Q: There are two different rafter tables in the IRC code book: Live Load and Snow Load. What is the difference and how do I know which one applies?
A: Victor Staley, a building official in the town of Brewster, Mass., responds: This issue of Live Load vs. Ground Snow Load might seem somewhat confusing until you read the basic premise that is identified in IRC Section R301.6, which states: “The roof shall be designed for the Live Load indicated in Table R301.6 or the Snow Load indicated in Table R301.2(1), whichever is greater.”
Using the term “live loads” when referring to roofs might make you scratch your head initially because roofs, unlike floors, aren’t asked to bear the weight of things such as furniture and occupants of a home—what you would normally think of as a live load. But when we discuss roofs these days, live loads are generally associated with the weight that the roof structure might be asked to endure while the roof is under construction or renovation (reroofing). These loads can include the construction crew and their tools, as well as roofing materials, like sheathing, roof shingles, and underlayment, that may be stacked in concentrated areas of the roof for short periods of time before they are installed and are evenly distributed (photo 1).
The ground snow load is pretty much what it sounds like: A predictable load situation that is derived from snowfall records over the years in a particular region (photo 2). These figures are established by the local jurisdiction, in my case the state of Massachusetts, which lists ground snow loads for each town in the state.
I find that code commentaries do a good job of explaining the basis of a code requirement, and in the case of Section R301.6, the commentary suggests that 20 psf is a pretty severe live-load condition and is most likely to be concentrated in a small area of the roof, as with a stack of shingle bundles. I take this as meaning that stacking the sheathing or roof shingles would initially concentrate the load (if stacked in one spot), rather than having that same live load acting on the entire area of the roof. Table R301.6 also takes into account that the steeper the roof, the less chance higher concentrated loads will occur. In other words, on a steep roof, less material would be stacked in a concentrated area at one time, to avoid having it slide off.
In my region on Cape Cod, we have a ground snow load design of 25 psf, which is a greater design load than the live load (12 psf to 20 psf) over an entire roof system. The bottom line is that the IRC requires you to design the roof structure to the greater roof load, and in my region, that’s the ground snow load.