Why are we still cutting birdsmouths in rafters? We now have power tools that can easily rip a continuous beveled strip that can be added to the top plate to attach the rafters. Plus, we have steel connectors to ensure the attachment.
A. Mark McKenzie, an engineer in Brewster, Mass., responds: Although this alternative framing method could work in some instances, I think that the issues raised outweigh the benefits when compared with conventional (and properly executed) birdsmouth rafter cuts.
To begin with, how well the rafter was attached would depend directly on how well the ripped strip was attached to the plate, which would require an engineered attachment schedule to ensure that the ripped strip stayed put. And my guess is that the narrow strip would most likely split when you tried to nail through it.
Another problem with this method occurs when the rafters attach to the top of a wall instead of to a plate on the floor deck. The angled strip on top of the wall plates would prevent the ceiling/attic joists from bearing properly on the top plate, and the strip itself would not provide adequate bearing for those joists. Placing the joists on the strip above the plate would also make it more difficult to attach the top edge of the wallboard.
Structurally, a rafter sitting on an angular bearing point (the inclined plane of the ripped strip) would require that the rafter-plate connection deal with the horizontal and vertical components of the force differently than with a birdsmouth. A lot of force is transferred down the length of the rafter, and the seat cut on the birdsmouth transfers that force directly to the top plate. With the ripped strip installation, there would be no horizontal bearing surface to resist that force. Granted, a structural ridge can lessen the amount of force on a rafter — but even then, I would not attempt a ripped-strip rafter installation without having the entire system approved by an engineer.
The solution of simply adding metal clips can also be problematic. Again, an engineer should be consulted to ensure that any metal clips used are rated for the additional loading from the elimination of the birdsmouth, in addition to dealing with any standard regional conditions such as wind uplift or seismic.
In general, while the ripped-strip installation does eliminate the need to make two cuts for the birdsmouth, it’s debatable whether there are any real labor savings. Ripping the attachment strip requires a completely different setup with a different tool, whereas most framers I know cut the birdsmouths as just a small additional part of rafter-cutting “production.” Those two cuts add just a few seconds to the whole process.
As a final admonition, make sure that any framing detail — such as the rafter-plate connection — is done in a properly engineered manner. It’s always better to have an engineer review and approve your methods before you start cutting and nailing.