Waterproofing an exterior laminated beam

[techinstructor]

Our contractor built a laminated beam using three 2x12 with two pieces of plywood sandwiched in between the boards. There are 2 of these beams, one on each of the gable ends of the house, spanning across the porch opening, under the roof. The beam sits on top of 2x6 treated porch columns.



Unfortunately he did not use treated lumber to construct the beam. He did put a 1x12 facer board around the beam that was treated with Thompson's Water seal, but the facer is constructed in such a manner that water can wick through the edges to get to the beam. We are planning to redo the facer boards using a different method to create a drip edge, but before we cover up the beam we want to treat it with something that will preserve it from any moisture that it might be exposed moisture from the air since we live in NC where the humidity is high in the summers.


I am looking for recommendations as to what to use on the beam. The facer board will cover it so I won't be able to reapply this preservative later so it has to be something that will last. My husband prefers something that will penetrate, such as linseed oil. The beam will not be exposed to sunlight, so UV protection is not an issue. Are there advantages to using Cabot's Clear Solution or Australian Tree Oil other than the UV protection that would make them worth the extra cost? I'm also concerned about how the plywood will react to an oil-based preservative. Are there other products that would work better in this application?

Thank you in advance for your responses.

[Don_P]

Humidity in our climate does not cause the moisture content of wood to rise to the level that would cause rot but can slow drying if rainwater gets in there.

Look up borate, Nisus is one of the bigger suppliers. Borates diffuse through wet wood and interrupt the life processes of fungi and wood consuming insects (they gotta eat it though, termites but not carpenter bees) Bora-care is a formulation designed for dry wood, they basically dissolve borate in a water/glycol mix that dries slowly so it lets the borate diffuse in deeper. It would take multiple wet on wet, saturating coatings to get to a concentration that would do much. Copper napthenate, usually in the paint supplies area at the big box would help as well.

Finishes do not go in very deep and inside the assembly in this application would probably just inhibit drying. Whenever I've cut stained wood, even those finished with "penetrating" finishes the oil had only gone in 1 or 2 cell layers deep.

Put the bottom, then side facings on, cover the entire top with window flashing tape and then the top, bevel it and overlap the sides a bit if possible. A metal cap would be even better. If your HVAC guy is still around they could probably brake something, copper would look nice. If the facings are spaced off the beam on thin strips it would help keep leaks from saturating the beam.