Glossary · P
Purlin
Also called purlins
A horizontal beam running the length of a roof, carrying the rafters; in East Asian frames the purlins rest directly on posts or beam-ends and their heights set the profile of the roof.
Rules that use the word
The ang: reach further without climbing. 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.
Raise the ridge by a third of the depth. For a great hall, measure the distance between the front and back eave purlins and raise the ridge purlin above them by one third of it; for lesser halls, by about a quarter.
Depress each purlin: 1/10, 1/20, 1/40…. Draw a straight line from the ridge purlin to the eave purlin and lower the first purlin below the ridge from it by one tenth of the rise; from that point draw a new line to the eave purlin and lower the next purlin by one twentieth; continue, halving each time.
Eaves in proportion to the rafters. 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.
The raised-beam frame: purlins at any height. Instead of a triangle, stack beams one above another, each shorter than the one below and raised on short posts, and lay a purlin on each beam-end; the heights of the purlins then set the section of the roof, which may be curved at will.
Where it appears in the book
In the chapters The Roof, China, Japan and Korea.
Nearby in the glossary
Prostyle · Protome · Pseudisodomum · Pseudoperipteral · Pycnostyle · Pylon · Pyramid · Pyramidion