A reader's Roofs and roofing question: Rafter birdsmouth?

Mark D.   from   Australia   had this Rafter birdsmouth question.

Bill's answer

Hi Mark,
Short answer, it is a carpenter's easy location device.

Have a look at the glossary page for a couple of birdsmouth sketches.

• The birdsmouth provides a flat surface for the rafter to sit on when the rafter is first placed into position.
• It stops the rafter sliding downhill while it is being fixed.
• Later on of course, if the rafter is bolted to the ceiling joists, or fixed firmly in another way, then it just about becomes almost redundant.
• That is why we don't make the birdsmouth too deep. Just enough to do a job, but not enough to weaken the rafter.
• The vertical face of the birdsmouth is a locating surface. That is, if the walls are straight, then by bumping the rafter to be fixed up into contact with the wall, then we know it is in the right position.
• In general with modern stronger fixings they are going out of fashion, they are essential with nails only fixings.
• In position it helps to support the dead weight of the roof structure and added loads like snow.

Where I am, (cyclonic construction) birdsmouths are redundant.
Invariably timber rafters are bolted to steel brackets or steel cleats welded to steel beams. In this case each rafter is clamped, drilled and bolted with no need for birdsmouth cuts.
We don't waste time or more importantly strength with them.