Figure 2-16 re roofing starter course (C) J Wiley, S Bliss Re-Roofing Asphalt Shingle Roofs - Best Practices

  • ASPHALT SHINGLE RE-ROOF GUIDE - CONTENTS: How to re-roof an asphalt shingle roofed building
    • Proper asphalt shingle roof preparation for a roofover job
    • Roof-over details for multiple layer asphalt shingle roofs
    • Characteristics of asphalt shingle roofing materials
    • Best practices for roofing material installation, flashing, ventilation, nailing, underlayment
  • POST a QUESTION or READ FAQs about re-roofing, tear-off, shingle-over with asphalt roof shingles

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This article discusses how to prepare an existing asphalt shingle roof for a roof-over or for re-roofing with new asphalt shingles. This article series discusses best practices in the selection and installation of residential roofing. This article includes excerpts or adaptations from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons.

Also see the roofing article links listed at the "More Reading" links at the bottom of this article and our roofing home page: ROOFING INSPECTION & REPAIR.

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Reroofing Options on Asphalt Shingle Roofs

Adapted/paraphrased with permission from Best Practices Guide to Residential Construction, chapter on BEST ROOFING PRACTICES:

Reroofing saves the cost, trouble, and risks (water damage while the roof is exposed) associated with a tear-off. If the roof is structurally sound, most building codes allow for two layers of asphalt shingles and some allow for a third on roofs with a 5:12 or steeper pitch. If the original shingles are not badly curled and the sheathing is sound (check for bouncy areas), then a reroof is a good alternative.

Shingle Type Recommendations for Re-Roofing Asphalt Shingles

The heavier the shingle on the new layer, the less likely it is that irregularities in the surface below will telegraph through. Laminated or other heavy-textured shingles work well, as they do not need to be carefully fitted to the existing shingles, and the irregular texture will conceal any small bumps or dips from the original roof.

Prep Work for Re-Roofing Over Asphalt Shingles

Shingles 3 layers (C) Daniel Friedman
  • Inspect the condition of the existing roof and roof deck for numer of roof layers, soundness of roof decking, smoothness of existing roof installation, and other factors that determine whether or not you can do a roof-over or if a roof shingle tear-off is required.

    Our asphalt roof shingle layer photo (left) shows that this roof already has three layers of shingles installed - more layers are not permitted. More warnings are below at "When is it Dangerous to Roof-Over Existing Roof Coverings?". - Ed.
  • Watch for damaged or un-sound roof decking; investigate, strip shingles entirely, and replace roof sheathing as needed before proceeding with the roof-over on the rest of the roof.

    If removal of shingles is required on some sections of a roof-over job, be sure that you properly build-up or shim meeting points between the removed-shingle area and the rest of the roof, so that the roof surface remains both cosmetically acceptable and so that you do not create a wear point in the new shingles that have to lap over these roof shingle thickness transitions. - Ed.
    Continuing from from Best Practices Guide to Residential Construction:
  • Clip any curled shingle corners and remove any curled tabs, replacing them with new shingle scraps as shims.
  • Install new drip edge on rakes and eaves. Specialty drip edge profiles designed for retrofitting wrap around the exposed roof edge, leaving a neat protected edge.
  • If the roof had no eaves flashing and one is needed, use a retrofit membrane such as AC Evenseal (NEI, Brentwood, New Hampshire).

When is it Dangerous to Roof-Over Existing Roof Coverings?

Unsafe roof deck means tear-off (C) Daniel Friedman[Addition to the original article - Ed.]

Watch out: Check roof deck condition: if you are stripping roof shingles entirely from a roof deck, either because of their poor and rough-surface condition (not suitable to shingle-over), or because portions of the roof deck itself need to be replaced (perhaps due to rot, insect damage, water damage), be sure that you properly identify the type of roof sheathing used and its condition over the entire roof deck.

Don't even think about roofing-over a structure whose roof deck is in the condition shown in our photograph at left. But a careful inspection may be required to detect more subtle but dangerous roof deck conditions.

Watch out: Life Safety Issues: Walking on an old water-damaged roof with an un-sound roof deck can result in serious injury if a worker steps into a soft area and falls through the roof - a hazard more likely with thin plywood sheathing (on some low-cost homes as thin as 3/8" or even 1/4") and also more likely where the roof is known to have been leaking, or finally, where fiberboard roof sheathing has been used.

Roof deck fall through report: In the 1980's when we [DJF] were working on a roof-over job for a 1960's house we had observed from the building attic that thin 1/4" plywood had been used for roof sheathing and we already had a concern about the nail-holding ability of that material to prevent a roof blow-off.

We had also seen evidence of prior leaks on the north slope of the roof. But we failed to spot badly damaged roof sheathing in one area of the roof. After our in-attic inspection, our worker who weighed the least, D. S., was walking over one slope of this modestly-pitched hipped roof when she stepped right through a section of soft roof decking. Luckily D.S. was not hurt, but a fall off or fall-through injury could have been very serious.

  • see Roof Shingle Tear-Off Procedure for Homasote Products [PDF], reproduced with permission of the Journal of Light Construction. More about fiberboard roof and wall sheathing is at FIBERBOARD SHEATHING, Celotex Homasote & Other .
  • See "When reroofing, should you tear off or recover?" [PDF copy] Thomas L. Smith, director of NRCA's technology and research, originally published by the NRC's Professional Roofing magazine. This article concludes that in some roof jobs a roof recover approach can be a viable operation and re-covering existing roof surfaces has been successfully used on many roof jobs, but it involves greater risks than a tear-off. The author points out that some roofing industry trade publications cite the roof-over option as a means of reducing waste disposal costs, but the author points out some concerns with roof-over re-roofing jobs in some conditions, including
    • life safety issues (as we explain above)
    • leaving high-moisture-content insulation in place when re-roofing
    • the structure may lack adequate strength to support the weight of an additional layer of roof covering

Best Practices Guide to Residential Construction continues:

Starter Course for Re-Roofing Asphalt Shingles

Figure 2-16 re roofing starter course (C) J Wiley, S BlissIf laying three-tab shingles over three tab shingles, it is important to nest the new shingles against the old to create a flat surface. This process starts with a 5-inch starter strip fit along the eaves and set against the second course of existing shingles (see Figure 2-16).

Next install a course of shingles cut down to 10 inches wide, so they fit against the bottom edge of the existing third course (this creates a new 3-inch first course). After that, shingling should proceed normally, fitting each course up against the bottom of an existing course.

Fastening / Nailing Details When Re-Roofing with Asphalt Shingles

Use galvanized roofing nails long enough to fully penetrate the sheathing, typically 1 1/2 inches for a second roof and 1 3/4 inches for a third. Nesting each new row below an existing one keeps the new nails 2 inches below the existing, which will help minimize any splitting of the sheathing.

Flashings for Re-Roofing Asphalt Shingles

Depending on their condition and accessibility, some flashings can be reused. New shingles may be able to tuck under existing step flashing, chimney flashings, and front-wall flashings. If they are deteriorated, they must be replaced along with vent boots.

See FLASHING ROOF WALL DETAILS and FLASHING ROOF-WALL SNAFU for specifications of roof-wall flashing and for some photo-examples of troubles with existing step and counter flashing when re-roofing an existing structure.

Valley Details for Re-Roofing Asphalt Shingles

Any type of valley flashing will work and simply lays over the existing flashing (except in a tear-off, where all flashings should be replaced). Unless a metal valley flashing is used, the first step is to line the existing valley with a new underlayment consisting of either 90-pound roll roofing or a more durable modified bitumen membrane. Then install either a closed or woven valley as described above.

See roof valley details at ASPHALT SHINGLE VALLEY FLASHING.

-- Adapted with permission from Best Practices Guide to Residential Construction.

Resources: Roofing Materials & Equipment Suppliers


Asphalt Shingles

Atlas Roofing Corp. Fiberglass and organic felt shingles

Certainteed Roofing Fiberglass shingles

Elk Premium Building Products Fiberglass shingles

GAF Materials Corp. Fiberglass shingles

Georgia-Pacific Corp. Fiberglass and organic felt shingles

IKO Fiberglass and organic felt shingles

Owens Corning Fiberglass shingles

Tamko Roofing Products Fiberglass and organic felt shingles

Ridge Vents

Air Vent/A Gibraltar Company A complete line of roof ventilation products, including shingle-over and exposed-ridge vents with exterior wind baffles and internal weather filters. Also soffit and drip edge vents and passive and powered attic turbine-type vents.

Benjamin Obdyke Shingle-over ridge vents. Low-profile Roll Vent uses nylonmatrix. Extractor vent is molded polypropylene with internal and external baffles.

Cor-A-Vent Shingle-over low-profile ridge vents, including Cor-a-vent, Fold-a-vent, and X-5 ridge vent, designed for extreme weather. Corrugated core.

GAF Materials Corp. Cobra vent: roll-out shingle-over ridge vent with a polyester-matrix core 102 CHAPTER 2 | Roofing

Mid-America Building Products Ridge Master and Hip Master shingle-over molded plastic ridge vents with internal baffles and foam filter

Owens Corning VentSure corrugated polypropylene ridge vents; also passive roof vents and soffit vents

Trimline Building Products Shingle-over low-profile ridge vents, Flow-Thru battens for tile roofs

Elk Premium Building Products Highpoint polypropylene shingle-over ridge vents

Tamko Roofing Products Shingle-over ridge matrix–type Roll Vent and Rapid Ridge (nail gun version) and Coolridge, which is molded polypropylene with external and internal baffles

Venting Underlayments

Benjamin Obdyke Cedar Breather, a 3/8 -in.-thick matrix-type underlayment designed to provide ventilation and drainage space under wood roofing

More Information about Roofing Materials, Methods, Standards

Asphalt Roofing Manufacturers Association (ARMA)

Cedar Shake and Shingle Bureau

Metal Roofing Alliance

Tile Roofing Institute


-- Adapted with permission from Best Practices Guide to Residential Construction.

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