The Building log can be obtained by using a Primitive saw or a superior metal Saw on any cut-down tree (Hard/Softwood log). It yields a number of building logs based on the size and quality of the tree the log came from. Hardwood trees have a higher yield than Softwood trees. A higher quality tree will yield a higher number of logs - it's likely with low quality trees to not get a single building log, it's all about cut-down tree durability (which depends on tree quality). If cut-down tree durability is less than 100, you won't get any building logs, if it's more, for every building log you saw, cut-down tree durability (or "Stones") decreases by 100.
With the Artisan skill Inspect Objects you can inspect a log to see its current durability ("Stones"). Since a building log uses 100 durability, a board uses 20, and a billet uses 10, you can avoid wasting some of the raw log by careful cutting. For example, if a log has 120 durability you could cut 1 building log which would leave the log with 20 durability. If you try to cut a second building log from it you will just waste the remain wood and get nothing, but you could use it to cut 1 board or 2 billets instead. That same 120 log could also be cut into 1 building log + (2 billets or 1 board), or 6 boards or 12 billets, or some combination of the two. To stop sawing (say, going for the 2nd building log) press ESC before the progress bar completes.
Major tree logs give twice the lumber of normal tree logs. Ill trees or saplings don't give logs and aren't worth cutting for lumber. If you have the ability to collect a sapling from high quality trees it is a good idea to collect some and replant them for future harvest.
- Click on Trees for a more detailed spreadsheet of how much logs are beeing obtained by cutting diffrent tree types.