I was told by a local builders merchants that I could use chipboard with a GRP roof, is that true?

You should never use anything else other than 18mm OSB3 decking, preferably tongue and groove. Aside from the problems chipboard has with exposure to condensation the surface is too smooth for the GRP roofing system to get a good bond too and it will delaminate.

Check out our guide on decking for more information or speak with one of our technical team for some free advice before you start your project.