Re-Roofing Asphalt Shingle Roofs - Best Practices
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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

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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 at page left 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

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?

[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 Sheathing Homasote & Other Board.
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

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 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

Manufacturers

Asphalt Shingles

Atlas Roofing Corp. www.atlasroofing.com Fiberglass and organic felt shingles

Certainteed Roofing www.certainteed.com Fiberglass shingles

Elk Premium Building Products www.elkcorp.com Fiberglass shingles

GAF Materials Corp. www.gaf.com Fiberglass shingles

Georgia-Pacific Corp. www.gp.com/build Fiberglass and organic felt shingles

IKO www.iko.com Fiberglass and organic felt shingles

Owens Corning www.owenscorning.com Fiberglass shingles

Tamko Roofing Products www.tamko.com Fiberglass and organic felt shingles

Ridge Vents

Air Vent/A Gibraltar Company www.airvent.com 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 www.benjaminobdyke.com Shingle-over ridge vents. Low-profile Roll Vent uses nylonmatrix. Extractor vent is molded polypropylene with internal and external baffles.

Cor-A-Vent www.cor-a-vent.com 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. www.gaf.com Cobra vent: roll-out shingle-over ridge vent with a polyester-matrix core 102 CHAPTER 2 | Roofing

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

Owens Corning www.owenscorning.com VentSure corrugated polypropylene ridge vents; also passive roof vents and soffit vents

Trimline Building Products www.trimline-products.com Shingle-over low-profile ridge vents, Flow-Thru battens for tile roofs

Elk Premium Building Products www.elkcorp.com Highpoint polypropylene shingle-over ridge vents

Tamko Roofing Products www.tamko.com 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 www.benjaminobdyke.com 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) www.asphaltroofing.org

Cedar Shake and Shingle Bureau www.cedarbureau.org

Metal Roofing Alliance www.metalroofing.com

Tile Roofing Institute www.tileroofing.org

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

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InspectAPedia.com^® - Daniel Friedman - Publisher & Editor.
Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education. Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com.
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BEST PRACTICES GUIDE to Residential Construction
BEST ROOFING PRACTICES

ASPHALT ROOF SHINGLES
  ASPHALT SHINGLE DEFINITIONS
  ASPHALT SHINGLE FAILURE TYPES
  ASPHALT SHINGLE INSTALLATION
  ASPHALT SHINGLE LIFE / WEAR FACTORS
  ASPHALT SHINGLE PROPERTIES
  ASPHALT SHINGLE RE-ROOF GUIDE
  ASPHALT SHINGLE UNDERLAYMENT SPECS
  ASPHALT SHINGLE VALLEY FLASHING
  ASPHALT SHINGLES on VERY STEEP ROOFS
  ALGAE, FUNGUS, LICHENS, MOSS COMPARED
  ALGAE STAINS ON ASPHALT ROOF SHINGLES
  ALGAE STAIN TEST METHODS
  ALGAE, FUNGUS, LICHENS, MOSS on SHINGLES
  BLACK STAIN REMOVAL & PREVENTION
  BLISTERS on ASPHALT SHINGLES
  CRACKS in FIBERGLASS SHINGLES
  CUPPING ASPHALT SHINGLES
  CURLING ASPHALT SHINGLES
  EARLY ASPHALT SHINGLE FAILURE
  EXTRACTIVE BLEEDING on SHINGLES
  FISHMOUTHING ASPHALT SHINGLES
  GRANULE LOSS from SHINGLES
  HAIL DAMAGED SHINGLES
  LADDERING & STAIR STEPPING SHINGLES
  LIFE / WEAR FACTORS in SHINGLES
  MECHANICAL DAMAGE of SHINGLES
  MOSS & LICHENS on SHINGLES
  ORGANIC FELT SHINGLE DEFECTS
  SPLICE DEFECTS on ASPHALT SHINGLES
  STAINS on ROOF SHINGLES
  WHAT ARE ASPHALT SHINGLES
  WIND DAMAGE to ROOFS

ARMA - Asphalt Roofing Manufacturer's Association - http://www.asphaltroofing.org/
750 National Press Building, 529 14th Street, NW, Washington, DC 20045, Tel: 202 / 207-0917
ASTM - ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA, 19428-2959 USA The ASTM standards listed below can be purchased in fulltext directly from http://www.astm.org/
NRCA - National Roofing Contractors Association - http://www.nrca.net/, 10255 W. Higgins Road, Suite 600, Rosemont, IL 60018-5607, Tel: (847) 299-9070 Fax: (847) 299-1183, Office hours: 8 a.m. to 5 p.m. CST
UL - Underwriters Laboratories - http://www.ul.com/
2600 N.W. Lake Rd.
Camas, WA 98607-8542
Tel: 1.877.854.3577 / Fax: 1.360.817.6278 E-mail: cec.us@us.ul.com
"Roof Shingle Tear-Off Procedure for Homasote Products", PDF document, Homasote Co., 932 Lower Ferry Road, West Trenton, New Jersey 08628-024, U.S.A. 800-257-9491 website: at www.homasote.com
"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: professionalroofing@professionalroofing.net. 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

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The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop, and from the InspectAPedia bookstore. The 2010 edition of the Home Reference Book 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. InspectAPedia.com ^® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.

Best Practices Guide to Residential Construction, by Steven Bliss. John Wiley & Sons, 2006. ISBN-10: 0471648361, ISBN-13: 978-0471648369, Hardcover: 320 pages, available from Amazon.com and also Wiley.com. See our book review of this publication.
Decks and Porches, the JLC Guide to, Best Practices for Outdoor Spaces, Steve Bliss (Editor), The Journal of Light Construction, Williston VT, 2010 ISBN 10: 1-928580-42-4, ISBN 13: 978-1-928580-42-3, available from Amazon.com
The Journal of Light Construction has generously given reprint permission to InspectAPedia.com for this article. All rights and contents are ©Journal of Light Construction and may not be reproduced in any form.
Architectural elements: the technological revolution: Galvanized iron roof plates and corrugated sheets; cast iron facades, columns, door and window caps, ... (American historical catalog collection), Diana S Waite, available used out of Amazon.
Asphalt Roofing Residential Manual,
Building Pathology, Deterioration, Diagnostics, and Intervention, Samuel Y. Harris, P.E., AIA, Esq., ISBN 0-471-33172-4, John Wiley & Sons, 2001 [General building science-DF] ISBN-10: 0471331724 ISBN-13: 978-0471331728
Building Pathology: Principles and Practice, David Watt, Wiley-Blackwell; 2 edition (March 7, 2008) ISBN-10: 1405161035 ISBN-13: 978-1405161039
Built-Up Roof Systems, Manual, C.W. Griffin, Mcgraw-Hill (Tx); 2nd edition (July 1982), ISBN-10: 0070247838, ISBN-13: 978-0070247833
Concrete Folded Plate Roofs, C. Wilby PhD BSc CEng FICE FIStructE (Author), Butterworth-Heinemann, 1998, ISBN-10: 0340662662, ISBN-13: 978-0340662663
Concrete Shell Roofs, C. Wilby PhD BSc CEng FICE FIStructE (Author),
Concrete Dome Roofs (Longman Concrete Design and Construction Series),
Concrete Roofing Tile, History of the, Batsford, 1959, AISN B000HLLOUC (availble used)
Copper Roofing, by CDA
Copper Roofing, Master specifications for copper roofing and sheet metal work in building construction: Institutional, commercial, industrial, I.E. Anderson, 1961 (hard to find)
Corrugated Iron, Building on the Frontier, Simon Holloway
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
Handbook of Building Crafts in Conservation, Jack Bower, Ed., Van Nostrand Reinhold Company, NY 1981 ISBN 0-442-2135-3 Library of Congress Catalog Card Nr. 81-50643.
Historic Preservation Technology: A Primer, Robert A. Young, Wiley (March 21, 2008) ISBN-10: 0471788368 ISBN-13: 978-0471788362
Historic Slate Roofs : With How-to Info and Specifications, Tina Skinner (Ed), Schiffer Publishing, 2008, ISBN-10: 0764330012 , ISBN-13: 978-0764330018
Low Slope Roofing, Manual of, 4th Ed., C.W. Griffin, Richard Fricklas, McGraw-Hill Professional; 4 edition, 2006, ISBN-10: 007145828X, ISBN-13: 978-0071458283
- Roof failure causes in depth (and specific methods for avoiding them)
- Roof design fundamentals and flourishes, based on voluminous industry research and experience
- New technologies and materials -- using them safely and correctly
- Comprehensive coverage of all major roofing systems pecifications, inspection, and maintenance tools for roofing work
Metal Roofing, an Illustrated Guide, R.A. Knowlton , [metal shingle roofs],
Patio Roofs, how to build, Sunset Books
Problems in Roofing Design, B. Harrison McCampbell, Butterworth Heineman, 1991 ISBN 0-7506-9162-X (available used)
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).
Slate Roofing in Canada (Studi4es in archaeology, architecture, and history),
Smart Guide: Roofing: Step-by-Step Projects, Creative Homeowner (Ed), 2004, ISBN-10: 1580111491, ISBN-13: 978-1580111492
Solar heating, radiative cooling and thermal movement: Their effects on built-up roofing (United States. National Bureau of Standards. Technical note), William C Cullen, Superintendent of Documents, U.S. Govt. Print. Off (1963), ASIN: B0007FTV2Q
Tile Roofs of Alfred: A Clay Tradition in Alfred NY
"Weather-Resistive Barriers [copy on file as /interiors/Weather_Resistant_Barriers_DOE.pdf ] - ", how to select and install housewrap and other types of weather resistive barriers, U.S. DOE
Wood Shingle Roofs, Care and Maintenance of wood shingle and shake roofs (EC), Stanley S. Niemiec (out of print)
...

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Reroofing Options on Asphalt Shingle Roofs

Shingle Type Recommendations for Re-Roofing Asphalt Shingles

Prep Work for Re-Roofing Over Asphalt Shingles

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

Starter Course for Re-Roofing Asphalt Shingles

Fastening / Nailing Details When Re-Roofing with Asphalt Shingles

Flashings for Re-Roofing Asphalt Shingles

Valley Details for Re-Roofing Asphalt Shingles

Resources: Roofing Materials & Equipment Suppliers

Manufacturers

Asphalt Shingles

Ridge Vents

Venting Underlayments

More Information about Roofing Materials, Methods, Standards

Questions & Answers regarding this article

Ask a Question or Search InspectAPedia

Technical Reviewers & References

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair