You can harvest water from any non-permeable roof type. This includes metal, tile, slate, concrete, and others, but the roof must have gutters. The larger your roof area, the more water you will be able to harvest, limited by the storage area, and what you use it for.
The roof type is important because you can collect more water off some roofs than others. This is called the runoff coefficient. The runoff coefficient is also affected by the type of rainfall you have in your area, and what time of year it rains.
To find out how much water you can harvest from your roof, check out our Rainwater Calculator: Calculate Savings.