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 this 1896 home from Sarasota County, Fla., shows, house designers understood the basics of protecting wood siding, wood framing, and wooden windows by using roof overhangs to shed rain water. Note, for instance, how the runoff from the roof is concentrated at the corners, thus away from the all-wood windows that benefit by the protection from roof 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 at various locations throughout the U.S. is beyond common sense. Here is a summary to use on one- and two-story residential buildings 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.
Equilibrium Moisture Content
When wood is manufactured into various building products and installed into buildings, the moisture escapes or is absorbed until an equilibrium moisture content (EMC) is reached between the wood and the surrounding atmosphere. The EMC of wood is dependent on the temperature and humidity of the surrounding environment. Typical EMCs for exterior framing in the U.S. range from 7% to 14%, depending upon location.
Wood products become susceptible to rotting when their moisture content rises above 25% and to insects and molds when MC is 20% and higher. Lumber and wood products are typically dried to moisture contents below 19% in order to stop the proliferation of deterioration inducing organisms and provide structurally stable materials. Once in service, though, it is critical that items made from wood not be subject to conditions that will allow the moisture content to rise above 20% for any length of time, particularly in those regions where temperatures are optimal for the growth of organisms responsible for decay in wood.
The Climate Index
Rainwater provides the most significant source of excessive moisture to exterior woodwork. Gravity flow and capillarity of rainwater create the potential for increasing the moisture content to dangerous levels, possibly leading to decay. How much water from rain accumulates on exterior wood and wood products depends on the amount, frequency, and type of rainfall, as well as the amount of wind during rainfalls. Studying the interactions of these factors has led to the development of guidelines by which builders and designers of structures, particularly residential, can reduce the risk of deterioration to exterior woodwork.
In 1971, T.C. Scheffer developed a formula based on U. S. Weather Bureau summaries to determine an index of decay potential for above ground wooden structures depending on location within the United States.
This formula integrated experimentally obtained growth and decay rates for rot inducing organisms, monthly rainfall duration and quantity, and monthly temperature measurements into yearly indexes for three diverse climatic regions:
In general, the larger the index for a given location, the more potential there is for decay propagation and the more protection that is needed for exterior woodwork to remain unharmed. The map clearly suggests that the extent of protection needed in different climatic locations is extremely variable from coast to coast—the decay hazard index varies from 0 to 130.
Designing Adequate Overhangs for Durability
In addition to proper flashing around windows, doors, and openings, an effective method for avoiding the wetting of exterior woodwork is to provide adequate overhangs in order to keep rain from soaking into and splashing onto the wood. Here’s a table with recommendations for appropriate lengths of overhangs, based on three ranges of the Climate Index, eave gutters, and stories:
When this table first was published in 1980, it only gave roof overhang widths for one-story buildings. We expanded the table to cover two-story buildings based on the assumption that the overhang width-to-wall-height ratio that adequately protects a one-story building will also protect a two-story building.
Since eave gutters provide additional protection against wetting from rainwater, using them makes it possible to reduce overhang lengths. Gutters should be incorporated into the roofs of wood buildings to help direct water away from the foundations as well as the building envelope. It is important to note that gutters must be maintained to provide the needed protection from rainwater—overflowing gutters could actually exacerbate the problem due to splash.
At first glance, the overhang (total) widths of 40 and 60 inches for two-story buildings might seem too long to be of practical use, especially in high-wind areas. For the case of an excessively wide overhang, designers and builders have an opportunity to exercise architectural creativity in employing these durability design recommendations. One option for two-story structures is to offset the second story, as shown below, in order to help meet the roof overhang width requirement needed for protection against water exposure. Here is an example from Dr. David Carradine, now at the Building Research Association of New Zealand, Inc. (BRANZ):
Both research and field experience have provided methods for specifying architectural detailing in order to maximize the life of wood products used for the exterior envelope in one- and two-story buildings. Regional differences in temperature and precipitation have been accounted for by Scheffer’s decay hazard index that provides for a location-specific guide to help prevent a common wood building problem: decay of wood products in the exterior envelope.
This article is a revised and condensed version of Wood Bits: Improving Building Durability, by Carradine, Woeste, and Loferski, published in 2001.
