noun
flat roof
Refers to a roof that is horizontal or nearly horizontal, as opposed to a pitched roof. Common in architectural contexts.
このビルは陸屋根なので、屋上に出られます。
This building has a flat roof, so you can go out onto the rooftop.
勾配屋根 is a pitched or sloped roof, the opposite of 陸屋根.
Compound of 陸 (roku, 'flat land') + 屋根 (yane, 'roof'). The exact historical derivation is uncertain, but the kanji combination directly conveys the meaning of a flat roof.