Flashings leaking around roughcast chimney - is GRP the answer

Dont know the answer to those question.It might be worth while finding out where and why the lead is leaking and have it re done.Lead is the perfect material for flashings.It is mallable and lasts forever.It's never a good idea to mix materials best to use one or the other.

Grp is a good roofing product but is predominately used on flat roof areas. It does not bond onto rough uneven surface very well especially if dirt from loose mortar is present. If you have a problem at the junction of your chimney my recommendation to you would be to re-new the existing lead flashings. These should either be chased into the chimney to a depth of 25mm and sealed with lead sealant or fixed with a bellcast and rendered down on top of with a 3:1 sand/cement mortar mix with an integral waterproofer added. Lead is an ideal product to weather junctions it is very malluable and can easily be dressed into shape, it can be welded to over come difficult detail, it expands and contracts with the weather and has a life expectancy of approx 25 years. The flashings around your chimney should be installed in Code 4 lead with a coat of patenation oil applied

From my experience, GRP will adhere to lead if correctly primed, however I would not recommend this as a way forwards. GRP adheres best to wood. G4 primer would need to be used if you were to GRP onto the lead. I'd recommend getting this looked at to see why it's leaking.