question on the roof structure of my garage

Question

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:

Answer 1

In order:

a) You can do it! You didn't mention anything about trying to move the existing structure, so this is based on my understanding that you are simply adding the tie back in to prevent further spreading. Personally I would use something more like a 2x8 or 2x10; a 20 ft. span is a long way for a 2x4, and you'll likely see some sag. I would put 5 nails into each rafter, in a rough square pattern with one in the center. But, you'll want to offset the nails a bit from each other so that they aren't going through the same grain (which can make it easier for a crack to split the wood where the nails are). You can cut the ends at the same angle as the rafters to get a longer surface to nail through.

b) If you do end up hiring someone else, a handyman could certainly handle it.

c) From your pictures, one tie every other is fine: the plates of the walls aren't going to move with a tie every 4 feet, and additional ones aren't going to do much.