Translation guide
To pile things into a heap, or to accumulate in a disorderly pile. In Japanese, the verb depends on whether the action is intentional or natural, and whether the focus is on the process or the resulting state.
To deliberately stack or pile objects into a heap.
Literally 'pile up'. Used for stacking things neatly or into a heap. Transitive verb.
彼は落ち葉を庭の隅に積み上げた。
He heaped up the fallen leaves in the corner of the garden.
Literally 'make into a mountain pile'. Emphasizes the resulting heap-like shape.
彼女は机の上に書類を山積みにした。
She heaped up papers on the desk.
General verb for 'pile up' or 'stack'. Can be used for heaping, but less specific.
石を積んで壁を作った。
They heaped up stones to make a wall.
To become piled up, often without deliberate action, or to describe the state of being heaped.
Intransitive verb for things piling up, especially snow, dust, or leaves. Focuses on the accumulation process.
雪が一晩で積もった。
Snow heaped up overnight.
Intransitive version of '山積みにする', meaning 'to become a heap'.
未処理の仕事が山積みになっている。
Unfinished work has heaped up.
To pile things carelessly or into a messy heap.
Literally 'pile and scatter'. Implies a messy heap. Rare.
彼は部屋に服を積み散らかした。
He heaped up clothes messily in the room.
To pile up casually or carelessly. '無造作に' means 'without care'.
彼は本を無造作に積んだ。
He heaped up books carelessly.
積む (tsumu) is transitive (someone piles things up), while 積もる (tsumoru) is intransitive (things pile up by themselves). Use 積む when you are doing the heaping, and 積もる when describing the state or natural accumulation.
Avoid directly translating 'heap up' as 'ヒープアップする'. This is not natural Japanese. Use the verbs above depending on context.
Intransitive verb for things piling up in layers. More formal/literary.
書類が机の上に積み重なっていた。
Papers had heaped up on the desk.