Help with structural calculations

Hi,

We have designed a loft conversion and have been asked to provide calculations for the proposed spreader plates. Specifically: 'Also the thickness of the plate at 25 mm needs justification because bending stresses and deflection might exceed allowable limits'

I wasn't sure how to answer this question so instead simply changed the plates to 2 no. 20mm plates tack welded together however building control are still insisting that we check the 'bending and deflection of the plates' which I've no idea how to do.

I attach drawings and calculations for the project (which specify 25mm thick plates but we have used 2 no. 20mm plates for each spreader as per above).

Please let me know if you can assist with this.