acutally fix to is not a phrase, you need to separate the sentence into two parts, fix and to the building, the to here is more like an action or directioins of the boards done to the building. I've read some of your posts, you had similiar questions like east in london or something , my advice is don't think two words as one phrase but do analyze the meaning of the words first when reading, then you might get the relationship of the words.

It is short for affix and means "to attach". Ex. The boards were attached to the building.