I've done the rafter trick also, but re-enforced the joists with another 2x4 and also cut a couple posts and wedged them between floor and joists for added strength.
That's a good point so I should have added to make sure your joists can carry the weight. I stick built my garage and code had me using 2 x 8's for joists and rafters. Factory made trusses are generally made of 2 x 4's so added support is definitely needed.
