All deck builders should refer to the DCA6 — prescriptive
guidelines for residential deck construction — and coastal
builders should pay extra-close attention to the material choices
and structural connections outlined in its pagesby Frank
A casual inspection of just about any wooden boardwalk or
ocean-side deck will provide ample evidence of what a harsh coastal
environment can do. Heat, ultraviolet light, elevated moisture
levels, and salt spray all take a toll. The apparent degradation of
materials evidenced by splintering, splitting, and rusting call
attention to one of the coastal contractors' highest priorities:
designing and building safe and durable decks. Fortunately, there
is help at hand to guide deck builders toward code-conforming deck
design and good construction practice.
The reference document for safe residential construction is the
International Residential Code (IRC), which has been adopted with
some modifications by most states and jurisdictions. While the 2006
IRC provides comprehensive prescriptive design guidance on the
foundation and structural framing of the residence from the walls
inward, the available guidance on the design of an attached deck,
until recently, has been very limited. In recognition of this
deficit of information, the American Forest & Paper Association
(AF&PA) developed and published Design for Code Acceptance 6
(DCA6): Prescriptive Residential Deck Construction Guide, which is
available on the Web at www.awc.org/Publications/DCA/DCA6/DCA6.pdf.
The document is 20 pages and is branded by the AF&PA, the
International Code Council, and Fairfax County, Va. The reader's
attention is directed to the bottom of page 1, where the basis of
the document is stated—IRC sections are bracketed when they
form the basis of a section, while other sections are considered
"good practice recommendations."
While every section of DCA6 is important to deck safety, the
purpose of this article is to give background on sections of the
DCA6 that may be of special interest to the coastal deck
contractor. All discussion is referenced to bracketed page numbers
and sections of the DCA document.
Minimum Requirements: Decay Issues for Lumber
Building a safe deck starts with selecting appropriate materials.
The DCA6 [item 2, page 2] provides builders with two options for
real wood decking: naturally durable species, such as redwood or
western cedars, or pressure-treated lumber. Both deserve careful
study by the deck contractor who is focused on durability.
Essential reference. The Design for Code
Acceptance 6: Prescriptive Residential Deck Construction Guide,
provides practical guidance for designing and building safe,
durable decks. Download and print the document at www.awc.org/Publications/DCA/DCA6/DCA6.pdf.
Naturally durable species. While such woods as
redwood and cedar are widely considered to be naturally durable
species, only the "heartwood" of redwood, cedars, black locust, and
black walnut is actually considered decay resistant by the IRC. The
sapwood, the outside part of a log, does not qualify. According to
the USDA Wood Handbook (1999), "Untreated sapwood of substantially
all species has low resistance to decay and usually has a short
service life under decay-producing conditions." Only the average
heartwood of species is rated in Table 3-10 of the Wood Handbook.
Corner sapwood is permitted if 90% or more of the width of each
side on which it occurs is heartwood.
Given these restrictions, it is unlikely that the typical decking
lumber available will consistently meet the IRC's definition of
"naturally durable wood." To ensure deck durability, the deck
contractor should consider special-ordering "all heartwood" in
conformance with model code definition of naturally durable
Deck disaster? This is not an atypical
condition of the materials and connections for decks built near the
ocean. Heat, ultraviolet light, elevated moisture levels, and salt
spray exact a stiff toll on wood decks in relatively short periods.
A condition like this should raise safety concerns for both the
deck builder and the general contractor.
Pressure-treated lumber. Pressure-treated
lumber works well for both the substructure and the decking.
However, contractors should pay close attention to the retention
level used for lumber on all parts of the deck.
Citing the IRC, the DCA6 is clear: "All lumber in contact with the
ground shall be rated as ‘ground-contact.' " The difference
in retention levels between "ground contact" material and "above
ground" material is significant, as shown in the table below.
This table gives the American Wood-Preserver's Association
(AWPA) required minimum retention levels for dimension lumber.
Treatment levels for both the "old" CCA and the new preservative
chemicals are provided. Note that required retentions for the
"above ground" treatment are substantially less than those required
for "ground contact" materials.
To ensure the right treatment level, deck contractors must
carefully examine the treatment tags on the lumber they purchase,
making sure that "ground contact" material is being ordered and
delivered when required. Once material with the proper retention
level is purchased for the job, I recommend that contractors keep a
sample of the lumber treatment tags, along with a copy of the PT
lumber invoice, as part of the job file. This will help to
substantiate the contractor's due diligence should a construction
defect complaint related to the deck ever arise.
Even in "above ground" applications, however, there is a strong
argument for using materials suited for ground contact. At this
time, the adequacy of the new PT chemicals at above-ground
treatment levels is largely unknown. Since about 1985, all the PT
lumber in our area was CCA-treated to the ground-contact level. As
such, deck contractors have vast experience using the
ground-contact southern pine in deck applications. After the
transition to the new chemicals, the above-ground material came to
be readily available for deck construction in the Virginia area and
along the eastern seaboard. However, the durability of the new
preservatives and lower treatment levels in deck applications is
unknown at this time. Faced with uncertainty on this subject, I
would strongly recommend that contractors consider returning to the
practice of using "ground contact" lumber for all above-ground deck
The lateral load connection required by
section R502.2.2 of the 2007 IRC requires hold-down tension devices
like those shown here. This type of connector is required in at
least two locations per deck, and each device must be designed for
an allowable stress capacity of at least 1,500 pounds.
Solid-sawn deck posts (timbers) embedded in the ground may even
require a preservative treatment level that is above the "ground
contact" treatment level typically used for 2-inch dimension
lumber. (For additional information on the recommended treatment
for timbers in the ground, please visit www.southernpine.com/pdf/tpsp_table4.pdf
and refer to category "Lumber/Timbers," subcategory "Ground Contact
or Fresh Water." Structural deck posts covered here are in the
realm of "Critical Structural Components.") Also keep in mind that
regardless of the PT wood materials selected, decks should be
inspected annually for evidence of decay and unsafe conditions.
Based on Table 5 of the DCA6, the table
above shows the required fastener spacing for southern yellow pine,
Douglas-fir larch, or hem-fir deck ledgers that will be secured to
a 2-inch nominal spruce-pine-fir band joist or engineered-wood rim
board. While the spacing values for 1/2-inch lag screws and bolts
are easy to select from this table, careful study of footnotes 1 to
9 is crucial before using the data. Noncompliance with any of the
footnote provisions could produce a disastrous in-service result.
For example, footnote 3 directly relates to the photo (above),
which shows the result of overlooking the Z-flashing on a
Corrosion protection of metal parts. Item 4 of the
Minimum Requirements in the DCA6 covers the corrosion resistance of
fasteners and metal hardware. Coastal contractors should skip right
to the fourth bullet point, which trumps the previous three bullets
wherever the job site is located near the ocean:
Fasteners and connectors exposed to, and located within 300 feet
of, a saltwater shoreline shall be stainless steel grade 304 or
"Fasteners" include all nails, screws, lag screws, and bolts.
"Connectors" are typically joist hangers and other framing hardware
that is fabricated from rolled steel. In the interest of deck
safety and increased durability, the provision should be considered
by the contractor and owner for decks greater than 300 feet from a
Assumed design loads. DCA6 is based on an
assumed live load of 40 psf and the 10 psf dead load.
The 40 psf live load (LL) anticipates occupants only with typical
deck furniture such as lightweight tables and chairs. It does not
anticipate heavy loads such as large planters, portable pools, the
water in a hot tub, or any other load beyond the weight of
Dead load (DL) is defined as the self weight of the structure and
the weight of fixed objects. The use of a 10 psf DL anticipates
wood framing with wood or plastic decking. It does not include the
dead weight of a hot tub or any other permanently attached
For coastal contractors, especially, it should be noted that the
DCA6 does not address lateral loads on decks produced by wind or
seismic events. For this, builders should turn first to the 2007
IRC, which does address the issues of lateral loads on decks as
shown in the illustration on page 3. These new deck provisions to
the 2007 IRC can be viewed at
As a matter of good construction practice, the deck contractor
should formally communicate these load assumptions to the homeowner
by reviewing the DCA6 Minimum Requirements [items 5 and 6, page 2].
If the customer's expectations call for loading conditions that go
beyond occupant loading, you will need to get a design professional
Joists and Beams
Maximum joist spans provided in the DCA6 [page 3] are based on No.
2 grade lumber and wet-service conditions. The tabulated spans are
less than common residential floor spans because the assumed
wet-service conditions reduce the strength properties of the
All carrying beams (multiple deck girders) must bear fully on
supporting posts, while the joists above should fully bear on the
beams. Such "wood-to-wood" bearing provides the most efficient
connection possible between wood framing members. Bolting the beams
to the sides of the post is not an efficient connection. In the
case of a 2x beam bolted to a 6x6 post, for example, the force is
applied to the beam members perpendicular-to-grain and the bolt
forces are applied parallel-to-grain in the post. In this
situation, the capacity of a typical 1/2-inch bolt in the
perpendicular-to-grain loading is relatively low under wet-service
conditions. Splitting is also likely to occur when two (or more)
bolts are aligned vertically in the beam, because the shrinkage
rates between the beam members and the post are dramatically
different (about 40:1).
Premature decline. On a beachfront home in
Virginia, no Z-type flashing was installed over the ledger-to-house
connection, which led to extensive decay.
Ledger Attachment and Flashing
Based on deck collapses and injuries reported in media sources, the
connection of the deck ledger to the house band joist is the most
critical structural element of a deck. But the integrity of the
connection is based on the assumption that the house band and deck
ledger has no decay [DCA6 Table 5, page 12]. Therefore, flashing
the connection is easily as important as the connection design
(fastener size and spacing).
Ledger fastening. The fastener spacings
from DCA6 Table 5 are based on tests of simulated deck-ledger to
band-joist connections performed at Virginia Tech and Washington
State University that form the basis of an IRC code proposal. The
design data (except for engineered wood product [EWP] rim boards)
with footnotes were adopted into the 2007 IRC. Since the
publication of the ledger fastener table in the 2007 IRC, AF&PA
has added EWP rim boards to the DCA6 Table 5 based on tests at
Ledger flashing. The critical role of
effective ledger flashing cannot be overemphasized. The photo at
the top of page 5 underscores the result from overlooking the
Z-flashing on a beachfront home. If for any reason the wall
sheathing and house band joist are exposed to water, decay will
follow because the typical ledger connection is "water trapping,"
and significant decay is the likely outcome.
Best practice calls for not only including the Z-flashing
recommended by DCA6 but also providing self-adhesive flashing
against the house. This type of membrane is strongly recommended in
coastal conditions to help protect against water infiltration
resulting from wind-driven rain.
Ledger connection to cantilevers. Without a
connection detail by a professional engineer, a deck ledger should
never be supported off the end of a cantilevered floor [DCA6 Figure
18] because the load path is not complete. It is extremely
difficult to transfer the vertical load from the deck ledger to the
floor joist of the house using fasteners, because the rim
joist/house band is bearing on "air" instead of a plate with a high
wood-to-wood bearing capacity. This situation is depicted in the
photo below. It is extremely dangerous and likely to collapse under
relatively low deck loads (possibly dead load only).
Deadly rail. The photo above shows an example
of a grossly deficient guardrail system that produced a fatality
and another injured party. In this case, a young man died after
falling through the guardrail with posts that were nailed only to
joists on the end of the deck. Note the notched 4x4 posts — a
detail that is not permitted in the DCA6.
In the vernacular of DCA6 [pages 15-16, 18-19], a deck's "guard" is
the guardrails or handrails surrounding the deck. All deck surfaces
greater than 30 inches above grade are required to have a guard.
Details for a proper guard can be found in DCA6 Figure 24. At stake
here is the protection of the occupants against severe injuries and
fatalities (see photo). Falls through decks guards are generally
grave, and among problems with decks, only deck ledger failures are
reported more frequently in the media.
DCA6 Figure 26 shows the rail post construction needed to meet the
intent of the IRC code (shown in the illustration "Guard Post to
Rim Joist Detail", below). By code, the top of the rail must safely
resist a 200-lb. outward force. "Safely" in this case means that a
safety factor is applied to a test of a detail to prove that it can
work in the field, because tests rely on virgin material and
perfect fabrication that doesn't always exist on site. In an effort
to determine what constitutes a safe rail post connection to the
end of deck joists, we tested numerous details at Virginia Tech.
All lagged and through-bolted connections failed to meet the
500-lb. test load, despite numerous attempts to reinforce the
connection with blocking. Only the connection that relied on a
metal hold-down anchor, such as the Simpson HD2AHDG (www.strongtie.com/products/connectors/HDA-HD.asp)
or DeckLok (www.mtdecklok.com/railpost.htm),
passed the test (as illustrated in "Guard Post to Rim Joist
Hold-downs required. To meet code, guard posts
for deck guards that run perpendicular to the deck joists must be
attached to the rim joist as shown at right. This connection relies
on hold-down anchors, which in a saltwater environment should be
made from stainless steel.
As mentioned previously, stainless steel connectors and fasteners
are recommended in DCA6 for saltwater shoreline exposure. DeckLok
is one source for 304 and 316 stainless steel hold-down anchors
that resisted the 500-lb. load used in the Virginia Tech guard post
Wrong! Deck joists (framed with PT framing
lumber) have been hung from a ledger that is lagged into the rim
board of a cantilevered I-joist system. Even if the lag screws were
to be positioned to penetrate the I-joist flanges, the connection
would be dangerously weak. It is likely to collapse under very low
loads, possibly the dead load of just the deck structure itself.
DCA6 strictly prohibits the connection of a deck to a cantilevered
The publication of DCA6 by the AF&PA is a landmark step in
advancing the cause of safe decks in order to prevent injuries and
fatalities from deck collapses. While I have only commented on a
few sections with respect to deck safety and durability, the entire
DCA6 is important and deserves careful study and consideration by
the professional deck contractor. For decks applications and
conditions that fall outside the scope of DCA6, the contractor
should seek professional design input and evaluation by the local
jurisdiction through the permitting and inspection program.
Frank Woeste, Ph.D., P.E., is Professor
Emeritus at Virginia Tech University, Blacksburg. All photos by the
author. A list of his publications can be viewed at www.vtwood.forprod.vt.edu/resumes/woeste1101.pdf
. Questions, comments, and suggestions are welcome and may be sent
to the editor at firstname.lastname@example.org.