Your roof is twenty years old, the shingles are curling, and the first roofer through the door says the cheap way is to leave the old roof where it is and lay the new one straight over the top. It sounds reasonable. It saves a dumpster, a day of labor, and a few thousand dollars. Whether it is actually allowed depends on what is already up there.
Sometimes it is fine — but the code is picky. California's rules let a roofer install a new covering over your existing roof only in specific situations, and flatly prohibit it when the roof already carries two or more layers of any covering, when the existing roof is water-soaked or deteriorated, or when the covering is slate, clay, cement or asbestos-cement tile (§ 705.3).
Key numbers
| What | The rule | Where it comes from |
|---|---|---|
| Existing layers that rule out going over the top | 2 or more applications of any covering | § 705.3 |
| Layers left after a true roof replacement | None — strip down to the roof deck | § 705.2 |
| Maximum buildup of self-adhered underlayment at walls, valleys, edges and laps | 2 layers | § 705.2 |
| Minimum slope waived on low-slope re-roofs that already drain properly | 1/4 unit vertical in 12 horizontal (a 2 percent slope) | § 705.1 |
Two different jobs, two different rule sets
The code splits re-roofing into two jobs, and almost every argument with a contractor comes down to which one you are actually buying.
A roof replacement means the old roof comes off. The code defines it as removing all existing layers of roof coverings down to the roof deck — the structural sheathing your roof is built on (§ 705.2). The building code repeats the same definition for the roof assembly materials (§ 1512.2). A handful of narrow exceptions let certain adhered membranes stay put when the sheathing beneath them is sound and the manufacturers approve it, but the starting point is a clean deck.
A roof recover is the cheap option: a new covering installed on top of the old one, left in place (§ 705.3). This is what your roofer is proposing when they say "we can just go over it."
Either way, the materials and the way they go on have to meet the roofing requirements of the building code (§ 705.1). Going over the top is a shortcut on labor, never a shortcut on materials.
When going over the top is allowed
A recover is permitted where any one of these is true (§ 705.3):
- The new covering is installed following the roof covering manufacturer's approved instructions.
- The new roof is a complete, separate system — a standing-seam metal panel system, for example — that carries its loads straight into the building's structure and does not lean on the old roof for support.
- The new covering is metal panel, metal shingle, or concrete or clay tile going over existing wood shakes (with an important catch, below).
- A new protective coating is going over an existing coating, a metal panel, a built-up roof, spray polyurethane foam, metal shingles, mineral-surfaced roll roofing, modified bitumen, or a single-ply membrane. Coatings do not require tearing anything off.
When it is off the table
Those allowances all collapse if any of the following is true (§ 705.3):
- The existing roof is water-soaked or deteriorated to the point that it is no longer an adequate base for more roofing. A rotten deck does not get better with a new layer of shingles on top of it — it just gets harder to see.
- The existing covering is slate, clay, cement or asbestos-cement tile. These have to come off.
- The roof already has two or more layers. This is the one that catches most homeowners:
Where the existing roof has two or more applications of any type of roof covering.
That is the whole test. Not two layers of the same shingle — two applications of any type. If your house has been re-roofed once already by going over the top, the next job is a full tear-off, and any bid that says otherwise is quoting work the code does not permit.
The wood shake trap
Going over old wood shakes is explicitly allowed for metal and tile coverings — but shakes sit on spaced boards, and laying a new roof over them can leave a hollow gap of open air against dry, decades-old wood. The code calls that a combustible concealed space, and where a recover creates one, the entire existing surface first has to be covered with gypsum panel products, mineral fiber, glass fiber or another approved material, securely fastened in place (§ 705.3.1).
This is a real material and labor cost, and it is the step most often missing from a low bid. Ask about it before you sign.
What cannot be reused
Even on a tear-off, not everything that comes down goes back up (§ 705.4):
- Slate, clay and cement tile may be reinstalled — but not any piece that is damaged, cracked or broken.
- Vent flashing, metal edgings, drain outlets, collars and counterflashings may not go back on if they are rusted, damaged or deteriorated.
- Aggregate surfacing from an old built-up roof — the loose gravel — may not be reinstalled at all, and damaged or cracked ballast is out too.
Flashings, the metal that seals where the roof meets a wall, chimney or valley, have to be rebuilt to the manufacturer's installation instructions, and metal that will have tar or bitumen stuck to it has to be primed first (§ 705.5). Reused flashing is the most common source of a leak on a brand-new roof.
What this does not settle
The sections above govern the method: what comes off, what may stay, what may be reused. They do not answer three things people usually ask in the same breath, so do not read them as covering it:
- Fire ratings. California has separate requirements for roof covering fire classification, and stricter ones again in wildfire-prone areas. Nothing quoted here addresses them.
- The underlayment you specifically need. Which underlayment goes with which covering is set by the residential code's roof-covering requirements (§ R905.1) and by the manufacturer's instructions for your product. It varies by material.
- Permits, inspections and fees. These are set locally and vary by city — check with your building department before work starts.
And the general rule holds here as everywhere: the state code is the floor. Your city or county can require more, and many California jurisdictions do. It cannot require less.
