Tear Off the Old Roof or Shingle Over It?
What a roof-over actually saves
Only one line of the bill goes away: tear-off and disposal. Everything else — shingles, underlayment where it is used, flashing, labour to install — is still there. On the 15-square example in the roof replacement cost estimator, tear-off at the placeholder rate is $1,502.64 of an $8,948.72 total, about 17%. Real quotes land in the same band: expect a roof-over to save something like a sixth of the job, not half.
What it costs you instead
- You never see the decking. Rotten or delaminated plywood, popped nails and soft spots around penetrations stay hidden under two layers. The one moment in decades when the deck is exposed for inspection is skipped.
- The new shingles wear faster. Laid over the old surface they telegraph every curled tab and butt joint, trap heat, and seal less evenly. Manufacturers commonly reduce or void the warranty on a second layer; ask before you buy.
- Weight. Architectural shingles weigh roughly 230–280 lb per square. On the 1,503 sq ft example that second layer is about 3,500–4,200 lb sitting on the rafters permanently, plus whatever the old layer already weighs.
- No third layer. The model building codes cap asphalt roofing at two layers. A roof-over now means the next roof must be a full tear-off of both, and tear-off is priced per layer — so part of the saving is only deferred.
- Flashing compromises. Step flashing, valleys and drip edge are hard to do properly without removing the old roof; many roof-overs reuse them.
When code rules it out
Under the International Residential Code, a roof-over is not permitted when the existing roof is water-soaked or deteriorated to the point that it is not a sound base; when it is wood shake, slate, clay, cement or asbestos-cement tile; or when there are already two layers of any kind. Some jurisdictions are stricter, and some require ice barrier and drip edge that can only be installed with the old roof removed. Check locally before pricing the option at all.
When a roof-over is defensible
- A single existing layer of asphalt shingles, lying flat, with no curling or blistering.
- Decking known to be sound — no interior stains, no soft spots underfoot, no history of leaks.
- A short ownership horizon or a genuine budget constraint, where the choice is a roof-over now or a leaking roof for another two years.
- A roofer willing to write the reduced warranty into the contract rather than glossing over it.
Pricing both properly
Run the estimator twice: once with one layer of tear-off and once with none. The difference is the saving. Then ask what a tear-off of two layers would cost at the next replacement, because that is the bill the roof-over is passing forward. For most houses that arithmetic ends the debate.
Turn a house footprint and pitch into roofing squares, then into an itemised re-roof estimate — tear-off, shingles, underlayment and accessories, labour, flashing, decking repairs and contingency — priced with numbers you collect locally rather than a national average.
Frequently asked questions
How much do you save by roofing over old shingles?
Only the tear-off and disposal line, which is typically about 15 to 20 percent of a shingle roof job. The shingles, underlayment, flashing and installation labour are all still paid for. On a 15 square roof at typical rates that is on the order of a sixth of the total.
Can you put a third layer of shingles on a roof?
No. The model building codes limit asphalt roofing to two layers, and many jurisdictions and manufacturers are stricter. A house that already has two layers must be torn off to the deck, and tear-off is priced per layer.
Does a second layer of shingles void the warranty?
Often it reduces it and sometimes voids it, depending on the manufacturer and the product line. Ask for the warranty terms for installation over an existing layer in writing before choosing a roof-over, and expect the labour warranty from the roofer to be shorter as well.
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