Wood Decay from Water Flowing off the Roof


Without kick-out flashing and proper gutter placement, water flowing off the roof runs down the siding (Photo provided by Steve Easley, Steve Easey & Associates, Inc.).

In Build Green: Wood Can Last for Centuries, Carol Clausen and Sam Glass write about yet another way that moisture decays wood. Many decay problems are caused by roof construction that does not allow rain to move off the roof and away from the house. In some houses, water flow over the roof is interrupted by chimneys, dormers, or adjoining walls. Such features are not problematic if they are flashed properly to make the roof watertight. A common water management detail that is often neglected is kick-out flashing. Kick-out flashing is used where a roof terminates adjacent to a wall to direct water from the roof away from the wall and usually into a gutter. Without the flashing, water runs down the wall.


Properly installed kick-out flashing. (Photo provided by Scott Butler, DryFlekt Products, Inc.)

Even slight leaks can produce serious decay problems in OSB or plywood roof sheathing because a large volume of water run-off occurs during each rain, and wood that is wetted by the leak has little opportunity to dry rapidly. Adequate flashing is essential in such areas.

Wood trim at the edge of the roof (fascia) is continually exposed to moisture and is prone to decay, particularly in areas of high-decay hazard. Water tends to “curl” under the edge of the shingles and flow back toward the trim. Shingles must be extended far enough beyond the edge of the roof, and metal edgings must be carefully positioned so that water flowing off the roof clears the trim. Leaking and overflowing gutters further wet the trim. Homeowners should keep rain gutters clean and in good repair. In areas of high-decay hazard, use of decay-resistant or preservative-treated wood for roof trim is recommended. This is especially true for roofs with low pitch. For additional protection, wood trim is often clad with vinyl or aluminum.


Serious problems can occur in the roof of a house if flashing is not properly installed between the roof and dormer. Note that the wood siding on the dormer is wicking water from the roof because it is touching the shingles.

Good tips for the homeowner or your contractor.