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I have access from unfinished basement to floor trusses and had a very good result eliminating a significant creak off one corner of our fireplace in the living room. I took measurements upstairs so that I was able to locate the creak from the basement. After inserting a couple of quality wood shims in a gap between the floor truss and the subfloor the creak was gone. I had other locations that shims had no effect, and screws were effective. Hope this helps, good luck with your project.