Cement on Roof Slates

Cement from the ridge tiles ran down the roof of my renovated cottage when it rained,and seems to have permanently marked the (natural) slates. The builder has offered to paint over the staining, but I'm afraid this may show up as a darker colour. Has anyone any experience to share on this topic?

Dorff,

I certainly wouldn't paint them. Matching in paint with the natural colour variation in your slates will be nigh impossible I'd imagine & be very noticeable. Furthermore, the paint itself will invariably wear or bleach leaving the concrete marks staining exposed again.