by Patricia McDaniel Hamilton
Building the First-Floor Deck
Houses built on pilings are in essence hybrid post-and-beam houses.
Ultimately, all loads must resolve to the pilings by way of beams, girders,
and cross-bracing. Because of the complex load paths involved, and the likelihood
that the home will be tested by high winds, water, and impact from
waterborne debris, it is essential that the entire structure be engineered.
Although we can design very simple houses ourselves in accordance with
the SBCCI Standard for Hurricane-Resistant Residential Construction (formerly
called Deemed to Comply), larger houses typically require an engineer
familiar with the unique demands of coastal areas.
Figure 2. The gas-powered chain saw is the weapon of choice for notching pilings (above). Girders are tacked in place for drilling the bolt holes (below). The author likes treated Parallam girders (further below) because they come in long lengths, making them quicker to install than built-up girders.
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Girders. The foundation girders - treated Parallams or built-up treated
2x12 beams - are attached to the pilings with bolts (Figure 2). These
connections must be engineered. The piling should be notched only deep
enough to provide a shelf for the
beam to bear on, while still leaving at
least 50% of its cross-section for
securing the bolts. If the piling is cut
flat instead of notched, steel straps are
usually required to secure the beams
to the pilings.
Notching. As a practical matter,
laying out and cutting flat notches in
round piles while working 6 to 10 feet
above grade can be extremely difficult.
We build a working platform by nailing
framing lumber from piling to piling
about 3 feet below the final top elevation
of the poles, and use walk planks
and guardrails where necessary.
The gas-powered chain saw is the
tool of choice for cutting pilings.
Sharp chains are essential. We usually
have to resharpen the chain about
every 20th piling. And though it's easy
to forget, we try to avoid placing the
saw in the sand.
The first cut establishes the top of
the piling. We always mark and cut
each piling 3 /4 inch below the elevation
of the bottom of the floor joist to
make it easier install the crawlspace
ceiling in later stages of the project.
The next step is to establish the house
corners and girder lines on the tops of
the pilings. We use conventional batter
boards and string lines for this, and
extend the corner point up to a block
nailed to the top of the piling with a
plumb bob. The outer pilings are usually
placed 6 to 12 inches in from the
edge of the footprint. At outside
walls, we cantilever the floor joists
3 1 /2 inches beyond the girders. This
leaves us plenty of room to attach
enclosure walls or lattice below to
enclose the girders - a zoning
requirement in some communities.
After establishing the corners, we
establish a string line for each row of
pilings. The layout marks for the
notches at each piling are then measured
from the string lines. The girder
lines sometimes need to be adjusted
slightly to ensure adequate bearing at
each piling. Once the line is established,
we snap it across the tops of
the pilings.
Next, we locate the seat of each
notch with a transit. After marking
out the notch cut, it's then a matter of
holding the chain saw steady enough
to create a flat, plumb vertical cut and
a smooth, level horizontal cut. It takes
care not to overcut. Although it might
seem easier to simply flat-cut the tops
of the pilings and use metal straps to secure the girders, this entails the
added cost of additional bolts and
metal straps, plus the added labor of
drilling the extra bolt holes and cut-ting
flat sections on the sides of the
pilings for the metal straps. However,
there are occasional pilings where this
detail cannot be avoided - where
girders meet at corners, for instance.
Once the pilings are notched, the
beams can be bolted on. We tack or
clamp the beams in place on the
pilings for drilling. Drilling one-inch-
diameter holes through 12 or 14
inches of piling and girder is no pic-nic.
A 1 /2-inch or 3 /4-inch electric or
air-powered drill is required. An air-powered
drill is expensive but less
dangerous to use, because there's no
kickback from torque when the bit
seizes in the hole.
We typically use two 3 /4- or 7 /8-inch
bolts at each piling/girder connection.
If two girders meet at their ends on a
piling, each one needs two bolts. With
built-up 2x12 beams, we overlap at
joints. Though they're more expensive,
Parallams are easier to install because
one continuous beam can run the
length of the house. On occasion, concentrated
loads will dictate the use of
Parallam beams, or sometimes even
steel I-beams. Steel beams and flitch
plates have to be protected from corrosion,
of course, by hot-dip galvanizing.
