When we were planning the barn, I posed this question to every builder we interviewed: how do you know how to design the loft to be strong enough to support filling it with hay? I knew this was an important question. I’ve read and heard horror stories of people having barns collapse when loaded with hay.
And I remember my dad saying that the county had kicked back his proposed pole barn design, insisting on an interior footing wall to support the hay load in a Monitor-style barn.
The answers I got were inconsistent enough to encourage me to do a little more research and thinking. A few barn building outfits downplayed the subject, saying, aw, 50 pounds-per-square-foot should be plenty. Others said 85, or 120 pounds-per-square-foot is “standard.” One of them supposedly asked the county, and didn’t get a clear answer there either. Residential construction is apparently 40 lbs in most jurisdictions (10 lbs “dead load” = weight of the building itself plus 30 lbs “live load” = the stuff in the building); I think? So what should it be for hay? I wondered.
I finally realized that this is the wrong question. The right question is, how much hay volume can you fit in your loft? Because that could vary a lot, depending on your roof pitch, and whether the loft wall height goes down to zero, or starts with 2’ high pony walls, as ours does. If you could imagine yourself, or some future building owner, attempting to cram the loft full to the gills with hay, floor-to-ceiling, neatly stacked, how much volume could you get in there? And then how much would that weigh? Divide by the square footage (because the weight will be distributed across the floor joists, even if the hay is stacked in a pyramid), and that should tell you what kind of live load for which to engineer the floor.
So before we were even sure what the final dimensions of the barn would be, I did some simple calculations. A “slice” of the loft would have both a rectangle and a triangle. So, recall from Jr. High math, the area of a triangle is half that of the rectangle two of them would make. Add that to the area of the rectangle it sits on. Then multiply that area by the length of the barn, and that gives the volume. We wanted a 9:12 roof pitch, which is steep. I figured just in the first couple of bays of the loft, there would be over 9,000 cubic feet of space to fill with hay.
So how much does hay weigh? This link suggests figuring 8-14 pounds per cubic foot. To verify that, I weighed and measured a bale of my own hay. Just sampling one bale, I found it to weigh 11.7 pounds per cubic foot. That means I could potentially squeeze about 53 tons of hay in the front 24 feet of our barn. If I went with the worst-case calculation of 14 lbs per foot (maybe hay with higher moister content or denser baling could weigh more?) then it would be more like 64 tons.
That seems like a ridiculous amount, but it’s really not. Just my small flock of 33 sheep will have consumed about eight tons of hay over this 3-4 month winter. If I increase the herd, not only would they eat more hay, they’d eat it for a longer season, because the grass would run out sooner in the fall, and not be ready for such a grazing load so early in the spring. If I were to cut hay on another lot and store and sell it, in addition to storing some for myself, it wouldn’t be inconceivable to put up that much hay.
So, that weight, spread out over the area of the floor, definitely gets us into the ballpark of 120 pounds-per-square foot live load, which is what we ended up specifying for our barn when it was engineered. The loft seems extraordinarily stout in the way it was designed, and maybe it’s overbuilt, but better safe than sorry. I’m glad I didn’t listen to the salesman (of a company we did not choose to use) who confidently asserted that 50 pounds would be more than enough; or we might have pancaked our barn! Now we can load ‘er up, and not ever worry whether we are pushing the limits of what it can handle.