If you are going that far I would check for cracks or termite damage, water damage etc. I would play it safe, I once tore off a porch roof and found a tin roof underneath, everyone said just go over it, I said rip it off, good thing, the beams were all damaged and cracked, and needed to be replaced or reinforced...better safe than sorry, and at least you will have peace of mind.
Dont forget to use some peel and stick waterproof rubber sheeting under the shingles (archetectual shingles cost a couple dollars more but are definately the way to go), you need to overlap it starting at the bottom of the roof with the next layer overlapping the one underneath, careful its slippery when you stand on it. If you do this you new roof will last longer and be very sturdy, and dont forget to let some rubber and shingles hang over the sides, of the roof, usually 3-4 inches is good. Add a gutter, on the bottom of the roof let the shingles lap into the gutter and then your all set... Hope you found this helpful.... Good Luck
Ps. Metal Tecos are very good and save some time. They are used to join your beams together
and make them much sturdier.