Split Jacks When Framing a Window Opening
Split jacks let you work faster, but they introduce another hinge point in the wall, which could be problematic in high-wind and high-seismic-risk zones.
When framing a window opening, should the jack or trimmer studs be continuous, or should they split around the rough sill?
—Jerry Riga, via email
This is an interesting question with no clear answer. Jack studs (or trimmer studs—the name depends on where you’re located), provide the load path from the header to the bottom, or sole, plate. Not even the IRC is clear about whether they can be split. Section R602.3 reads, “Studs shall be continuous from support at the sole plate to a support at the top plate,” but then includes an exception for “jack studs, trimmer studs, and cripple studs.” The wording is tricky. It could be that the IRC makes this exception because jacks are always cut shorter than regular stud height in order to fit below the header, or it could mean it’s OK for the jacks to be split around the rough sill. While all the drawings in the IRC show continuous jacks, and many inspectors will fail split jacks, this appears to be a regional preference.
Some carpenters like split jacks because it allows them to nail through the king stud into the end of the rough sill (with continuous jacks, the connection to the sill is normally toenailed), and the lower portion of the split jacks support the rough sill without having to add cripple studs. Simply put, using split jacks lets you work faster.
However, split jacks do introduce another hinge point in the wall, which could be problematic in high-wind and high-seismic-risk zones. Another issue is the horizontal wood grain in the rough sill. Wood has much more compressive strength parallel to the grain than across it, so there’s a chance of the rough sill crushing under, say, a heavy snow load. The counter to this point is that all the plates and the header run horizontally, so what’s the difference?
Although it is frowned upon by advocates of advanced framing—who strive to avoid all unnecessary framing—you could also run the jacks continuously and add cripple studs below the ends of the rough sill. This supports the sill and avoids unwanted conversations with your inspector, although placing lumber where insulation could be installed increases thermal bridging.