Summary Falls from heights are still one of the main causes of workplace injuries in Quebec, and were the second leading cause in terms of costs over the period 2010–2012, accounting for an average of $397 million annually. According to CNESST data, ladder-related injuries, in both construction and organizational settings, have been increasing over time (from 2007 to 2012), and from 2009 to 2013, 20% of injuries due to falls from heights involved a fall from a ladder. One of the main criteria affecting ladder stability is the angle of inclination. It determines a ladder’s stability with respect to foot slippage and tipping backward. The optimal angle for preventing foot slippage is around 75°, but the greater the angle, the greater the likelihood of the ladder tipping backward. In Quebec, under the Safety Code for the Construction Industry, the angle should be between 75.5° and 70.5°. This same guideline is also given in the Regulation respecting occupational health and safety. The purpose of this study was to determine the stability limits when using portable ladders and stepladders. The limits were determined based on the type of surface the foot and the top of the ladder were resting against, the height of the worker on the ladder and, for angles within the limits set by the provincial regulations (70.5° and 75.5°), different worker positions. The tests were conducted in the lab. Two 24-foot (7.3-m) commercial ladders were used. One was made of aluminum and the other of fibreglass. The two ladders, compliant with standard CSA Z11, were grade 1A (extra heavy duty). Two aluminum stepladders, a 12-foot model (ESC12) and a 6-foot one (ESC6), were also used for testing and, like the ladders, were grade 1A and compliant with standard CSA Z11. Two experimenters of different height and weight took part in the testing, and their weight was adjusted by having them wear a weighted vest and belt. Forces were measured with force plates and recorded using data acquisition software at a frequency of 100 Hz. First, preliminary tests on the different support surfaces were conducted to identify critical cases. At the foot of the ladder, smooth concrete and tile surfaces were examined, both wet and dry. At the top of the ladder, gypsum, wood, steel and aluminum were tried out, and some tests were performed with the ladder resting on its side rails. The critical surfaces at the foot and at the top of the ladder were, respectively, tile and steel. Subsequent tests were carried out on these two support surfaces. The order in which the tests were conducted was partially randomized to eliminate experimental bias. In all, just over 300 stability tests were carried out on ladders and stepladders. The experimenters performed a sequence of movements in the same order for each test: Initial position: standing, straight, at rest Position 1 (P1): leaning back, arms extended Position 2 (P2): leaning to the side, hand on a rung of the ladder Position 3 (P3): turned around, holding on with one hand, arm extended if possible. In the case of an unextended ladder, for experimenter E1 (the taller and heavier of the two experimenters), there was a risk of instability at rungs 2 and 3 for all angles of installation and all positions; at rung 4 for all angles of installation in the case of positions P1 and P3; and at rungs 5 and 6 in the case of position P3 for angles of installation of 72.5° and 75°, respectively. In other words, the greater the angle of installation, the greater the risk of tipping backwards, and this risk applies until quite high up on the ladder. The material a ladder is made of also has an influence on the risk of instability: the heavier the ladder (fibreglass), the lower the risk of instability. While choosing a fibreglass ladder over an aluminum one lowers the rung for which a risk of instability exists, a heavier ladder is harder to set up and move around, which are other factors that must also be considered. Subsequently, an analytical model was developed and validated on the basis of the experimental results as a way of generalizing the study conclusions. The parametric study conducted using analytical models served to validate the observations made during testing and to generalize the conclusions for extended and non-extended ladders. The heavier the worker, the greater the risk of instability on the upper rungs (5th or 6th rung from the ground depending on the worker’s position). A worker’s height is also a factor influencing the risk of loss of stability: the taller the worker, the greater the risk of instability, but this parameter is less of a determining factor than the worker’s weight. Last, a lighter ladder, especially an 8-foot one-piece ladder, has a greater risk of loss of stability, up to the 4th or 5th rung depending on the angle of installation, even for a very light worker (50 kg). According to the ladder climbing tests, at angles of 72.5° and 75°, there is a risk of tipping backwards if the worker climbs up facing the ladder. Using an adapted technique, which involves climbing up the first few rungs of the ladder sideways on, helps to limit or even eliminate the risk of tipping backwards. Proper training is therefore necessary to ensure workers know how to use and climb a ladder safely. Small, very light stepladders with a reduced footprint can be quite unstable, even for short, light workers. They should only be used by people who are fully aware of the hazards, and workers should be reminded that a fall, even from a limited height, can result in serious injury and require time off work.