How to Measure a Bird's Mouth

A bird's mouth is an angled cut used in construction. Bird's mouths are used to fit wall joists into rafters or fashion brick architecture. The trick to succeeding with a bird's mouth measurement is to make all measurements as accurate as possible. When in doubt, always err on the side of making the cut too small as opposed to too wide. You can always go back and trim the angled cut but you cannot add back what you have already removed.

Measuring tape
Pencil
Saw
Safety glasses
Gloves

Instructions

- 1
  Measure the horizontal cut, called the seat. Trace a horizontal line straight across the surface of the rafter or stable brick. This line marks where the top of the joist (or other brick) must meet flush with the rafter (or stable brick).
- 2
  Subtract the thickness of the wall. Measure the wall -- six inches, for example -- and subtract it from the length of the seat line. Dot this mark on the seat line.
- 3
  Trace the plumb, or vertical, line. Align the measuring tape vertically with the dot on the seat line. Trace the vertical line, straight down to determine where the side of the joist (or other brick) meets flush in the rafter (or stable brick). Cut out the "bird's mouth" angle to fit in the frame.

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