When a roof wears out, there are two ways to put the new one on. You can tear the old roof off down to the deck and build the new system on bare wood, or you can leave the old roof in place and install the new one right over it — a roof-over, also called an overlay or a layover. The roof-over is cheaper on the day it’s installed, and that price difference is the whole sales pitch. Here’s why, on almost every house we look at, we recommend the tear-off anyway — and the narrow set of cases where we’ll say an overlay is defensible.
What A Roof-Over Can’t Show You
The deck is the foundation everything sits on, and a roof-over buries it unexamined. Rot, delamination, and water damage don’t announce themselves from the attic in every case — plenty of bad decking looks fine from below and crumbles from above. When we tear off, we see every sheet, and bad wood gets replaced before the new roof goes on. When a roofer overlays, whatever is under there stays under there, and the new roof’s lifespan gets built on it anyway. Trapped moisture between the layers makes it worse: the old roof keeps rotting, quietly, under the new one.
An overlay also telegraphs the old roof’s problems upward. Old shingles that are curled or uneven print through the new layer, shortening its life and its looks. Valleys, flashings, and edges — the details that actually keep water out — are harder to rebuild properly over an existing layer than on a clean deck. Fasteners are another quiet problem: nails for the new layer have to be long enough to reach through two roofs and still bite into the deck, and when they miss or land in softened wood, wind uplift finds them first.
Code And Weight Set Real Limits
California code caps an asphalt shingle roof at two layers total, and no code anywhere allows a new layer over a deck that is deteriorated or water-soaked — which is precisely the condition you can’t verify without tearing off. Many cities and HOAs in Southern California are stricter than the state floor. Tile, for the record, isn’t an overlay material at all: the options are lift-and-relay or full replacement.
Weight matters too. One layer of asphalt shingles weighs roughly 200 to 300 pounds per square. A second layer doubles that dead load on the framing — 24 squares of roofing becomes an extra two to three tons the structure carries every day, for decades. Most framing handles it; some older framing shouldn’t be asked to.
Warranties And Manufacturer Rules
Shingle manufacturers warranty their products installed the way their instructions describe — over a clean, sound, properly prepared deck. A roof-over can reduce or void coverage depending on the manufacturer and the condition of the layer beneath, and shortened shingle life over an old layer is a well-known pattern: the old roof’s heat and uneven surface age the new shingles faster. The money “saved” by skipping tear-off often comes back as years shaved off the new roof.
What An Overlay Does At Resale
Inspectors flag multiple layers, and buyers discount for them — because a two-layer roof means the next roof is a full tear-off of everything, at full tear-off cost, on the buyer’s watch. If you plan to sell, know that roof certifications weigh the whole system; a clean single-layer roof with a verified deck is simply an easier roof to certify, finance, and sell than a layered one with an unknown deck under it.
The Rare Case Where An Overlay Is Defensible
Almost always isn’t always. An overlay can be the honest choice when all of the following are true: the existing roof is a single layer, lying flat and verified sound; the deck condition is known, not assumed; the framing clearly carries the added weight; and the goal is a deliberate short bridge — a few years of service life on a secondary structure, for instance — with eyes open about the trade-offs. What we won’t do is sell an overlay as the equal of a tear-off at a friendlier price. It isn’t, and pretending otherwise is how homeowners end up paying for two roofs and getting one.
How We Price The Wildcard
The standard objection to tear-off is the unknown: what will the deck look like? That’s a proposal problem, and it has a proposal answer. Every Mars Roofers tear-off proposal states the deck terms in writing up front — how many sheets are included or the per-sheet price if rotten decking is found — so the day of tear-off holds no surprises. Read our sample estimates and you’ll see the deck line in plain language, before any signature.
Getting bids on a re-roof? Ask each roofer one question: tear-off or overlay, and why. Then call 310.880.9718 and ask us. We’ll walk your roof, show you the photos, and put the recommendation — and the deck terms — in writing.