In this case, Kawara refers to Chinese unglazed earthware such as 'Kawarake' (unglazed earthenware), therefore this idiom does not describes roof tiles falling down, but an unglazed clay pot broken into pieces (according to 'Bushu no hanashi' (a story about radicals) published by Cyuko new book).
この場合の瓦とは、「瓦笥（かわらけ・素焼きの食器）」などの中国での素焼き土器のことであり、屋根瓦が崩れ落ちる様子ではなく素焼きの土器が砕ける様子であるという（「部首のはなし2」中公新書）。