How to Modify Wood and Attic Trusses
Here I am alongside a truss being used over a giant garage. This truss has a special elevated bottom chord. Photo credit: Todd Fratzel
Modify Wood and Attic Trusses TIPS
- Trusses can be modified or changed
- Structural engineer must be consulted
- WATCH truss videos below!
- Replace with common rafters
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Mike, from Cleveland, Ohio, wrote to me:
Tim, great reading your tips. I've got a question, I would like to convert the attic above my garage into a living space. Access is not an issue as it's also in front of another room upstairs that I can put a door in the wall.
There's plenty of room to make a 12 x 12 room with 10-foot ceiling at its peak.
Common Trusses
Unfortunately, my home builder had framed out the attic using a common truss system utilizing 2x4's (see attached picture of exact type, although my roof is much steeper, and no middle non bearing wall below).
This is a graphic Mike supplied to me. I don't know if he drew it, but I sort of doubt it. Image credit: Unknown skilled person
There's enough space to switch the floor joists that are 2x4s with 2x10s or 8s, and/or sistering them. The main issue I have what to do about the truss itself (2x4 webs, 2x4 chords, etc.). From what I imagine larger rafters would need to be in place of those 2x4 top chords.
Is this possible, and how costly would a project like this be? (just the structural engineering and framing aspect to allow for live loads and support the roof). I'm trying to decide if it's all worth it."
Changing Trusses
You can modify the trusses, but it's an enormous amount of work. Also, if you decide to do this, you MUST work hand-in-hand with a licensed structural engineer that's familiar with wood trusses.
Huge Challenges
Here's one challenge. If you feel you'll be sliding new larger common rafters up inside the attic alongside the existing trusses, you're dreaming.
You'll have shingle nails in your way that extend through the roof sheathing. You'll also be dealing with a low heel height on the existing trusses.
Heel Height
Most common trusses have a miserable heel height of just over four inches.
The heel height is the vertical distance up the truss face where it passes over the OUTER EDGE of the building line or supporting wall. This is typically where the face of the exterior wall sheathing is.
Wood Truss Videos
Watch these two videos to see very unique things truss engineers can do to create higher ceilings and large overhangs.
Attic Trusses
The sad fact is your builder could have installed attic trusses over this garage for just slightly more than what the existing trusses cost. Attic trusses have a rectangular shape inside each truss.
If the roof slope is steeper than 9 inches in 12 inches of run, it doesn't take much width to end up with a nice room 12 feet wide with an 8-foot ceiling. I have this above my own garage.
Look at this photo of the space created by attic trusses at my home. You can't see the floor, but the window sill is just about 32 inches up off the floor. The flat ceiling that connects the two sloped parts of the truss is just out of the photo frame.
This is the end wall of the attic above my garage. The space is 12-feet wide and ceiling height is 8 feet. I didn't build this house, but the architect did everyone a favor by making the vast space above the 40-foot-long garage out of attic trusses. I took part of this giant room and made a cozy man cave ham-radio shack. © 2017 Tim Carter
Demo Roof
You may find it easier, as crazy as this sounds, to rip off the existing roof and start over. I guarantee you it would be FASTER.
You'd be able to use regular 2x10 or 2x12 rafters and create a gigantic room where now exists useless space.
If you have limited manpower, you'd do this in sections. I'd just rip off about six feet of existing roof at a time, set the new rafters, sheath them and cover them with tar paper.
You'd have a small gap between the new and old roof you'd need to weatherproof in case rain was in the forecast.
It would help TREMENDOUSLY if you increased the new roof pitch so the new roof ridge board would rest on top of your existing trussed roof.
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