by Kenton Shepard and Nick Gromicko
The purpose of the series “Mastering Roof Inspections” is to teach home inspectors, as well as insurance and roofing professionals, how to recognize proper and improper conditions while inspecting steep-slope, residential roofs. This series covers roof framing, roofing materials, the attic, and the conditions that affect the roofing materials and components, including wind and hail.
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’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.
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.
Learn how to master a roof inspection from beginning to end by reading the entire InterNACHI series: Mastering Roof Inspections.
Take InterNACHI’s free, online Roofing Inspection Course
Mastering Roof Inspections
Roofing Underlayment Types
Inspecting Underlayment on Roofs
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