I have been using Sketchup Pro for some time now and have found a little issue that I wonder if anyone knows the answer to solve.
If I have a face with (for example) a wood siding material, then I decided to enlarge that face by adding another part (not by moving a line, but by drawing in other lines), when I delete the line going through the middle of the enlarged face sometimes it will make the new area to have the wood siding material, and other times it will make the old wood siding material disappear and go with whatever the new section’s material is, which is usually the default if it is a newly drawn area.
I have never figured out why it does one way one time, and the next time it will do the opposite. Any ideas of how the software chooses which material to use? Any way to control that (meaning, I would like it to always fill the newer section with the older material)?
Thanks, Mike
As I recall it has to do with surface area. The material on the larger surface wins.
I have wondered that in the past, but it does not always prove true. Just made a long “wood” beam, then added an angled cut on the end. The 2% newer section won the battle. Which is why I decided to ask if anyone had an insight. 
I have been working with and against this for a year now, in various situations, and have not figured it out.
Probably not worth investing too much time in. Clearly there isn’t one hard and fast rule affecting it.
If there was a way to choose, I would like to know. It would not save me hours, but perhaps minutes. Here is a sample I came up with where the little area won. It took a little time to draw in various areas and finally find one where the little area did win, so I cannot tell you what I did or did not do.
I have neve posted a gif and am a newbie but I will try here …
Well, you could save those minutes and likely more by not applying the materials until the geometry is completed. I don’t apply textures until after all of the geometry is complete. The textures don’t help me create the model and they sometimes mask incorrect face orientations and other problems.
If I could just tell my customers not to change their floor plans when the project is 97% done! 