BIRCH REPLACEMENT SEAT BOARD, CIRCLE PATTERN. Circle Pattern Drilled Seat Board 19 7/8" sq. approx. size Pattern Dia. 10". Birch seat boards are sold untrimmed due to the different shapes and various sizes of chairs to be fitted. They can be cut with a band saw, scroll saw or saber saw. We also carry Decorative Nails. To determine the number of nails necessary, plan on spacing nails from 3/4" to 1 1/2" apart. For example, a chair seat 13" in diameter using a 1 1/2" spacing would require 28 nails.