Have you had a look at the DIY Projects area, especially the project entitled "Flat Roof Repairs": https://www.diydoctor.org.uk/projects/repairflatroof.htm This may provide you with the information you need or you can also contact the presenters of this forum Permaroof as they have much experience in this field.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!