chicken ladder

crawling board

A plank with cleats spaced and secured at equal intervals, for use by a worker on roofs; not designed for transporting material.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.
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Slate and metal roofs are so slippery, I use what some roofers call a "chicken ladder"--two wooden ladders pinned together at their top narrow ends, and draped in an upside-down "V" over the roof peak so you can get up either side.