft for storing stuff and it runs 4m in both directions from the hatch. The photo shows it in one direction.

The span between the joists is 1.2m and all 4 joists are 8 inches x 3 inches. The insulation has been mostly topped to the height of the joists. In case it matters, the rafters look to be 6" x 2".

Can I simply screw plywood sheets to board these gaps with turbogold screws? If so what thickness? 25mm? I was thinking of buying 2ft x 4ft x 2.5mm ply. Then stagger the left and right rows.

Thoughts please.

 

[Bouy]

Plasterboard weighs about 10kg/m2, 25mm ply about 15kg, so in effect you are increasing the design load on the joists by 150%, without doing the joist calcs it looks pretty iffy, but it depends if the joists were over designed and how much storage you are looking at

 

[cdbe]

If those joists are 8x2 then your insulation is inadequate, I would run some minimum 4x2's across the top and add more insulation. If the boarding is only for storage/access you don't need to worry about it deflecting etc - only your knee going through it - I've used some old sapele doors in parts mine of mine for the Xmas decs.

Can't comment on the structural side but everyone seems to get away with light storage and put anything heavy well away from mid span.

 

[Dereekoo]

Ran the calcs and even assuming C16 grade joists and a loading of 75kg/m2 the joists are easily within allowable loading and just in defection limits. Deflection comes in at 11.5mm but there are all sorts of FoS included in the calcs so would likely be less, obviously less loading = less deflection