Millions of wood decks are built every year. Unfortunately,
many of them will deteriorate long before they should. As a
deck builder in the Pacific Northwest, I have a lot of
experience demolishing and replacing rotted decks. I want what
I build to last, so when I tear down a rotting deck, I try to
figure out what went wrong.
Know Your Enemy
Rot is a fungus, an organism that feeds on and destroys natural
materials like lumber. The spores it uses to reproduce are
nearly everywhere and will grow wherever conditions are right.
There are many different types of rot, but they all require
food and moisture to survive. If you understand how these
organisms grow, you can build decks that are less hospitable to
Food source. Most decks
— particularly the structural framing — are made
from woods that are susceptible to rot. Because the companies
that treat lumber use local materials, the wood you use depends
on where you work. From Denver east, builders use
pressure-treated southern pine, a kind of wood easily
penetrated by chemical preservatives. On the West Coast, we use
pressure-treated hem-fir, a species group that does not accept
chemicals very well. Frequently, only the outer surface
contains preservatives, so the interior of the lumber is
unprotected and susceptible to rot (see Figure 1).
Figure 1.Pressure-treated hem-fir lumber is
incised to increase penetration, but often the colored
preservative barely soaks in at all, as is evident on this
piece of blocking. The end cut should have been coated with
preservative; the brown staining visible on the end grain is a
sign of rot.
Moisture. Wood won't
deteriorate unless moisture is present. Green lumber often
contains enough moisture to rot, but even materials that start
out dry can become wet when they're exposed to the weather,
washed, or used as a surface for potted plants.
Some kinds of rot fungi, the wet rots, require a wood moisture
content of 30 percent or more to survive. Many types of rot
will do just fine as long as the wood has a moisture content of
at least 20 percent, a threshold easily reached in damp
climates like ours.
People often speak of "dry rot," but if lumber is dry, it will
not decay. So-called dry rot fungi are a specific type of brown
rot that sends out hyphae, strands of tissue that can transport
moisture from surrounding wood. But this can't happen unless
the surrounding wood is wet. If you can keep a deck below 20
percent moisture content, it won't rot.
A typical deck harbors hundreds of places where organic matter
can collect and sit. I call them collection zones. Every time
it rains or the wind blows, leaf litter — leaves,
needles, twigs, dirt, dust, and grass clippings — finds
its way into the gaps and cracks in the deck. The collected
material holds moisture, so that a deck full of collection
zones mimics the natural habitat of fungus — wet fallen
wood on a forest floor.
A collection zone can be very small. For example, when a
baluster is nailed to the face of a rim joist, the area where
the two pieces meet is a collection zone. Poor deck design and
building practices increase the size and number of these zones.
Once the collected material becomes wet, rot will attack any
unpreserved wood in the area.
The best way to prevent this is by limiting the size and number
of the collection zones. If you prevent moisture and debris
from getting into places they can't get out of, you reduce the
potential for rot.
Fascia detail. Some common
building practices create perfect collection zones. For
example, many carpenters rim the deck with a fascia board
(Figure 2). There is usually no airflow between the fascia and
the framing, so the 20 percent moisture content required for
rot can be maintained over long periods of time. Water takes
debris down between the fascia and the rim joist. Once organic
matter gets into this collection zone, there is no way to get
Figure 2.On this deck (left), water got between
the fascia and rim, partially rotting the fascia and completely
destroying the rim. When the fascia was removed, the rim
collapsed. Here (right), the author installs composite decking
on a pressure-treated frame that will completely cover the
fascia and rim, avoiding this potential collection zone. The
lattice separating the fascia and rim joist will also help to
drain away water.
It's best not to use a fascia at all, but if one is required,
cantilever the deck boards one inch over it. This detail
greatly decreases the likelihood of rot because it reduces the
size of the areas that dirt and moisture can get into. One way
to compare construction details is by looking at how many
linear feet of collection zone each design creates (Figure
Figure 3.Wrapping a 12-by-16-foot deck with fascia
(left) creates 40 linear feet of potential collection zones at
the rim joist. Running the deck boards parallel to the house
and overhanging the fascia will reduce the collection zones to
a cumulative 12 1/2 inches (1/4 inch per gap x 50 gaps = 12 1/2
inches). While some water and debris will get in, the greatly
reduced amount will be much less likely to cause
Built-up beams. It's common
practice to double or triple up 2-by material to form beams.
Unfortunately, the space between the pieces is a perfect
collection zone. It's better to carry loads with solid 4-by or