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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.
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
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
Prep Work for Re-Roofing Over Asphalt Shingles
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
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.
roof had no eaves flashing and one is needed, use a retrofit
membrane such as AC Evenseal (NEI, Brentwood, New
When is it Dangerous to Roof-Over Existing Roof Coverings?
[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 "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
If 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
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
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.
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
Tamko Roofing Products
Fiberglass and organic felt shingles
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
Shingle-over ridge vents. Low-profile Roll Vent uses nylonmatrix.
Extractor vent is molded polypropylene with internal
and external baffles.
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
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
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
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
Cedar Breather, a
3/8 -in.-thick matrix-type underlayment
designed to provide ventilation and drainage space under
More Information about Roofing Materials, Methods, Standards
Asphalt Roofing Manufacturers Association (ARMA)
Cedar Shake and Shingle Bureau
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"When reroofing, should you tear off or recover?", Thomas L. Smith, AIA, CRC, Professional Roofing,[date tbd, p. 54], Professional Roofing Magazine, O'Hare International Center, 10255 W. Higgins Road, Suite 600, Rosemont, IL 60018-5607, Telephone: (847) 299-9070, E-mail: firstname.lastname@example.org. Professional Roofing Magazine is a publication of NRCA, the National Roofing Contractors Association. [Permission requested 9/26/10].
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter INSPECTAEHRB in the order payment page "Promo/Redemption" space.
Green Roof Plants: A Resource and Planting Guide, Edmund C. Snodgrass, Lucie L. Snodgrass, Timber Press, Incorporated, 2006, ISBN-10: 0881927872, ISBN-13: 978-0881927870. The text covers moisture needs, heat tolerance, hardiness, bloom color, foliage characteristics, and height of 350 species and cultivars.
Green Roof Construction and Maintenance, Kelley Luckett, McGraw-Hill Professional, 2009, ISBN-10: 007160880X, ISBN-13: 978-0071608800, quoting: Key questions to ask at each stage of the green building process Tested tips and techniques for successful structural design
Construction methods for new and existing buildings
Information on insulation, drainage, detailing, irrigation, and plant selection
Details on optimal soil formulation
Illustrations featuring various stages of construction
Best practices for green roof maintenance
A survey of environmental benefits, including evapo-transpiration, storm-water management, habitat restoration, and improvement of air quality
Tips on the LEED design and certification process
Considerations for assessing return on investment
Color photographs of successfully installed green roofs
Useful checklists, tables, and charts
Roofing The Right Way, Steven Bolt, McGraw-Hill Professional; 3rd Ed (1996), ISBN-10: 0070066507, ISBN-13: 978-0070066502
Slate Roofs, National Slate Association, 1926, reprinted 1977
by Vermont Structural Slate Co., Inc., Fair Haven, VT 05743, 802-265-4933/34. (We recommend this book if you can find it. It
has gone in and out of print on occasion.)
Roof Tiling & Slating, a Practical Guide, Kevin Taylor, Crowood Press (2008), ISBN 978-1847970237, If you have never fixed a roof tile or slate before but have wondered how to go about repairing or replacing them, then this is the book for you. Many of the technical books about roof tiling and slating are rather vague and conveniently ignore some of the trickier problems and how they can be resolved. In Roof Tiling and Slating, the author rejects this cautious approach. Kevin Taylor uses both his extensive knowledge of the trade and his ability to explain the subject in easily understandable terms, to demonstrate how to carry out the work safely to a high standard, using tried and tested methods.
This clay roof tile guide considers the various types of tiles, slates, and roofing materials on the market as well as their uses, how to estimate the required quantities, and where to buy them. It also discusses how to check and assess a roof and how to identify and rectify problems; describes how to efficiently "set out" roofs from small, simple jobs to larger and more complicated projects, thus making the work quicker, simpler, and neater; examines the correct and the incorrect ways of installing background materials such as underlay, battens, and valley liners; explains how to install interlocking tiles, plain tiles, and artificial and natural slates; covers both modern and traditional methods and skills, including cutting materials by hand without the assistance of power tools; and provides invaluable guidance on repairs and maintenance issues, and highlights common mistakes and how they can be avoided.
The author, Kevin Taylor, works for the National Federation of Roofing Contractors as a technical manager presenting technical advice and providing education and training for young roofers.
The Slate Roof Bible, Joseph Jenkins, www.jenkinsslate.com,
143 Forest Lane, PO Box 607, Grove City, PA 16127 - 866-641-7141 (We recommend this book).