Flat Garage Roof Joist Spacing

[old_shaky_hands]

Hi,

After several eye-watering quotes, I'm coming round to reroofing my garage myself. It's approx 3m x 5m flat roof and I'm either looking at either EPDM or GRP. One concern is the joists, a few need replacing which is fine, but they're at 1400mm centres.
Will I need to add more joists for either of these roofing types? The roof won't see any footfall.

Ta!

 

[sxturbo]

thats rather large centres, even without foot fall it should be at least half that imo