A build plate unit (400) for a three-dimensional printer includes a build plate (410) having an optically transparent member comprising a flexible, gas permeable, optically transparent sheet (414) having upper and lower opposing sides, wherein the flexible sheet upper side defines the build surface; and a flexible, gas permeable or impermeable, optically transparent base layer (412) on the lower side of the flexible sheet opposite the build region, wherein the build plate is configured to permit gas flow to the build surface.