The Obsidian Roof is a dark blue, nearly black slanted building-block that you can use as a roof-tile or for other purposes (it can be rotated).
It can be crafted in your Crafting Menu (default key "Q"), but only after the crafting-recipe has been unlocked by
- crafting blocks of Stone Roof for the first time (unlocked by Wood Roof)
- creating Obsidian Rods by putting Obsidian (bars) into a Processor
- creating Obsidian Slabs by putting Obsidian (bars) into a Processor
Crafting/taking blocks of Obsidian Roof is one of the unlocking requirements of the recipe for Iron Roofs.
To craft 8 (blocks of) Obsidian Roofs at a time, you'll need (as of R43 in May 2017):
- 1x Melted Wax made of Beeswax in a Forge or looted from Wood Treasure Chests or Stone Treasure Chests
- 1 Stone Rod made of Stone, Limestone or Bedrock in a Processor
- 2 Obsidian Rods made of Obsidian (bars) in a Processor
- 4 Obsidian Slabs made of Obsidian (bars) in a Processor
Since R42 in May 2017, inner and outer corners blocks can be created for Obsidian Roofs too by putting them into a Processor. No crafting recipes are required for this.