Stacking crafting and building materials (wood, logs, metal) is a little flawed:

• logs and firewood can't be combined (except for putting them into a building site/camp fire and taking them out again)
• wooden planks can be combined but not in the number displayed in the UI (display: 20, de facto: 15; if taken out of a construction site, only 10 are taken at a time)
• corrugated metal: if wooden planks and metal are in the same building site, selecting the respective building option will construct the opposite material (could also be a mistake in the German localization, but I didn't check it)