You can raise it up with more sand and gravel until you reach the desired height.
Raising my garage roof.
It has a pitched roof using modern w style trusses and concrete tiles.
You could brace the inside of your garage and use hydraulic lifts to raise the roof.
You might also try raising the actual garage itself but this obviously has structural challenges.
Lift garage poor small wall and set garage ontop of wall.
Consult a contractor for an estimate.
If you want to raise your garage roof you might as well just tear off the old roof scrap it including your top sill plates.
Ways that have crossed my mind but of which i have no idea that would pass muster would be.
Before applying the finish siding install metal flashing over the sheathing and let the flashing run 3 in.
Currently whilst the pitch roof is of substantial height the trusses get in the way of access the full height.
Below the top of the foundation wall.
If a gap doesn t close up try laying a plank across the roof and pound on it with a sledgehammer.
Additional support may be needed if the roof is not very stable.
Lift garage lay a few course of block and sit garage back down on blocks.
I have a detached brick built garage with the usual roof trusses.
The major downside to it is the height of the door opening at about 1 9m and the floor to ceiling height of approx 2 4m.
If you need to lift the walls again the gap will stay covered.
Build a small framed wall of desired height increase lift garage and secure to new mini wall.
Approaches to raising the roof.
Then run new studs up adjacent to your existing studs and put on new sill plates and a new roof.
If you are in a situation where you can t remove the posts then you can attach extenders to the top of the posts that are already in place.
I doubt if you can use your existing trusses.
Finally fill in the outside of the wall with tapered pieces of sheathing and siding.