I second Wills, for ease.
These are Wills. They are HO scale but look fine for 28mm, especially if using under scale buildings (as I do). Each sheet measures 80mm x 130mm, so for big roofs you'll need multiple sheets. The sheets (hard plastic) are easy to cut with a slice and snap technique along the tile edges.
Top tip: For the ridges, cut a strip of tile bumps 'down roof', bevel, and glue into the ridge gap. This works very well (see stilted granary below); it was years before I realised you could use the tiles like this.