Nothing wrong with a properly fitted torch on roof (as far as flat roofs go)
GRP again if fitted correctly is pretty neat and easy to repair should it get damaged.
I agree with what your saying roofer.I have never heard a 4 layer system for torch on felt, 3 layers should be enough.
As for EPDM i did the firestone contractors course, which allows me to fit firestone EPDM products i also did the polyroof GRP roofing course so i have experience of using all these systems. I my opinion they all have their good points and bad points but a properly laid felt roof using good quality torch on felt is as good as any other system.
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