Roof Truss Calculator: How Many Trusses Do I Need?

If you’re building or remodeling a truss-supported roof, one of the many things you’ll have to consider is how many trusses you need. Thankfully, there’s an easy way to calculate this—as long as you know the length of your roof and the amount of spacing between trusses. Here’s how to figure it out.

How Many Trusses Do I Need for My Roof?

In residential construction, there’s usually 24 inches of on-center spacing between roof trusses. Using that as a guideline, here’s how many trusses you would need for your home, depending on the length of your roof:

How to Calculate the Right Number of Roof Trusses

If you’re replacing trusses in an existing home, you can determine how many new ones you need simply by counting the old ones. However, if you’re building a new house, you can use a formula to figure out how many trusses you need.

Assuming you leave the standard 24 inches of space between trusses, your roof’s length is the only measurement you need to calculate the right number of trusses for your home. When you have that, you can plug it into this formula: 

For example, if your roof is 40 feet long, you’d need 21 trusses. Adding the extra truss at the end ensures you’ll have one truss at the front of your roof, then another truss for every 24 inches of roof length, and one more truss at the back of your roof.

How to Measure Your Roof for Trusses

Measuring your roof for trusses isn’t something you should do alone. Instead, you’ll need to work with a home builder near you or pay for the cost to hire a structural engineer to find your roof’s measurements, calculate the right number of trusses, and pull the necessary building permits. During that process, these professionals will provide all the guidance you need for measuring and installing trusses.

Who to Hire to Install Roof Trusses

If you have carpentry experience, you could build your own roof trusses instead of ordering them from a truss manufacturer. By doing so, you could save $70 to $560 per truss in material costs (not including labor), depending on the truss size.

However, when it comes to installing trusses, it’s far safer and easier to hire a professional. This is a job that requires you to follow local building codes, pull permits, work at tall heights, and lift heavy objects. 

Plus, trusses are a vital part of your home’s framework—and if you make a mistake during installation, it could affect the structural integrity of your house. On average, hiring a pro to frame a roof costs $13,000 to $22,200.