The thickness of the composite concrete deck slab on deck type bridges typically will be 250mm. An in-situ slab of this thickness, cast either on timber formwork or permanent formwork, will span around 3.5m, hence girder spacing (main girders in multi-girder decks , cross girders in ladder deck systems) will typically be at 3.5m centres, or a little more as the slab spans between the flange