There are regulations addressing the question. There are standards addressing the question. There are codes that address the question and there are industry customs and practice that address the question. But the real question isn’t how firm it is but rather what do we mean by firm? As in, scaffold legs shall bear on adequate firm foundation. It would seem that the statement (or more precisely, requirement) is somewhat of an oxymoron. Can a foundation not be firm? Can it be adequate but not firm? The Federal Occupational Safety and Health Administration (OSHA) regulation requires: “Supported scaffold poles, legs, posts, frames, and uprights shall bear on base plates and mud sills or other adequate firm foundation.”
At best, this regulation is confusing; at worst, misleading. To further confuse the issue, the sub-regulation mandates that: “Footings shall be level, sound, rigid, and capable of supporting the loaded scaffold without settling or displacement.” So, is a footing a foundation? What if the scaffold is supported by the sloping floor of a water tank? And how sound should it be? If nothing else, these regulations should tell the reader that the OSHA Standards surely are not instructions on how to construct scaffolds. A little research into the dictionary tells us that firm means solid, not easily moved or disturbed. That sure makes sense for a scaffold. So far so good, but what happens when we use a mud sill. If the sill is truly on mud, there goes our firm foundation. And the way the regulation is written, it sounds like that mud sill is firm all the time. If it’s sitting in the mud, it won’t be firm and besides, no matter how good the mud or the sill is, it still needs a foundation to support itself and the scaffold!
If you’re still reading, by now you probably figured out that the regulation addressing foundations wasn’t written too well. That’s why we need to look beyond the letter of the law and learn its’ intent. What is the intent? We don’t want your scaffold to fall over because your firm foundation became infirm. A scaffold leg on concrete doesn’t need a sill, unless of course you very recently poured the concrete. But that scaffold leg still needs a base plate. Obviously the leg won’t go through the concrete if it doesn’t have a base plate but the concrete can damage the bottom of the tube. Consequently, a base plate is required. Is a sill (notice I left out the mud) required when the scaffold, and base plate, is supported by concrete? No, but it’s not a bad practice to have a sill – it helps provide some friction and keep everything from sliding. Do we always nail the base plate to the sill? Not required, but why not hammer a nail in it; you won’t lose the sill that way.
For many of us, experience has demonstrated that if the scaffold is on soil, or dirt, (same thing except to soils engineers) a sill is required. The question is, how big should the sill be. The answer, in true engineering terms is; it depends. Hire that soils engineer and he/she can test the soil for you but that’s not too practical for most of us. How about using that other approach called the heel test. If you’re up to your knees in mud, chances are you won’t find a sill big enough to support the scaffold. But if your heel makes only a slight impression and the soil is compacted, you can be reasonably assured (the “reasonably” is lawyer jargon) that a sill 10 inches by 12 inches will function reasonably well. This rule of thumb (or heel) assumes the soil is compacted, protected from washout or other stability threatening environmental event, and the scaffold leg has no more than 2000 pounds bearing done on the sill. Remember, if you are not comfortable with your ability to make the decision about sills, get the help of a qualified person!
Now that we have briefly discussed a couple of things about base plates, sills, and firm foundations, read the regulation again. Think about its intent. Think about that scaffold leg load that has to be supported by the foundation. Does it need a sill and firm foundation to spread the load so we don’t lose the scaffold and hurt someone? Don’t forget, avoiding injury and death is the real intent of this regulation.