In the past few years, there have been several articles in this magazine, as well as in others, that discuss guardrail loading requirements. They all say that a guard railing has to be able to resist a 500-pound load in any direction. A question I get a lot is, Why is that, when the language in the IRC requires only a 200-pound load? The answer is that the code requires a 2.5 safety factor for unique assemblies that don’t otherwise have their own standardized test methodology (Question & Answer, May/June 2007). Here’s what it’s all about.

Engineered Designs Rely on Tested Materials

There are essentially two ways to know what loads a certain type of construction can handle: using an engineered design or testing the assembly. In engineered design, calculations are made using known values for various properties of the materials, which might be wood — taking into account the species, grade, and size of the lumber — or something else, such as steel, plastic, or concrete.

Engineers rely on these values in their designs but recognize that they are assumptions; they are stereotypes. There can be variation in the specimens, environment, and installation quality that make the standardized values questionable. So, while an assembly designed to just barely support the expected loading might be safe, generally speaking, we don’t want buildings of any sort to be right on the edge of failure. With these considerations in mind, engineering calculations are full of safety factors to cover possible flaws and other variables.

Where do the material values used in calculations come from? Testing, of course. In the case of lumber, the values are derived from a series of tests, using multiple samples and a methodical process. All common construction species and grades are tested, and you can find design values derived from these tests in the IRC. Similarly, the values for products like steel and concrete come from testing.

Lumber and other commodities aren’t the only materials tested — manufacturers can pay to have their proprietary products tested as well. When a material like composite decking or metal hardware is tested, safety factors are also used in determining the rated allowable load. These factors vary depending on the product or material at hand. For guards, there’s no specific standard, so the default is that provided in the International Building Code, which is the parent document of the IRC. When the IRC lacks detail, the default answer is often found in the IBC.

Testing Could Be Required for Site-Built Guards

There are really two ways of looking at the required loads for guards: before they’re installed and after. When guards are tested prior to installation, such as what a manufacturer of a guard system would have done, they must indeed be tested to a 2.5 times safety factor. This is required by Section 1715 of the IBC, and results in the 200-pound load in the code becoming the 500-pound load we hear about.

There’s a bit more to it, though, when talking about site-built guards. In theory, the building department could require the installed guard to be load-tested. Here, the IBC has a different requirement. Section 1714, “In-situ tests,” handles this situation and requires a safety factor of 2 (400 pounds). There is one catch — the 400-pound load has to remain in place for 24 hours. That might be hard to pull off on most deck jobs, and I doubt anyone has ever encountered a requirement to load-test a site-built guard.

Since none of these tests are exactly feasible or commonplace for typical field-constructed guards, what’s the takeaway? What do you do? You build the guards strong enough that they won’t be questioned by the building inspector. If you build them questionably and they attract attention, there is no feasible way to prove their strength. It’s best to get a confident hold of your guard design on your own — before you’re told how to do it. Make resisting a 500-pound load your goal, but know that 400 pounds is good enough.

Contributing editor Glenn Mathewson is a building official in Westminster, Colo.

This article originally appeared in PROFESSIONAL DECK BUILDER.