From Roof to Earth: How a Building Carries Its Weight
When you look at a house or a commercial building, you're seeing a lot of materials working together to hold everything up and keep it stable. And underneath all of that is one question that guides the entire structure: where does the weight go?
In construction, we call this a load path. Every piece of weight, whether it’s snow on the roof, furniture inside, or wind pushing on the walls, needs a way to get safely down into the ground. That’s the job of a load path.
What Are Load Paths?
A load path is exactly what it sounds like: it’s the route a force takes as it travels through a structure. Snow, roof sheathing, floor loads, wind pressure, even seismic activity, all of it has to find its way to the foundation and ultimately into the earth. If it doesn’t, you’ve got a problem.
Gravity Loads: Straight Down to the Ground
Let’s start with the most obvious: gravity. Gravity loads always want to go down. Think of a roof truss. It takes the weight of everything on it (snow, roofing material, its lumber) and transfers that weight out to its bearing points. From there, the weight passes through the wall framing, through the foundation, and into the soil.
In the Pacific Northwest, we typically use double top plates in the wall. That allows a truss to land directly over a stud or in between two studs, which works fine in light snow load areas. But if the truss is a girder truss, meaning it’s carrying multiple trusses, it needs more support. We match the number of plies in the girder with the same number of studs or posts underneath it.
If there’s a window or door opening below the truss, we add a header. That header transfers the weight out to the sides of the opening, where it can continue its path downward through the wall and into the foundation. Without proper support, like leaving out a header, that load won’t have anywhere to go. You’ll see bowing, sagging, and eventually failure.
Lateral Loads: Forces That Push from the Side
Gravity isn’t the only force we design for. Wind and seismic activity create lateral loads that push on the sides of a structure. In order to keep a building upright, those forces have to be resisted and redirected into the ground, too.
Sheathing, usually plywood, is one of the most common ways we handle this. When it’s nailed to studs, it stops the building from rocking or shifting under pressure. It stiffens the wall and pushes the force back into the foundation. Seismic forces work similarly. The only difference is that instead of wind pushing the building, the earth is shifting underneath it. The building has to stay rigid while the ground moves.
The Gable Truss Problem
Gable trusses can cause confusion when it comes to load paths. These trusses are wide in one direction but only an inch and a half thick when viewed from the side. That narrow dimension doesn’t resist wind very well. It’s why we always recommend bracing for gable trusses. You’ll usually find this called out in our truss packages or on your blueprint from the home designer or engineer.
To make that truss more stable, we add wood bracing, either up to the roof or down to the ceiling. That bracing helps take the wind force and transfer it back into the structural system, where it can safely get down to the ground.
A Trickier Example: Dormers in Room-in-Attic Trusses
Let’s take a more complex example. Say you have a garage with a room-in-attic truss system, so there’s usable space above the garage. If you add a dormer to bring in some light or a view, you’re introducing new loads right in the center of the span.
To support those, we use girder trusses on either side of the dormer. Those girders carry the dormer load and transfer it into the walls, through the top plate, and down into matching posts. And again, those posts carry the load through the wall and into the foundation. It’s always about getting the weight to the earth.
Uplift: When Wind Tries to Pull the Roof Off
Most people think of wind pushing on the sides of buildings, but wind can also create lift, kind of like an airplane wing. As wind passes over a truss, it can try to pull it up off the wall.
We counteract that force with hardware. On the truss, we install hurricane clips. In the foundation, we use bolts, rods, or tie-downs that connect the framing to the concrete. The goal is to make sure the uplift force, like all the others, is carried back to the ground.
Every Force Has to Go Somewhere
Whether it’s snow pressing down, wind blowing sideways, or uplift trying to peel your roof off, every force acting on a structure needs to travel through it and get anchored to the ground. No matter where the load starts, its final destination should always be the earth.
