Q&A / 

Can I Cut Attic or Roof Trusses?

Quick Column Summary:

  • How to create room in attic
  • Comparing roof trusses with roof framing
  • Get a structural engineer involved

DEAR TIM: My family is growing and I need to move my home office. I looked up into the attic over my garage that’s built with standard roof trusses. There’s plenty of room to create a room. How do I cut and modify the trusses to create an open space like the attic in the house I grew up in? It doesn’t seem to be that hard to do. I need to get started soon as the new baby will be here before you know it. Kevin B., Silver Spring, MD

DEAR KEVIN: If you want to live to see that new addition to the family, make sure your reciprocating saw stays in its case for now. The good news is you can convert that space above your garage into a room, but you’re going to go about it with some help from a registered structural engineer.

Let’s compare common roof trusses with the roof framing materials that were used in your parent’s house. While they look the same, you probably didn’t notice the subtle differences.

Attic trusses are complex structural elements of a building. They’re designed by registered engineers in almost all situations and you should never cut any part of a truss without consulting an engineer first. Photo Credit: Todd Fratzel

Attic trusses are complex structural elements of a building. They’re designed by registered engineers in almost all situations and you should never cut any part of a truss without consulting an engineer first. Photo Credit: Todd Fratzel

Most common attic trusses for homes and garages are made with 2x4’s. Typically the truss designer will specify a very strong wood species like southern yellow pine or even douglas fir. A 2x4 on it’s own can’t support lots of weight without cracking. So how do they do it when you consider how heavy shingles are and possibly 2 feet of snow on a roof?

Trusses will have angled pieces of 2x4 that connect between the sloped top chord member and the horizontal bottom chord. These are referred to as webbing. Each piece of webbing where it contacts the sloped top chord acts like a support beam to the otherwise flimsy 2x4.

You’ll notice a common truss is almost always a series of interconnected triangles that make up one giant triangle. A triangle is an incredibly strong structural shape. This is why you see this same design used in massive bridges that span hundreds of feet over rivers and canyons.

Let’s look at the attic in your parent’s house. Typically you’ll have a long common rafter that extends from the roof peak down to one of the exterior walls. This rough lumber is not a 2x4. At the very least it’s a 2x6, and it’s probably a 2x8 or 2x10. The attic floor you walked on as a kid was no doubt made with similar larger dimensional lumber. These pieces of lumber, when spiked together with long nails, also created a giant triangle.

The horizontal bottom chord of the truss, as well as the attic floor lumber at your parent’s home, hold the sloped top chords and common rafter is position so the roof doesn’t kick out and collapse.

An engineer who knows all about roof truss design may be able to come up with a design that allows you to modify all the small angled pieces of 2x4 that connect between the top and bottom chords. I guarantee you he’ll come up with a design that requires you to install a much larger bottom chord. This will not be easy to do, but it’s possible.

I suggest you take measurements of your garage as well as great photos of the exterior and photos of your existing trusses. You’ll need the width of the garage and the height of the trusses measured from the tip of the peak down to the bottom of the bottom chord.

Take all of this information to your local truss fabrication plant. You can locate these via a trip to your local old-fashioned lumber yard. The lumber yards order the trusses they sell from these local plants. Call and make an appointment to meet with the plant engineer. Most will accommodate you as they don’t want to read about you in the newspaper.

The engineer can look at your photos and measurements and tell you within minutes if there’s a chance at modifying your existing trusses. If you get good news, ask how much it would cost for him to complete a design drawing that you can follow. Be sure to ask for the step-by-step directions on how to provide temporary support to a truss as you start to modify it. You just can’t start to cut and hope all works out!

To get a quick idea of what you’re trying to do, you can get a drawing of the truss that should have been installed above your garage when it was built. Attic trusses don’t cost that much more than a common truss and it’s a shame more builders don’t use them all the time. Your local lumber yard often can produce this drawing while you wait using computer software. You’ll see what the engineer may come up with before you even meet with him.

I have attic trusses that span my garage giving me a room that’s 12 feet wide and has a ceiling height of 8 feet. Part of the walls of the room are sloped, but that’s okay. I’ve also used attic trusses on jobs where a standard rectangular room was created that had straight walls floor to ceiling. Good luck, take your time and be safe!

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2 Responses to Can I Cut Attic or Roof Trusses?

  1. Quick Question….I'm installing a Whole House Fan in my ceiling. I have a Truss-Style Roof. My question is can I cut one Joist to fit my 30" Whole House Fan. The joist are 24" on center. My plan is to use the "jack" stud and "king" stud system to support the joist that I cut. Is this all ok to do?

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