How to Modify Wood and Attic Trusses
Quick Column Summary:
- Can I modify trusses to add a room
- Work with a structural engineer
- Take off roof to make it manageable
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' ceiling at its peak. 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).
There is 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). Trying to decide if it's all worth it."
Here's my answer:
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.
Here's the 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 and you'll also be dealing with a low heel height on the existing trusses.
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 wall sheathing is.
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 ceiling is just out of the photo frame.
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.
If you have limited manpower, you'd do this in sections. I would just rip off about six feet of existing roof at a time leaving the rest in place for weatherproofing. 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.