Many homes in the Dallas area are built on crawlspaces and
use LVL beams. Most of these homes have dirt floors but are
built to code with regard to crawlspace ventilation. In many
cases, I'm finding that the engineered wood is rotting at the
point where metal joist hangers are fastened to the LVL beam.
Is this a problem in other parts of the country?
A.Paul Fisette, director of
building materials and wood technology at the University of
Massachusetts Amherst and a JLC contributing editor,
responds: Like any wood that is neither treated nor
naturally decay-resistant, an LVL — which is typically
made of southern pine or Douglas fir — will rot when
exposed to excessive levels of moisture. In this case, the
problem isn't the engineered wood itself, but rather the
conditions that the LVL beams are exposed to. You're probably
seeing rot at metal-connector locations and not in other areas
of the assembly because the metal hangers are functioning as
condensing surfaces and trapping moisture against the LVL
beams. When the crawlspace air becomes saturated, condensation
will form on the connectors first, creating the warm, wet
environment that causes wood to rot.
To prevent this condensation, you need to lower the relative
humidity in the crawlspace. The first step is to decouple the
crawlspace from the damp soil. Whether the crawlspace is vented
or unvented, it's critical to install a plastic ground cover
over the soil. You can cover the plastic sheet with a layer of
sand to hold the sheeting in place, which will also allow
people to move around in the crawlspace without damaging the
ground cover in the future. In addition, you should coat the
below-grade sections of the crawlspace walls with a
water-resistant, low-permeance material like Sto Watertight
Coat (Sto Corp., 800/221-2397, www.stocorp.com) to slow
moisture transfer through the wall.