@donmccurdy @mrdoob shouldn’t that be by default 1.0 if metalnessMap is defined (checked the docs and it is the case for roughness and roughnessMap) ? Sounds like a major overlook in three.js design 
Tbh, I had to double check just to be sure myself, and by some luck my code already had it set to 1.0 if metalnessMap is present, but it seems so easy to miss if someone doesn’t read the docs line by line.
(Edit: I realise setting it to 1.0 would spawn a 1000 issues and tearful complaints on github, but it kinda feels like a good idea to fix it now and avoid future users complaining about PBR not working at all.)