The Igneous Roof is a dark red slanted building-block.
Despite its name, this roof is not inflammable.
This slanted block can be placed to create corners of walls or for other purposes too when being rotated by pressing and holding R (as the default key) while moving the mouse.
It can be crafted in your Crafting Menu (default key "q") but only after the crafting-recipe has been unlocked by
- creating Iron Rods by putting Iron Bars into a Processor, made from Iron Ore that can be extracted from Nodes on the Stalactite layer or can be found in Iron Treasure Chests or Diamond Treasure Chests
- crafting or picking up Igneous Wall (unlocked after taking a block of Igneous Rock for the first time with at least an Iron Mining Cell equipped)
- crafting or picking up Iron Roofs
To craft 8 (blocks of) Igneous Roofs at a time, you'll need (as of May 2017):
- 1x Melted Wax made of Beeswax in a Forge or looted from Wood Treasure Chests or Stone Treasure Chests
- 2 Stone Slabs made of Stone, Limestone or Bedrock in a Processor