> You need planks that are crossed with each other, which require good quality saws and a tight fit. You don't need saws at all. Initial shaping with an adze,

> You need planks that are crossed with each other, which require good quality saws and a tight fit. You don't need saws at all. Initial shaping with an adze, and subsequent flattening/fitting of the two pieces with friction and an abrasive like sand would give you closely fitted wood surfaces, though not necessarily all that flat. Note that a somewhat similar technique was used to fit irregular stone blocks together with high precision by the Inca. You could also create a flat wood surface by abrasion against a flat stone surface, but that is more labor intensive to produce.