Complex sentence: The earliest extant example of hidden roof is Hōryū-ji's Daikō-dō, built in 990 and was discovered only in the 1930 during repair works.
Split into simpler sentences: The earliest extant example of hidden roof is Hōryū-ji's Daikō-dō, built in 990. The hidden roof there was discovered only in the 1930 during repair works.