I’m using a Glulam for the mainbeam on an outdoor deck project and need some finishing advice. The project is in Woodside, CA, where the climate is moderate with low to moderate rainfall for a couple of months of the year and long periods of dry weather, and the humidity is generally low. Basically, there isn’t a lot of weather to deal with.
Glulams rated for exterior use are special order and $$. This Glulam will not be exposed to elements (rain or UV) as it’s supporting a deck with connecticut bluestone on the top and T&G planks finishing the underside, not the typical board deck that is ‘porous’. Some portion of the Glulam will be visible, but not necessarily exposed to elements. I need 20′ plus some smaller sections, so I would like to go down to my local supplier and stock beam, one that is intended for interior use.
Can’t paint it, wouldn’t look right with the overall design and the exposed glulam finish is part of the appeal. I was thinking of rubbing in a couple of coats of spar varnish before it’s installed, that should protect well for UV and occasional moisture. I am concerned about the exposed end, could leave it exposed or create a cap that also serves as a design element. Glulam specs indicate high dimensional stability, but that rating is going to be for interior use.
Any thoughts, comments, critique?
Replies
I have a dumb question. Is the glue used in making an interior-rated Glulam beam suitable for exterior use? What is the difference between an interior-rated and an exterior-rated Glulam beam?
that's the question I had, and my research does indicate that the mfg uses a different glue for exterior beams. That's why I'm considering spar varnish, it would give me both UV and water resistance. As I indicated, this is not an exposed area where this beam will be. It's kind of like the front door on my house, I used interior grade paint on it and it's been fine for years... it's northwest facing and in a covered entry area, so it doesn't get exposed to sun or water (unless I hose it off).
JeffN7:
As a Wood Technologist (MS PennSt), I think there are several other issues beyond resin/adhesive types, and those include Building Code approval and manufacturer's warranty. This may go so far as the company may not even sell you the product if you tell them your intended use. Then again if you discuss your end use application with the manufacturer, they will likely have an alternative solution that is durable and safe.
In an exterior application the range of Relative Humidity may be signicantly greater than for interior uses and therein, there will be greater changes in moisture content and greater levels of changing MC stresses. There is also the issue of UV degradation, surface wetting and protection from the yeastie/beasties (fungi and insects).
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