Reroofing over vs tear-off
When a roof reaches the end of its life you have two paths: lay new shingles over the old layer, or strip everything to the deck first. The choice affects lifespan, warranty, cost, and what your code allows.
What each approach means
An overlay (also called a roof-over) installs a new layer of shingles directly on top of the existing one, after re-nailing flashings and adding new starter and ridge. A tear-off strips all existing roofing down to the deck, which then gets inspected, repaired as needed, and re-covered with fresh underlayment and shingles. The difference is simply whether the old material stays in place or comes off.
The layer limit and your local code
Building codes generally permit a maximum of two roof coverings on a structure — so an overlay is only allowed if your roof currently has a single layer, and a roof already carrying two layers must be torn off before re-covering. Many jurisdictions now require a tear-off even from a single layer, and some climates or districts have stricter rules. These specifics vary by location, so always confirm with your local building department before deciding.
Lifespan and warranty
Overlays tend to last meaningfully less than a fresh tear-off — commonly cited around 16 years versus 20–30 for a tear-off with new materials. Two stacked layers trap heat, which ages the new shingles faster, and any problems in the deck or old layer stay hidden underneath. Many manufacturer warranties only apply to tear-off installations, so an overlay can leave you without full coverage if a defect appears.
Cost and what gets skipped
An overlay is cheaper up front because it skips tear-off labor and dump fees and avoids deck work. But that saving comes from skipping the inspection a tear-off provides, so hidden rot or damaged decking goes unaddressed. On a cost-per-year basis, the shorter lifespan and warranty often make tear-off the better value despite the higher initial price. Tile, slate, and metal substrates almost always require full removal first regardless.
When an overlay makes sense
An overlay can be a reasonable choice when the existing roof has a single sound layer, the decking is known to be good, the structure can carry the added weight, and you have a near-term reason — a pending sale, an approaching season, or a short ownership horizon. Outside those conditions, a tear-off is usually the longer-lasting decision. Either way, expect a permit; see do I need a permit to reroof.