Collapsing building © Daniel Friedman Flat & Low Slope Roof Conversion to Pitched Gable Roofs
Roof Re-Cover procedures, roofing codes, fire hazard & moisture warnings
     


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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.

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Flat Roof Conversion: Retrofitting a Pitched Roof over a Flat or Low Slope Roofed Structure

Original flat ceiling is retained with a roof-structure-over roof conversion from flat to pitched slope (C) InspectApediaConstruction of a sloped roof over an existing flat roof is a common procedure and one used by many builders for a variety of reasons including these principal objectives:

  • Fix a chronically leaky flat roof at or past end of useful life
  • Avoid the extra cost of a complete tear-off of the existing flat roof
  • Avoid exposing the occupied building space to water damage should rain occur during the building project
  • Add insulation to the building envelope
  • Correct a difficult-to-vent moisture trap in a flat roof (FLAT ROOF MOISTURE & CONDENSATION)

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.

Gable roof constructed atop original flat roofed building (C) InspectApedia Gable roof constructed atop original flat roofed building (C) InspectApedia

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).

Gable roof constructed atop original flat roofed building (C) InspectApedia Gable roof constructed atop original flat roofed building (C) InspectApedia

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.[1][2]

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 roofing.

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.

Exceptions:

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.

2. Metal 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. - [1] CA Building Code Chapter 15

Gable roof constructed atop original flat roofed building (C) InspectApedia Gable roof constructed atop original flat roofed building (C) InspectApedia

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 [4]

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.

More Reading

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