Start with three basic forms
Roofs on old Japanese buildings are first told apart by three forms. A kirizuma-zukuri (gable) roof has two slopes falling away from the ridge, leaving a triangular face at the end wall. A yosemune-zukuri (hip) roof has four slopes, with hip ridges running down to the corners from the main ridge. An irimoya-zukuri (hip-and-gable) roof combines the two: a gable on top and a hip below.
Horyu-ji has an example of each. The bell tower and the sutra repository are gabled, the Tsunafuzo storehouse is a raised-floor hip roof, and the Kondo and the Lecture Hall are hip-and-gable, as the structure field of the Agency's database records. All are roofed in honkawara tile, so you can compare the forms with the material held constant.
Even buildings of the same kind do not share one roof form. The East Gate and the South Gate of Horyu-ji are both three-bay, one-door, eight-legged gates in honkawara tile, but the East Gate is recorded as gabled and the South Gate as hip-and-gable. With gates too, comparing roof forms shows the difference.
Even the Higashi-muro at Horyu-ji, a long building twelve bays wide, is recorded as single-story, gabled and honkawara-tiled. The basic form of the roof stays the same however long the building is.
The mokoshi under an irimoya roof
The Horyu-ji Kondo is recorded as two-story, with a mokoshi on the first story, hip-and-gable. A mokoshi is a small skirt roof added around the outside of a building, and it makes the building show one extra tier of roof. That is why a two-story hall looks like three layered roofs from the outside.
The Kondo of To-ji is likewise recorded as single-story with a mokoshi, hip-and-gable, honkawara-tiled, so a one-story hall carries two tiers of roof. When the structure field says mokoshi, counting the tiers of roof as you look up tells you a lot about the building's shape.
A porch projecting from the front is called a kohai. The Main Hall of Myoo-in in Fukuyama is recorded as hip-and-gable with a one-bay kohai and honkawara tile, and dates from the late Kamakura period. Whether a hip-and-gable building has a kohai on its front is another clue for telling roofs apart.
Telling materials apart by roofing method
Honkawara-buki is the tile roofing widely used on temples and castles, in which craftsmen produce the upturned eaves and the curve of the roof through the tiling itself. The Agency for Cultural Affairs regards it as one of the most important techniques in conserving historic buildings, and it became a Selected Conservation Technique in 1994.
Hiwada-buki is roofing made of cypress bark, a technique unique to Japan. It was already in use by the middle of the 8th century, and while it appears on many Important Cultural Properties, it is hardly used in ordinary buildings outside them. Kokera-buki is roofing with thin wooden boards; it descends from early board roofing and was established by the late medieval period. Both became Selected Conservation Techniques in 1976.
Some National Treasure buildings have copper-tile roofs. The main sanctuary of Nikko Toshogu is hip-and-gable with a copper-tile roof.
Comparing form and material within one temple
At Muro-ji, the Kondo is a hip roof in wooden shingle, while the Hondo (Kanjo-do) is hip-and-gable with cypress bark. At Daigo-ji, the Kondo is hip-and-gable in honkawara tile and the Yakushi-do is hip-and-gable in cypress bark. Even with the same hip-and-gable form, a different material changes how the roof looks, so seeing them side by side in one precinct makes the difference clear.
For plant-based roofs such as cypress bark and wooden shingles, the techniques are hard to pass on because ordinary buildings rarely use them, and a shortage of good domestic material has also been pointed out. The Agency for Cultural Affairs examined repair cycles from past records and forecast the amount of material needed, publishing a report in 2021. Looking at a roof's material is therefore also a clue to what kind of upkeep the building needs.
Reading terms in the structure field
The structure field also contains words that describe the form in more detail. Tsumairi means the entrance is on the gable-end side, hakamagoshi is a flared skirt base on a bell tower, kake-zukuri is a platform of stilts built against a slope or cliff, and karahafu is a gable with a curving profile.
The East Precinct bell tower at Horyu-ji is recorded as hakamagoshi, hip-and-gable, honkawara tile; the Shoryo-in as gable-roofed, tsumairi, honkawara tile; and the worship hall of Seiryu-gu at Daigo-ji as kake-zukuri, hip-and-gable, tsumairi, with a three-bay front porch and a karahafu on the eaves. Knowing these terms lets you read details of a building from the structure field even after you have checked its roof form and material.
At the Horyu-ji Kondo, the main body is recorded as honkawara tile and the mokoshi as plank roofing. Materials can differ from part to part within a single building.