This is a follow-up question on how to carry 20ft lumbers to home. Thanks for the advices and comments. I learned a lot. :) Because the question is now diviated from the original question, I made a new thread.
My garage is about 20 ft x 20 ft and has the hip roof structure. It has a long beam (rafter tie) that connects the front and the rear rafts and three beams that connect the right and left rafts. The three rafter ties are all located in the rear side. There was one rafter tie in the front side but was removed by a contractor about 3 months ago. The reason is that, we replaced the roof of the main house about 8 months ago and the roof workers put all the materials on the garage roof (which I still do not understand). Since then, the front side of the garage sunk a little bit, which caused the front rafter tie to be hit by the garage door when it was opened. When I talked to a contractor about this problem, he tried to lift up the front rafter tie but found that the nailed parts on one rafter got damaged and couldn't lift it up. So, he simply removed the rafter tie, telling me that it should be fine. Now that I learned that it is NOT ok, I plan to install rafter ties in the front side.
a) Firstly, is it something that amateur DIYers like me can do? Or, is it better to be done by a professional? Because it is a structure problem, I am concerend with any possible mistake which might lead to a severe consequence. But, if it is a simple job (for example, just nailing 20ft 2x4 to the rafters), I probably can do.
b) If I hire somebody else, whom should I contact? Would a handy-man be able to take care of this? Or, should I contact a professional structure engineer?
c) In general, how many rafters would be needed for 20x20 garage? Would it be even better if I add more rafter ties beside the removed one?
For the reference, I am attaching three photos of the roof structure: