HOW TO STOP CONDENSATION IN THE SHED?

How do I stop the metal roof on my shed from getting condensation, I bought a de-humidifier �180.00 and it still not working. Its cold outside, -3 last night so when I open the shed door droplets of condensation are inside on the roof and they fall down soaking everything. I have put an air vent in thinking that would solve it but no! Is there a way I could lag the roof inside and cover it or would that just mask the problem? Live in Leeds West Yorkshire England