Tear-Off vs Overlay

When a roof is replaced, the old covering can either be removed completely (a tear-off) or left in place with the new roof installed over it (an overlay, also called a recover or "layover"). Codes in many places limit when an overlay is allowed, because extra layers add weight, hide deck damage and can make the new roof less flat and less durable.

Code limits in the United States

The International Residential Code (IRC), adopted with local amendments across much of the US, treats overlays in section R908 (Reroofing). In the 2018 edition, section R908.3.1.1 does not permit a recover, so tear-off is required, when:

• the existing roof or roof covering is water-soaked or has deteriorated so that it is not an adequate base;
• the existing covering is slate, clay, cement or asbestos-cement tile; or
• the roof already has two or more applications of any type of roof covering.

In practice this usually means one existing layer of asphalt shingles may be covered once, giving two layers in total. Local rules can be stricter. The City of Norman, Oklahoma, for example, does not allow a recover over any existing asphalt shingle layer, while Bloomington, Minnesota, applying a code based on the 2018 IRC, allows a total of two layers before tear-off is needed.

Overlay technique

Where an overlay is permitted, Canadian guidance describes a "butt-up" or nesting method for strip shingles: old ridge caps are removed, edges are trimmed and fitted with new metal edging, and each new shingle is pushed up against the butts of the old course so the new roof lies flat. Nails must be long enough to pass through both layers into the deck.

Pros and cons

	Tear-off	Overlay
Deck inspection	Full	Not possible
Flashing renewal	Straightforward	Often compromised
Weight	Lower	Higher
Cost and waste	Higher	Lower

Many manufacturers' warranties and high-wind programmes favour or require a tear-off. A tear-off also allows the deck to be re-nailed and sealed. Overlays are mainly used for asphalt shingles and for some low-slope and metal recover systems designed for that purpose.