Don’t Cut That Truss Until You Read This
Suppose you’ve got a set of pre-manufactured trusses on site and something isn’t quite right, maybe a piece got damaged during delivery or it doesn’t match the field conditions exactly. In that case, you’re probably wondering: Can I cut or modify this truss?
The short answer is: sometimes, yes. But don’t grab your saw just yet.
Always Start with the Truss Manufacturer
Before making any field changes, contact your truss manufacturer. It’s tempting to fix the issue on the spot, but an engineer needs to review the situation first. If you skip this step and make an alteration on your own, it could result in more work later, including removing what’s already been done. Once an alteration is made, the options for repair become more limited and more complex.
What Happens When a Truss is Too Long?
Let’s say you’re dealing with a roof truss that’s six inches too long. Maybe it was supposed to stop at the back wall of a garage, but now it extends past it. This is a manageable problem. You’d report the issue to the truss manufacturer, who would send it to the engineer. The engineer might give you the green light to use the truss as-is, or they might send back instructions to cut and reinforce the truss.
In this scenario, part of the truss, maybe a web at the end, might need to be removed. The engineer would then provide repair details, like adding a plywood or OSB gusset with specific instructions on size, nailing patterns, and reinforcement methods.
When Plumbing or HVAC Interferes with a Truss
Floor trusses are another common issue area. For example, a plumbing drop might need to go right where a floor truss is installed. Ideally, this would be planned ahead of time, and trusses would be spaced or designed to accommodate the drop. But in real life, plumbers often show up after trusses are in place and drill through a top chord. Now what?
Again, send a diagram of the modification to the truss manufacturer. The engineer will return a specific repair detail that shows how to restore the truss’s structural integrity. This might involve adding lumber, plywood gussets, or steel angle reinforcements.
Don’t Cut That Web—Yet
Attic HVAC installations are another frequent reason people want to modify trusses. If a web is in the way of the furnace or access platform, it may be tempting to remove it. But remember, trusses function using triangular geometry to distribute loads. Remove a web, and that triangle (and the load path) is compromised.
In these cases, the engineer may provide a redesign that still supports the load. This could involve installing a new web in a different direction or reinforcing the top and bottom chords to maintain strength without the original triangle.
Drilling Through Bottom Chords? Not So Fast
Sometimes you’ll find someone’s already drilled holes through a bottom chord to run plumbing or electrical. It happens more often than it should. The repair might involve adding another member alongside the existing one or reinforcing the area with steel. Whatever the solution, you’ll need specific guidance from the engineer.
A Final Word on Truss Modifications
Modifying or repairing trusses is usually possible, but only with approval from the manufacturer and clear, engineer-stamped instructions. That’s the best way to avoid extra work, prevent liability issues, and make sure the truss still performs as designed.
Got a modification question? Always go straight to the source: your truss manufacturer. It’s the simplest way to get it done right the first time.
