Measure the area you would like to cover carefully. Take wastage into account to ensure you will have enough material to complete the project.
Also, keep in mind that the material from a quarry today may be different from what was available six months ago or will be available in the future. Additionally, there may be more than one quarry of the material. It is always recommended to include “attic stock,” which is a small amount of material - approximately 10% extra - to be stored in the event future repairs are needed.