· by Michael Blair, Nashville's Roofers
A roof-over puts new shingles directly on top of the old ones, skipping the tear-off and the disposal. It is cheaper, it is faster, and in most cases it is permitted. It is also the decision most likely to be regretted, and the reasons are worth understanding before the quote arrives.
What the code allows
Building codes generally allow two layers of asphalt shingles on a roof and prohibit a third. That is a structural limit as much as a roofing one, because each layer adds dead load to a frame that was designed for a particular weight.
Allowed is not the same as advisable, and the code sets other conditions. A roof-over is not permitted over shingles that are curled, buckled or wet, over more than one existing layer, or where the existing deck is unsound. Those conditions rule out a fair number of the roofs it gets proposed for.
What you give up
The single biggest loss is information. Tearing off exposes the deck, and that is the only opportunity anyone will have to see rotten sheathing, old leak staining, failed flashing and inadequate nailing. Covering it means the next roofer finds those problems, at the next replacement.
The second is life. A layered roof runs hotter, because the old shingles insulate the new ones from the deck and heat cannot dissipate downward. In Middle Tennessee summers that shortens the life of the new shingles measurably, and most manufacturers reduce or void the warranty on an overlay.
How it looks and how it seals
New shingles laid over old telegraph every irregularity underneath. Cupped edges and worn ridges show through the new surface, and the roof reads as uneven from the street in a way that is obvious on a low-slope roof and in raking evening light.
Flashing is the other issue. A proper replacement renews the step flashing at walls, the boot at every pipe, and the valley metal. On an overlay these are typically reused or built up around, and since almost every roof leak starts at a penetration or transition, that is where the compromise costs you.
- No inspection of the deck underneath
- Higher shingle temperatures and shorter life
- Reduced or void manufacturer warranty
- Old flashing usually reused
- Telegraphing of the old surface through the new
When an overlay is a reasonable call
On a roof with one flat, sound layer, on a house being sold or transitioned within a few years, where the budget genuinely does not stretch to a tear-off, an overlay buys real time. It is a defensible short-horizon decision made with the facts in hand.
It stops being defensible when it is sold as equivalent. If a quote presents a roof-over and a tear-off at similar life expectancy and similar warranty, the quote is wrong. Ask specifically what the shingle manufacturer warrants on an overlay, and get the answer in writing.