Is $10,000 to repair a leaking flat roof deck over a garage a fair price?

I’m dealing with a leak in my deck that’s built over my garage. My deck has wood boards on top of an EPDM rubber waterproof membrane. Part of the deck rotted. We removed the damaged wood and cut out part of the EPDM to patch it. We had an individual contractor do the job. He fixed the EPDM with a EPDM tape and Lapel sealant and it’s still leaking! I want to repair it properly so it lasts through harsh winters with very cold temperatures and heavy snow. I'm also wondering if $10,000 CAD seems reasonable for a deck around 300–500 sq ft over a garage. I need to know what building code requirements I should be aware of when replacing a flat roof deck like this and how to make sure the new work meets code. I'm also considering if felt roofing is ever a better choice than EPDM for cold, snowy climates, or if I should stick with EPDM. I want to make sure I’m handling this the right way and protecting the garage below! Our deck sits on top of 2x4 wood, which sits on top of the membrane, which sits on top of plywood and that sits on top of joists.