Extension Ladder Safe Working Angles

We all (should) know that the safe working angle for a leaning ladder is 70-75 degrees but did you know that most manufacturer's labels have a thick, straight line through them called the 'Vertical Line indicator' that should be plumb when the ladders are set at the correct working angle. Posher ladders may come with a 'pyramid' or similar angle indicator but these days there really is no excuse for not getting it right. (Ps a Pyramid is a triangle usually slotted into one of the rungs with a tiny metal ball in it. When the ball is in the centre of the triangle the ladders are at the right angle).