Build the visible eaves as low and wide as the design requires, and above the visible ceiling raise a second, steeper roof on cantilever beams (hanegi) to shed the rain.
In the Chinese system the rafters seen from below are the rafters that carry the tiles, so the pitch of the eaves and the pitch of the roof are the same. Japanese builders, working in a wetter climate with heavy tile roofs, separated them: the noyane (field roof) is framed independently above a hidden space, and great cantilevers pivot on the column line to carry the eaves out. The visible roof can be almost flat and deep in shadow, the actual roof steep enough to keep out the rain.
It is the principal structural invention of Japanese carpentry, and it freed the silhouette of the Japanese temple from the Chinese rule of jǔzhé (Depress each purlin: 1/10, 1/20, 1/40…).
Build the bracket set (puzuo) of blocks (dou) and arms (gong) crossing at right angles, each tier projecting no more than 30 fen beyond the one below; a set of n projecting tiers is called an (n + 3)-puzuo, from four to eight.
projection per tier ≤ 30 fen · rise per tier = 21 fen · puzuo = tiers + 3
Where the eaves must project far, replace the outer arms by slanting levers (ang) that run down and out beyond the bracket set and rise inward to be held down under an interior purlin.
From the intermediate bracket set of the second bay the rafter ends advance progressively outward, so that at the corner the eave projects further (by 4 cun for a hall of one bay, 5 for three bays, 7 for five bays) and rises with the corner beam.
Project the eaves from the centre of the eave purlin by 3½ chi for rafters of 3 cun diameter and 4 to 4½ chi for rafters of 5 cun; beyond them add flying rafters projecting six-tenths as far again.
eave projection ≈ 3½–4½ chi · flying rafters = 0.6 × eave projection
Build the sanctuary of unpainted cypress on posts set directly in the ground, with a raised floor and veranda, a straight thatched gable roof entered on the long side, crossed finials (chigi) and ridge billets (katsuogi), and a free-standing post at each gable carrying the ridge.
Raise the god’s dwelling high on thick pillars planted deep in the bedrock, with crossed finials reaching up to heaven.
Cite this page Chris Paton, “The hidden roof: separate the eaves from the weather”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/hidden-roof. Text, drawings and models licensed under CC BY-NC 4.0.