Roof ventilation

[Guitar_man]

I want to ventilated my unventilated cement tiles/felt roof. The house is an small end terrace.
Can someone advise they best way to proceed?
Should add vents in the soffits and the ridge in order to get a flow of air through? What products would you recommend?
The loft has 100mm of insulation.
Any idea what an approx cost for this job would be?
Thanks,

 

[jeds]

Soffit vents front and rear should suffice. I'd also add another 200mm insulation if I were you.