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Re: pressure-treating glu-lams

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The problem with pressure treating large pieces of wood is that the
pressure treatment only extends about an inch into the wood even when
incising is used.  Thus much of the interior of the section is untreated.  

When wood checks it can allow water to get to this untreated wood which in
turn results in the center of the piece rotting.  The surface does not rot
because it is treated but the interior will be worthless.  This is a real
problem with large pieces of lumber.

Thus for pressure treatment to be fully effective in glu-laminated lumber
you want to treat the laminations prior to gluing.   When this is done it
is necessary to plane the laminations after the treatment process.  The
problem is that the shavings from the planing process are considered
hazardous waste because they have been treated, and as a result most
laminators are not willing to go to the bother, so you may have to settle
for treating  the laminated member.

Mark Gilligan


From: "Brian M. McMahon" <brian(--nospam--at)>
To: <seaint(--nospam--at)>
Subject: Re: pressure-treating glu-lams


How a glu-laminated beam is to be treated depends on the manufacturer,
specifications if they already exist regarding required retention
percentage, material type, use, architectural issues, etc.

Treating prior to lamination is a very effective way of ensuring a good and
long lasting protection.  Typically Southern Yellow Pine is treated in this
way in the Southeast United States.  Probably the most expensive

Douglas fir is typically treated by incising the members, the little "cuts"
in the beam, to ensure a deep penetrating treatment.  But, architects don't
like to see the incising marks on beams.  You'll typically see D.F. on
outdoor play structures, girders for wood bridges because they're hidden
underneath, etc.

Both of these examples are the difference between open and closed cell
Without getting too deep it's a material property issue and how the
absorbs the chemicals.

Remember, the odors of treated beams are usually pretty strong.  Make sure
there is plenty of outdoor ventilation.  For example, don't treat a whole
rafter that sticks out of a building by only a couple of feet.  In that
you would apply a coat of treatment by hand if necessary.

Hope this quick explanation helps.

Best Regards,

Brian McMahon, E.I.T.
Universal Timber Structures, Inc.

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