Shingles shed water; they do not seal a roof. The waterproofing system is underneath: underlayment (the membrane over the deck) and flashing (the metal work at every joint, valley, chimney, and wall). Most "new roof leaks" are flashing or underlayment failures, not shingle failures.
Underlayment options
- Synthetic underlayment — the modern default: tougher and more water-resistant than felt, standard in our published rates.
- Asphalt felt (15/30 lb) — the older standard; still code-legal in many areas, less durable under exposure.
- Ice-and-water shield — a self-adhering membrane for eaves, valleys, and penetrations. Code requires it at eaves in cold climates; in wet climates like the Northwest, generous coverage is cheap insurance.
Flashing: replace, don't reuse
Step flashing, counter-flashing, valley metal, and drip edge are included in our published installed rates as standard flashing. Reusing old flashing to save a few hundred dollars is the most common corner cut in roofing — flashing ages on the same clock as the old roof. If a bid is mysteriously cheap, ask whether new flashing is included. Chimney re-flashing on complex masonry can be a legitimate separate line; ask for it itemized rather than bundled.
What this means for your estimate
Every range on this site assumes new synthetic underlayment, ice-and-water shield at eaves and valleys, and new standard flashing. Bids that beat our low end by a wide margin are usually subtracting exactly these invisible items.
Run your own numbers: the satellite estimator and the roof cost calculator are both free, and both use the published dataset on our methodology page.