Structural Help

Asphalt Shingle Roof Recovering or Replacement

2012-03-31

One or several roof repairs, recoverings, or replacements usually become necessary over the life of a typical home. The word 'roof' in this context refers to the waterproofing assembly that is applied to the structural roof sheathing or decking, as opposed to the roof structure. The focus of this discussion will be on asphalt shingle roofs.  Asphalt shingles are widely used because they are relatively cheap and easy to install.  Asphalt shingles are classified as either organic-based or fiberglass-based. Organic-based shingles are manufactured with celluse (paper and wood) saturated with asphalt.  They are typically more flexible than fiberglass shingles, which use fiberglass mats coated with asphalt.  Weather resistant mineral granules are applied to the exposed surface of both organic and fiberglass asphalt shingles. In residential applications, 'strip' and 'laminated' are the most widely used types of asphalt shingles. The '3-tab' strip shingle is perhaps the most common, resulting in an exposed rectangular tab area of approximately 5"x12" placed in the familiar step pattern. The laminated asphalt shingle is sometimes referred to as 'architectural', and uses more than one layer of tabs for additional thickness and visual depth. The need for roof work may arise due to one or a combination of several factors. These include damage from hail or wind; long term deterioration from normal exposure; and improper original installation. Any one of these factors may cause or contribute to a roof leak and interior ceiling stains. The 25% replacement rule should be considered prior to any roof being repaired recovered, or replaced. The 2010 FBC, Residential states the following with regard to roof decking, minimum roof slope, and underlayment for asphalt shingle roofs:

R905.2.1 Sheathing requirements. Asphalt shingles shall be fastened to solidly sheathed decks.

R905.2.2 Slope. Asphalt shingles shall only be used on roof slopes of...2:12...or greater. For roof slopes from...2:12...up to...4:12...to layers of underlayment...is required...

R905.2.3 Underlayment. Unless otherwise noted, required underlayment shall conform with ASTM D 226 Type I or Type II, ASTM D 4869, Type I or Type II, or ASTM D 6757. Self-adhering polymer modified bitumen sheet shall comply with ASTM D 1970.

A roof 'recovering' or 'roof-over' are terms used to describe the installation of new roofing material over an existing roofing material.  The 2010 FBC, Existing Building states the following with regard to roof recoverings:

611.3 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, clay, cement or asbestos-cement tile.
    3. Where the existing roof has two or more applications of any type of roof covering.