。
Deciding how to pitch the roof is extremely important.
Literal
Roof [genitive-の] pitch [object-を] how do [question-か] decide [nominalizer-の] [topic-は] very important is.
An architecture-ish declarative. The embedded indirect question どうするか ('what to do,' 'how [to handle]') is nominalized as the object of 決める. This 'wh-clause + か + 決める' pattern is the standard way Japanese expresses 'decide what/how/when to X.' The whole phrase is then nominalized by の and topicalized with は to become the subject of the predicate 重要です. 勾配 (こうばい) is the technical term for 'slope, pitch, gradient' — applied to roofs, hills, roads. Slightly specialized vocabulary, more common in construction or engineering contexts than in everyday speech.