You may be looking at your downriver roof and wondering can you lay new roof shingles over old? The answer is yes, you can lay new roof shingles over old ones. However, now that we’ve taken care of that initial question, let’s look at a couple of reasons why you might want to do this. Then we will review lot more reasons why this may not be the best idea. Before we jump in, one thing should be made clear: adding new roofing over old (sometimes this is referred to as a “re-roof”) is possible only with asphalt shingles (also known as “composition shingles”). You cannot attempt to do this with wood or slate, for example, and you should never mix materials, like laying asphalt shingles over cedar shakes. Also, your existing roofing must be in decent condition. Otherwise, you are just putting lipstick on a pig that may lead to costly repairs down the road.

Why Add New Roof Shingles Over Your Existing Roof?

Despite what might seem like an obvious benefit, having multiple layers of shingles does not automatically make a roof more waterproof, and it often creates its own set of issues. So why would you want to lay new roof shingles over your old, existing one? The answer is twofold and simple:  convenience and cost. If you keep the old shingles on, you skip the loud, messy, labor-intense removal step, known as the “tearoff”, and you’ll save a bit of money on the roof job.

However, both benefits have caveats. It’s clearly easier and cheaper to just to leave your old shingles in place, but there’s some special preparation involved with a re-roof job (namely removing vents, ridge caps, and misshapen shingles, among other things) and you still have to replace or add new flashing, which is often tricky over old roofing. Eliminating the tearoff could save $1,000 or more on a new roof, but in reality, you are simply delaying the cost: when it’s time to replace the roof again, you will have no  choice but to start fresh  (two layers is the maximum allowed in most areas), and then you’ll have to pay extra for the two-layer tearoff and disposal.

Why Not Add New Roof Shingles Over Old?

As we noted above, the potential cons of re-roofing very often outweigh the potential pros, but it is all dependent on your specific situation. The following list includes pretty universal reasons to not reroof, and also some factors or things to check out if you’re really considering adding new roof shingles over old ones.

• By design, shingles are intended for flat surfaces. They do not do a good job of bridging over gaps, dips or bumps, and this includes the stepped texture that is created by overlapping shingles. Re-roofers have techniques they use to lay new roof shingles over existing ones so that the same step-ups between courses are maintained, but if any old shingles are cupped, curled, or otherwise misshapen, those defects will transmit through to the new layer. Many roofers recommend using laminated, or dimensional, shingles for re-roofs because they’re thicker than standard shingles and typically have a staggered edge profile (for appearance) that helps disguise any dips or high spots in your old roofing.

• Shingles add weight. Good-quality composite shingles may weigh 350 to 450 pounds per “square” (meaning 100 square feet of roofing, installed). Chances are, your roof structure was designed for a dead load of only one layer of shingles, plus extra for snow weight as well as a safety margin. Installing new roof shingles over existing ones essentially doubles the weight of the roofing, so you have to ensure your roof structure can bear the extra load. That is also why building codes generally limit re-roofing to no more than two layers. On some houses, the extra weight of these multiple layers can cause the roof sheathing (decking) to sag between the rafters or trusses.

• There is no opportunity for visual inspection of roof sheathing. Without a tearoff, roofers are unable to see what the decking looks like. A good roofer performs a careful “walking” inspection looking for any spongy areas and or other issues, and they can make specific, localized repairs before re-roofing. A sub-par roofer might be less diligent.

• You cannot replace underlayment. The building paper (known as tarpaper) that’s laid between the roof sheathing and the shingles is there for a reason — because water can find a way to get under shingles, no matter how many layers of roofing you have. If the underlayment is old and weakened, it will not protect your roof deck from water that gets in past the shingles. In snowy climates, most new roofs get an ice-and-water shield (made up of a rubbery moisture membrane) applied to the sheathing along the eaves to protect against any ice dams. Adding this membrane cannot be done with a re-roof, and the extra layer of shingles offers almost no protection against ice dams.

• Re-roofing may impact the warranty on the new shingles. This one is self-explanatory. Be sure to check with the specific shingle manufacturer regarding all warranty issues and installation requirements before taking on re-roofing.

• Local codes may not allow re-roofing. Two layers is the maximum in most areas, but there are some building codes allow only one layer. For example, towns in areas with heavy hailstorm activity or significant ice dam issues might not allow multiple roofing layers.

• A re-roof may not look good on an inspection report. Adding new roof shingles over old ones is very tempting to many people looking to sell their house in the near future: why pay for a tearoff if you don’t have to? Of course, the new owners will be stuck with the extra expense of a two-layer tearoff down the road, as well as any problems that arise from an improper re-roof job. Because of this, home inspectors typically report double-layer roofs when they find them, and they can warn the buyers of potential issues associated with re-roofs. If you are planning to sell your house after installing a new roof, it’s a good idea to review your options with local real estate pros and/or home inspectors.