Installing new roof shingles over your old roof shingles will void your manufacture warranty.
The double layer of shingles will cause excessive heat build up in your attic which will make your home
hotter in the summer along with years off the of your roof shingles.
The added weight is that of a small car on your roof will cause structural damage to your roof like
sagging hips, rafters, valleys and ridges of your roof structure leaving your roof and home more venerable
to high winds and storm damage.