Flat & Low Slope Roof Conversion to Pitched Gable Roofs
Roof Re-Cover procedures, roofing codes, fire hazard & moisture warnings
LOW SLOPE ROOF CONVERSION - CONTENTS: low slope roof re-cover procedure, converting a low slope roof to a steeper pitched slope: building a new roof over a low-slope or flat roofed building or mobile home
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Flat or low slope roof conversion to sloped roof design: this article describes & illustrates the construction of a sloped gable roof over the flat-roofed home shown above. The original tar and gravel low slope roof lasted many years but ultimately a combination of roof age, wear, ponding, and leaks led the owners to construct a gable roof over this building. Photographs of the gable roof conversion structure are illustrated below. We use flat roof conversion photographs from several homes to discuss fire and building code concerns when a new roof is constructed atop an existing structure.
Our photo (left) makes clear that for this roof-over construction project the original flat interior ceiling and its original structure were retained. The new pitched roof was constructed to bear on the top plates of the original building walls.
Note that there is an important distinction between adding a pitched roof, typically 4 in 12 or more, over an existing flat or low-slope roof - leaving the original roof structure and covering in place - and converting a flat roofed structure to a cathedral ceiling. On a flat or low sloped roof building that includes parapet walls, the builder may construct the new roof to bear on the parapets.
A cathedral ceiling conversion will require complete removal of the original structure, while the simpler addition of a pitched roof over an existing flat roof is typically handled by bearing on the existing building structure, its walls, and possibly by including sleepers and an "overframing" approach.
At below left you can see the intersecting gable roofs that were constructed atop the flat roofed structure shown at page top. Converting to a pitched roof helped eliminate chronic roof leaks in the original structure.
At above right and in additional photographs below construction details include temporary bracing below a valley rafter (below left), and the construction of a knee wall to address original differences between two building section roof heights (below right).
The photos above and at below left show that in these different homes from the one shown at page top the builders left the original tar and gravel built-up roof (BUR) in place, along with original flat roof vents (below left). Should a building fire occur the fire department may not be happy to discover that there are multiple layers of roof structure upon which older roofing was left in place in the building. Building codes refer to this approach as a "Roof Re-Cover". As you will see, there are some easy solutions that avoid having to perform a horrible old-roof tear-off in confined space.
Roof recover: the process of installing an additional roof covering over a prepared existing roof covering without removing the existing roof covering.
1510.3 Recovering versus replacement. New roof coverings shall not be
installed without first removing all existing layers of roof coverings where
any of the following conditions occur:
1. Where the existing
roof or roof covering is water soaked or has deteriorated to the point that
the existing roof or roof covering is not adequate as a base for additional
2. Where the existing roof covering is wood shake,
slate, d clay, cement or asbestos-cement tile.
3. Where the
existing roof has two or more applications of any type of roof covering.
1. Complete and separate roofing
systems, such as d standing-seam metal roof systems, that are designed to
transmit the roof loads directly to the building's structural system and that
do not rely on existing roofs and roof coverings for support, shall not
require the removal of existing roof coverings.
panel, metal shingle and concrete and clay tile - roof coverings shall be
permitted to be installed over existing wood shake roofs when applied in
accordance with Section 1510.4.
3. The application of a new protective coating over an existing
spray polyurethane foam roofing system shall be permitted without tear-off of
existing roof coverings.
1510.4 Roof recovering. Where the application of a
new roof covering over wood shingle or shake roofs creates a combustible
concealed space, the entire existing surface shall be covered with gypsum
board, mineral fiber, glass fiber or other approved materials securely
fastened in place. -  CA Building Code Chapter 15
At above right the black stains on the roof sheathing of the "new" gable roof constructed over the original flat roof show that this roof was not adequately vented.
Clearing up the Definition of Roof Re-Cover or Roof Recovering
Building codes and facilities management experts use the term roof re-cover generally to refer to covering an existing roof with additional layers of roofing materials without removing the original material. As expert sources point out,
Re-covering can postpone more expensive roof replacement projects, but if a re-cover is performed without due consideration, the new membrane can exacerbate existing problems within the roof. On the other hand, a new roof is a better way to guarantee protection, but it comes at a cost.
Whether a re-cover or replacement project is best depends on many factors. Insurance considerations, building codes or other regulations might prevent re-covering a roof. For example, the International Building Code prevents more than two systems on a roof deck. A need to replace insulation or portions of the deck are among the other factors that can force replacement. - http://www.iccsafe.org/ and facilities net 
Watch out: when retrofitting a sloped roof over an existing low slope or flat-roofed building be sure to check with your local building department. Don't just plow ahead building a non-conforming structure. Building permits will almost certainly be required and you may require the services of a design professional, an architect or structural engineer whose drawings will confirm the adequacy of the new structure. Also see FIRE RATINGS for ROOF SURFACES or select a topic from the More Reading links shown below.
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"Choosing Roofing," Jefferson Kolle, January 1995, No. 92, Fine Homebuilding, Taunton Press, 63 S. Main St., PO Box 5506, Newton CT 06470 - 800-888-8286 - see http://www.taunton.com/FineHomebuilding/ for the magazine's website and for subscription information.
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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).