Deciding whether to re-roof or completely replace your roof is an important decision; there are a number of factors to consider. One consideration is the age and expected lifespan of your current roof. Also, the costs involved. A repair is quicker and will always be more cost-effective in the short term, but the long-term costs are a serious consideration.
During re-roofing, contractors layer a new shingle overlay over the existing shingles, there is no way of knowing if any damage exists underneath, such as rot or sagging, re-roofing can only be performed over a single layer of shingles.
During complete roof replacement, contractors remove all of the shingles, exposing the deck underneath. If the deck is badly damaged it has to be repaired, otherwise, a hole will form. Underlayment is applied to the deck, which protects it from water penetration and subsequent leaks, then new shingles are laid on the top. Roof replacement allows any damage under the shingles to be identified and repaired before serious structural problems can develop. A complete roof replacement lasts longer than re-roofing. The new roof will have a lifespan of at least 20 years. Long term, a complete replacement reduces the probability of expensive future repairs.