Q. Do the listed weight limits for ladders and scaffold planks include a safety factor? If so, how much weight can be added beyond the listed weight until failure occurs?

A.Alan Kline, president of Lynn Ladder and Scaffolding Co., Lynn, Mass., responds: As a general rule of thumb, OSHA requires ladders and scaffold planks to be designed with a 4-to-1 safety factor. That means that under test conditions, they must be able to support four times their weight rating without failing.

As to when any particular model might fail, that depends on many factors. Overloading any ladder or scaffold plank is a violation of OSHA regulations.