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Re: Wood Beam repair

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Tony Luisoni

Decay requires 4 elements to sustain growth: 1) Air, 2) Moisture (water), 3)
food (wood cells) and 4) suitable temperature (above freezing). By eliminating
any one or more of these elements, decay will not grow.

Generally the most frequently used method for existing outdoor conditions is
to keep the wood from getting wet. This can be done by sheltering the wood
from direct or windblown rainfall or high humidity (such as near the ocean or
in humid climates such as the Southeast section of the United States). The use
of sealers and or paint is generally not effective since the wood will have
seasoning checks or other openings that moisture can penetrate and wet the
wood. This in turn causes swelling that can then cause additional openings in
the "protective" envelope (paint or sealer). During dry periods (no rainfall,
winter when humidities are lower) the wood will shrink and cause more openings
(seasoning checks) that will continue to deteriorate the protective envelope.
All the while, the decay continues to devour the wood.  

Field treatment using "brush on" preservatives can be beneficial, but a
continuing maintenance program must be established to keep adding
preservatives and to monitor possible further decay.

A more effect method is to shelter the wood using impervious materials that
are placed around the top and sides of the wood members. The protective
materials, often sheet metal, is spaced from the wood surfaces to allow air
circulation around the wood. The bottom should be left open to prevent
trapping moisture due to rainfall or condensation from collecting and wetting
the wood.

"Dry rot" is a product of decay of wood and is somewhat of a misnomer. The
wood was wet when the decay fungi attacked the wood. 

It is not clear from your post as to the stress (tension or compression) that
is in the portion of the beams that have the decay. Generally, adding
section(s) of wood similar to the method you described are successful. I would
need more information before I could provide more in depth details for the
repair.

Good luck and keep the wood dry.

Bruce Pooley, PE
Timber Design
Lakewood, Colorado