There are times in life where we get anxious because we want to do something. We stand up on our new roof or in the framework of a deck. We look around and see that there is lots of places for gravity to work against us. Gravity wants to pull down. It wants to bend and break steel and lumber and will do so unless careful math are done.
And most people believe that if something feels solid, well then it must be strong. Not true. All structures depends upon an unseen hierarchy of forces. Knowing that hierarchy make all the difference as to whether a home stands proud for years or cracks and sags.
How Weight Travels Through Your House
This can be reduced to a workable system as shown in the infographic above. From the roof downward, these loads move along their path to the foundation. And that’s where it gets dicey. Weight alone isn’t the problem. It’s the path.
Dead loads (permanent), like the structure itself, are pulled down by gravity. So do live loads (temporary), including snow, furniture, and of course people. The diagram shows the pyramid. Everything up there has to be supported by what’s at the bottom. Get the load path? Then you know why a beam fails.
No, it doesn’t fail because it wasn’t made of good stuff. It fails because someone did the math incorrecty.
For example, let’s take a look at your home’s floor in the living room. According to the chart, the expected live load is 40 pounds/square foot for home use. If that seems random, think about what it represents. Think of all the objects that could be placed on a living room floor, like the coffee table, the piano, and the sofa. This include the weight of those things plus the people who use them. This is a conservative number assuming the floor won’t bend, bounce, etc., when used as intended.
Then there is the ‘dead’ load which is the sum of the finishes, subfloor, joists and such. Add those together following Load and Resistance Factor Design and suddenly the true demand increase greatly on the materials. See how this accumulates in the chart? The load factors multiplies the dead load by 1.2 and the live loads by 1.6. This isn’t an advisory to us do-it-yourself types; it’s a built-in safety factor in the code itself.
Then there are roofs. Roofs must support not only their own weight but also endure weather conditions. Rain, wind, and snow create forces against a roof. Building code directs us to design for the worst case of these three. The visual guide shows us that we can reduce roof live loads depending off what it’s supporting.
That’s a point many novices struggle with. The bigger the space, the lower the allowable roof load per square foot. There’s a good chance that none or very little will ever be under full load at one time. It’s a statistical thing that saves both materials and costs without sacrificing safety.
Attics are another weak spot. For many homeowners, the attic is just a free storage space for old boxes and holiday decorations. The chart differentiates between no-storage attic spaces and those with storage space. Ones with some storage space. It gives the no-storage space a factor of ten (per square foot) and the storage space a factor of twenty. So if your attic is meant for ten but you’re keeping really-heavy stuff up there, you’re effectively doubling the original design load.
And that’s where people screw it up. They assume that the ceiling joist over their bedroom were built to support a season’s worth of memorabilia.
Balconies and decks are similar to their interior counterparts but have stricter requirements based off minimum values. A minimum value of 60 pounds per square foot is common for exterior decks where people may access them. Because of this potential for crowd loading and public access, the deck needs to meet a higher requirement.
The load passes downward from the balcony/deck through beams, girders, and columns until reaching the footing. Each link in the chain has to be able to carry the load without failure.
A building code isn’t some red-tape bureaucratic hoop to jump through. Instead, it’s condensed history, drawn out of previous failures. The chart is your cheat sheet to those lessons. It takes complicated engineering concepts and turns them into something you can litrally see and understand.
If you learn how the weight travels and respect the numbers, then you’ve got yourself a sturdy structure. Ultimately, gravity will win. Let the math speak for itself. Your house depends on it.
