Economical new roof for outbuilding

If it were me, I'd re-timber the roof to make sure that all of the roofing timbers were good, then I'd use metal sheets for the roof itself. Depending on what you want to use the building for, it might be worth looking at having a translucent sheet somewhere as that lets a lot of light in. Tiles are very expensive if you don't have enough, slate even more so. There are bitumen sheets that you can use or even shed felt, but I don't recommend either if you are in an area that gets proper weather. You could sheet the roof with OSB sheets and then put a rubber cover on that, but I have found that the best bet is almost always metal sheets. You can get them in various colours and shapes to look like tiles or straight forward corrugated or box section. But metal wins for me everythime.

To make the stone outbuilding usable and economical for storage or as a utility space, you have several options for a new roof. Reusing the old tiles may provide a traditional aesthetic, but keep in mind that they can be heavy and may require sturdier timbers. Alternatively, opting for a lighter roof system, such as metal roofing or lightweight synthetic materials, can be more cost-effective and easier to install. Lighter weight timbers can reduce costs, as they require less structural support. However, it's crucial to consult with a professional contractor or architect who can assess the building's structural integrity and provide specific recommendations based on your budget, local regulations, and desired functionality. They will be able to guide you on the most suitable materials and construction methods for your specific project.