There is poyentially sufficient weight to justify 7mm holes, brown plugs and 5mm screws at 300 to 400mm centres IMHO. I recently fitted out the interior of an 8 person lift car with 25mm thick rough sawn timber (walls and ceiling - not my choice, but the architect's design, and he seems happy enough with it). Anyway the lift car engineer who came to balance the lift informed me that I had increased the weight of the lift car by nearly 200kg