Decay, or wood rot, requires three conditions to be present: liquid water, oxygen, and a favorable temperature (generally between 35 and 100 degrees F). Of the three, the only factor that can be managed by building design is liquid water. Hence, overhangs of sufficient width are critical to protecting modern wood-based materials used in wood-frame house construction.
As the 1896 home from Sarasota County, FL shows, house designers understood the basics of protecting wood siding, wood framing, and wooden windows by using roof overhangs to shed rain water. Being that wooden buildings around the world have survived for centuries by utilizing pronounced overhangs and knowing water is the driving force of wood decay, we consider overhangs to be a “common sense” design approach for protecting and extending the life of wood and wood products used in one- and two-story wood construction.
The use of overhangs for enhanced durability may be common sense, but the width of overhangs needed for protection of one- and two-story residential buildings throughout the U.S. is beyond common sense. A design method, based on location and stories, for utilizing overhangs and possibly offsetting floors in order to provide protection to the exterior building envelope (siding, trim, window and door frames) from rain water was published in Remodeling Magazine.
Overhangs as “Green Building”
The benefits of overhangs are also recognized by the ICC/ASHRAE 700-2015 National Green Building Standard that is available as a free “read-only” download: https://builderbooks.com/book/green-building/icc-ashrae-700-2015-national-green-building-standard.html. ICC/ASHRAE 700-2015 contains a Section 602 on Enhanced Durability and Reduced Maintenance. More specifically, Section 602.1.2 Roof overhangs gives the recommended overhang width (and credit) for One- & Two-story Buildings based on “Inches of Rainfall” for both eave and rake overhangs:
602.1.12 Roof overhangs. Roof overhangs, in accordance with Table 602.1.12, are provided over a minimum of 90 percent of exterior walls to protect the building envelope.
Table 602.1.12
Minimum Roof Overhang for One- & Two-Story Buildings
|
Inches of Rainfall(1)
|
Eave Overhang
(Inches)
|
Rake Overhang
(Inches)
|
|
≤40
|
12
|
12
|
|
>41 and ≤70
|
18
|
12
|
|
>70
|
24
|
12
|
- Annual mean total rainfall in inches is in accordance with Figure 6(2).
For SI: 12 inches = 304.8 mm
The Climate Index presented in our 2017 Remodeling Magazine article was based on 1) number of days in the month with 0.01 inch (0.254 mm) or more of precipitation, and 2) mean monthly temperature (°F). Even though the variables used for determining recommended widths of overhangs by the ICC/ASHRAE 700 “Inches of Rainfall” method and the “Climate Index” method aren’t the same, the recommended overhang widths in various climatic regions throughout the contiguous U.S. follow a similar trend, increasing with likely rainfall events or “Inches of Rainfall” as defined by footnote 1 of Table 602.1.12.
Visit https://nationalmap.gov/small_scale/printable/images/pdf/precip/pageprecip_us3.pdf for a color map to use with Section 602.1.2 Roof overhangs.
For a longer discussion of a design method for selecting an overhang length based on a specific location and the Climate Index, see http://www.remodeling.hw.net/how-to/a-common-sense-design-to-create-durable-overhangs_o to read our complete article in Remodeling Magazine.
When we combine common sense, wood science and design knowledge, we are much better able to build structures that will withstand the test of time.
Frank Woeste is Professor Emeritus, Virginia Tech University and a wood construction consultant.
Joe Loferski is professor of sustainable biomaterials at Virginia Tech University.
Comments may be sent to: fwoeste@vt.edu.