My house in Northern California has a gable roof. In the attic, there are two 2x4 angle braces (one at each gable end) that connect from the ceiling joist (gable-end) to the ridge board (mid-span). Please see photo to understand what I mean.
What is their purpose/name? Can I can do something with them since they cross the attic space and reduce head space. Thanks
It’s difficult to see, but I suspect your gable end wall is not ballooned framed, but rather has a double top plate. This creates a “hinge” in the wall.
The diagonal braces keep the end wall from bending like a hinge during a wind storm or seismic activity. (I know, you’re going to tell me the ceiling will keep everything square and plumb. However, ceilings can’t give you a load resistance value to maintain the wall.)
Get a framing manual for you location at look in the chapter on bracing and/or stick-built roofs,
You can get the equivalent shear resistance by bracing the rafters on both sides with the same sized lumber. you'll have to cut the brace where it crosses the underpurlin. And it'll need to be fixed to every rafter (some of which will be hard to reach) and to the underpurlin and the ridge board, and you'll need 4 braces instead of two so you'll end up using more wood)
there may be a way to use steel strapping instead of wood, you'll need to check the framing manual.
Some disassembly may be required to fix the corner ends.
