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Re: Wood Beam repair

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Tony,

Since the desired slope for a scarf joint should not be greater than 1:10, 
preferably, 1:12, your slight slope would be quite long.  Since the strength 
of glue is greatest in shear, any joint should subject the glue primarily to 
shear, hence the reason for finger joints in GLB construction.  (The slope of 
each "finger" is, I believe, 1:12.)  Such finger joints will get 
approximately 85% efficiency (from memory; reference USDA Wood Handbook) 
since the fingers are not shaped to a sharp point, which if they were, would 
cause another problem --- that of a sharp re-entrant corner.  To do a finger 
joint in the field would be a real problem!

As far as preservative treatment goes, Consumers Union, of all places, has 
been conducting a multi-year test on wood preservatives, such as Thompson's 
Water Seal, and others.  Although the test had not been completed, some of 
the preservatives performed so badly in less than a year, they issued an 
interim report already listing these as "Not Acceptable," including probably 
the best known brand which could be called the "Kleenex" of wood 
preservatives.

Hope this helps.

A. Roger Turk, P.E.
Tucson, Arizona

. > Hello fellow engineers:
. > 
. > I need your input (bright ideas) again.
. > A (prospective) client of mine has a pressure (treated) wood deck which is
. > supported on cantilevered sawn (untreated S4S ) wood beams. The 
. > cantilever is about eight feet. The front half of the beams have dry rot.
. > I am thinking of cutting the beams at a fairly flat angle, glue on a new 
. > piece (with one perpendicular lagscrew or bolt, side plates or scab-ons 
. > with thrubolts.
. > 
. > What will keep the untreaded wood beams from rotting again?
. > 
. > I appreciate any comments and/or alternative repair recommendations.
. > Thanks for your help.
. > 
. > Antonio S. "Tony" Luisoni
. > Consulting SE  
. >