Measurments and Quantities - I'm feeling thick!

I'm trying to clad my roof space to stop traffic soot coming in and I need to work out costs. But my brain has switched off again!
I have two spaces about 12ft by 13ft that need cladding and the 12ft is intersected at the 6ft mark. So four areas to work with at 6' by 13'
I hope to use 2 - 3mm ply and I'm trying to work out how many sheets I would need. I also know to get these in the loft they would have to go through the loft hatch so the widest point would have to be 2' so the boards will need to be cut which is also what I'm having trouble figuring out.
Any clues please?
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