Yungfong,
It has been a pleasure discussing your situation. I'm truly honored. And I will be glad to offer you my opinion. As long as you realize it's "only my opinion"... nothing more.
I hope I can call upon your help one day...smile.
I'm puzzled about your request to help you proving defect, fraud, negligence. I have no first-hand knowledge of your situation. Sorry if I misread the intent of your request.
It's up to you to provide the required proof.
But I, along with your fellow students here will help point you in the direction for finding case law, and procedural guidance. BTW, I hope some of the case law I provided you was useful.
Additionally, you might be careful on pursuing a fraud complaint. I came across some case law that appeared to make a distinction between deliberate fraud, and possible character slander. Basically, your contractor could have provided service in good faith. He could testify something beyond his control/knowledge...like substandard 3rd party building material was used.
In that situation, it's possible your complaint would have no merit. Unless you could prove something along the lines of a product recall was issued. Then it's possible you could argue that the contractor failed to perform adequate due diligence. In that case, you can't allege fraud!
Alleging fraud, where there is none, could open yourself to counter-suit! I forget the terminology; but they could state you impugn their good name, causing loss of business/customers. Prepare for that tactic to be used!
Again, this is all my opinion. Perform you own due diligence.
But it's something to consider...if you don't have irrefutable proof of fraud.
Finally, did you check the Better Business Bureau? You might find prior claims against the company.
Good Luck!