On paper, yes. You save $1 to $2 per sq ft in tear-off labour. In practice most Ontario contractors and building codes advise against it. A second layer traps moisture, adds structural load, hides underlying damage, and voids manufacturer warranties on the new shingles in most cases.
Most reputable Windsor-area roofing companies will push for a full tear-off before installation. Even where code allows the overlay, the new roof’s lifespan takes a hit of 5 to 10 years because ventilation is compromised and the shingles can’t sit flat on an uneven surface. The savings at the front end rarely survive that math.
This connects directly to whether you should put a new roof over an old one at all. The answer from both code and contractor experience is the same: a tear-off is the one chance you get to inspect and repair the decking before sealing it up for the next 25 to 30 years. Many homeowners also ask whether it’s cheaper to put new shingles over old ones specifically. Same answer. For full replacement cost context see how much it costs to replace a 2,000 sq ft roof in Ontario.