Foam insulation PSI loads and rooftop workers

Hi all — I’m a bit confused by the choices of different types of EPS and XPS foam, where the variable is PSI rating. If we want foam on the roof I assume that it needs to withstand workers — or myself — occasionally walking on the roof. What PSI rating is adequate for, say, a 250 lb worker walking or standing?

What I mean is how many square inches do you use to estimate this? I’m having visions of heel strikes and video-recorded running shoe fittings… 🙂 If we assume a 250 lb worker I assume that they can create a load much greater than 125 lbs per shoe area, when their weight is on one leg, heel strikes, etc.

I see very low PSI ratings for EPS (e.g. 20 or 30 PSI at the most) compared to XPS (up to 100 PSI). Are the EPS numbers good enough for heavy workers?

And what if a load exceeds the PSI rating? What are we talking about as far as damage? Dents in the foam? Erosion of R-value?

Thanks for your help.