Mastering Roof Inspections: Asphalt Composition Shingles, Part 28

When it’s time to install a new roof, homeowners face the choice of whether to have the existing shingles removed before installing new ones. Most jurisdictions limit the number of layers allowed on a roof before removal of all layers is required prior to installing new shingles.  In most places, the maximum is two or three layers. In a few places, it’s only one layer.

Installing new shingles over old has some disadvantages:

• It will void any manufacturer’s warranty.
• Shingles will dissipate heat more slowly, which will shorten their lifespan.
• Shingles may radiate more heat into the living space, increasing cooling costs.
• "Telegraphing" can be an issue when new roofs are installed over old. The new shingles can conform to the older shingles beneath them and follow any humps or low spots. Telegraphing is even more obvious when asphalt shingles have been installed over wood shingles. As runoff flows down the roof, this unevenness can create areas of increased friction, which results in premature failure of the new roof over those areas.

It’s important to understand that roofs with new shingles installed over old will not last as long as roofs with shingles installed over a proper substrate.

There are really only a couple of advantages to installing new shingles over old. One is the money saved by not having to pay for removal and disposal of the existing shingles. The other is the fact that the additional layers will reduce the chances of moisture finding its way through the shingles and causing leaks.

InterNACHI's Standards of Practice requires you to comment in your inspection report on the number of layers installed. It’s especially important to notify your client if the maximum number of layers allowed is installed. You should take the time to find out what is allowed in the areas where you inspect homes.

If the original roof is wood and installing wood is no longer allowed in that jurisdiction, it’s important to alert your client.

In addition to the cost of removal and disposal of the existing layers, the existing spaced-board roof sheathing will need to be overlaid with new, solid sheathing in order to satisfy the manufacturer’s warranty. That information should be included in your inspection report.

You can sometimes spot shingles installed over wood from the street because the roof looks sort of lumpy.

Usually, you can confirm it by looking at the roof edge at the eaves or rakes.

Reduced Impact Resistance 

New shingles installed over existing shingles will be less impact-resistant. This is important in areas that experience significant hailstorms. Newer shingles installed over old roofing will not be as well-supported as when they’re installed on a proper roof deck.

Areas where new shingles bridge the butts of old shingles are unsupported. Even these small voids make asphalt shingles more vulnerable to impact damage.

Bridging and Nesting 

Bridging and nesting are methods for covering standard-size shingles, such as 3-tab, with a layer of larger, metric-size shingles, such as laminated shingles.

Bridging

Using the “bridging” method, shingles are installed starting at the lower edge of the roof as if they were being installed directly onto sheathing. They’re installed with the exposure recommended by the manufacturer. There will be unsupported areas beneath the new shingles where they bridge the butts of the old shingles, as you can see here. These voids are especially vulnerable to damage from impact.

In addition to reduced impact resistance, the underlying shingles may “telegraph through” the new shingles and become visible. They’ll look like horizontal ridges in the newer shingles.

In this example of bridged shingles, you can see the tops of new shingles overlapping the butts of the existing shingles.

Nesting