The laws of physics are unvarying, especially the law of gravity. If the physics facts conspire to cause a problem, it will indeed happen. The use of portable ladders on the fireground always is hazardous and risky, whether there be electric wires in the way, inadequate ground anchor conditions, overloading, bad placement or other geometric problems.

The stability of the typical ground ladder when used in an upright position is a standard operation and is well-covered in texts, training manuals and practice. The relationships of ladder angle and the friction or anchor between the ladder and the ground and what the ladder is rested against are very straightforward and usually present a stable environment from which a firefighter can work.

But the use of the roof ladder can be a completely different story. The standard roof ladder is 14 feet in length and has a set of folding hooks that may be used to anchor the ladder over the ridge of a roof. The NFPA 1931 and 1932 cover the design, testing and use of such ladders. The concept is that the hooks will dig in to the roof surface and provide a mechanical anchor to prevent ladder movement when in use.

However, with hard-surface roofs such as slate or metal, as well as in the presence of ice, rain or debris, the hooks do not dig in and are merely a friction-type anchor. And certainly the geometry of the roof influences how well the combination of frictional forces and their direction combine to provide stability.

Unfortunately, the coefficient of friction between such hard roofs and the steel ladder hooks is low. As a result, there is a lack of stability under various circumstances. For example, the coefficient between an aluminum ladder beam and a metal roof is less than 0.3 under ideal, dry conditions. Obviously it is even lower in wet, icy or debris-covered surfaces. And if the roof has much of a rise, the ladder will simply slide all on its own if the hooks are not deployed. For a 4:12 roof (a 40foot rise in 12 feet) the angle is about 18°. The tangent of this angle is 0.33. Hence the ladder will slide if not anchored.

So the hooks are important, providing additional friction and allied forces to hold the ladder in place. However, the placement of weight on the ladder, specifically the position of firefighters and equipment, may or may not generate the forces necessary for stability. If the applied weight is above a critical position on the ladder it will be stable; if below the weight will cause the hook end of the ladder to slide toward the ridge, and the hook end of the ladder will rise up. If the hooks don't catch on the ridge the hooks will slide over the ridge. Then the entire ladder and whatever is on it will slide down and ultimately off the roof.

The position of weight on the ladder that will determine if it is to be stable or unstable depends on several things: the roof frictional characteristics, the geometry of the roof, and the ladder material and dimension. It's a given that ladder hooks will not “bite” into the roof structure but will simply slide across it, possibly leaving some scratches but not becoming mechanically connected.

So the bottom line is that on roofs that are of a hard surface, roof ladders should be anchored by ropes or chains to a fixed anchor. Don't count on the folding hooks to ensure the ladder's stability.

In the subject case in Ohio, the metal roof (with a coefficient of friction less than 0.3) and a roof angle of about 18° (the tangent of which is 0.33) was a potential problem. The firefighters reportedly got onto the ladder at the upper end, past the subsequently calculated critical stability point, and as they came down the ladder a distance they passed the critical point. The hooks slid, the upper end of the ladder actually raised, the hooks slid over the roof ridge, and the entire ladder/equipment/personnel weight slid down the roof and ultimately off the edge.