by Bobby Parks
This past year, the company I work for in North Atlanta, Ga., built
several elevated decks, two of which were 28 feet high. Because of
the height involved, caution and planning are critical when
building these decks.
The basic types are stacked multilevel decks (photo, left) and
single upper-level decks (Figure 1). Loading and footings are more
complex with stacked decks, but single decks can be more difficult
to start building because their columns are frequently taller. Both
are often built over sloping ground, which complicates
Figure 1. Elevated decks require more thought
than ground-level decks. Difficulties of working at a height,
safety concerns, increased footing loads, unfamiliar materials such
as steel, and a greater need for lateral bracing all contribute to
— and justify — significantly higher
Consulting both a geotechnical and a structural engineer is
crucial. It's not cheap, but the more details you can provide the
engineers for review, the less their charges will be. The
alternative, risking a deck failure 20 feet in the air, could cost
Designing for Height
It's important to remember that an elevated deck isn't simply a
regular deck on stilts; the large distance from the ground dictates
a number of special design considerations.
For example, all decks need lateral stability, but the taller
support posts on elevated decks increase the need for bracing.
Generally speaking, if the width of a deck (across the house) is
less than or equal to the depth (perpendicular to the house),
bracing may be required. The wider a deck is compared with its
depth, the easier it is to stabilize.
If the decking boards will be laid perpendicular or parallel to the
house, the deck will need angle braces between the columns, along
with bracing under the joists. I prefer diagonally installed
decking because it significantly stabilizes the structure by tying
all the floor joist structure back to the attachment to the house
— and it looks clean and cosmetically appealing.
Stairs are an integral part of an elevated-deck design. If stairs
will be connecting the upper and lower levels, I like to place them
outside and at the ends of the deck to conserve floor space. I find
that constructing the stairs perpendicular to the deck out to a
landing and then back to the lower deck often works best. To
minimize the length of the stair, I try to land it near the house,
where the ground should be at its highest grade. If there is no
lower deck, you may need to work a series of landings into the
design for the stairs to reach the ground (Figure 2).
Figure 2. Bringing stairs off the end of the
deck saves floor space. Very high decks require landings between
levels. Having the stairs reach the ground close to the house,
where the grade is highest, keeps them as short as
The higher the deck, the more critical railing design is. I
consider 42 inches (as opposed to the code minimum of 36 inches) to
be a standard rail height for decks more than 12 feet high, because
the extra 6 inches makes a significant safety difference. A 42-inch
rail hits even tall people at a level that can make the difference
between a fall and a recovery.
I also plan for what will be placed on the deck. We reinforce the
structure where a heavy grill or other item will add a constant
load; doubling or tripling the joists where the grill will be is
usually adequate. Supporting a spa requires additional columns and
beams that make a continuous load path down to the footings. We
also double the joists under spas, and shorten the span between
beams. Although these details aren't unique to elevated decks, the
greater consequences of failure on a high deck underline their
But height isn't all about reinforcement and safety; sometimes a
higher elevation offers additional design possibilities. On a
stacked deck, for example, it's possible to create a porch on the
lower level by building a watertight floor between the decks, and
by framing and screening the walls. Extending the waterproofed
upper deck beyond the lower deck will help to protect the porch
from blowing rain.
The importance of consulting both a geotechnical and a structural
engineer before beginning an elevated deck is worth repeating. The
structural engineer details the plans, but the geotechnical
engineer comes on site. Even though the local building inspector
checks the footing holes, I ask the geotechnical engineer to
determine the soil's bearing capability. With an elevated deck, the
last thing you want to deal with is a settling footing. It's much
better to spend the time and money up front to cover
It's likely that typical-size footings won't be adequate for a
single elevated deck — and it's certain that they won't
support a stacked deck. To handle the increased loads, the footings
need to be wider and deeper than usual, and reinforced with
additional rebar. (For more on footing design, see the
January/February 2007 issue of Professional Deck Builder.)
Width and footing thickness should be engineered, and here's where
the advice of a geotechnical engineer is invaluable. On sloping
sites, the footing has to be deep enough to transfer loads, without
having a tendency to slide or roll toward the outer slope. To
prevent sliding on a slight slope, the bottom of the footing may
need to be 2 to 3 feet below the downhill side of the pier; on a
steep slope, that depth may be as much as 6 feet, or even
To keep the base of the column from sitting in the dirt and
corroding away, I make sure the top of the footing is raised above
grade by extending it with 16-inch- or 18-inch-diameter piers that
project 6 inches or so above grade (Figure 3). These large-diameter
piers allow for a larger footing. They're also less prone to
toppling or rotating under load than are more slender piers. I use
# 4 rebar to join the footings and the piers.
Figure 3. Piers rising above grade keep steel
columns out of the dirt. Making them a larger diameter provides a
larger target for locating the column, and helps to spread the load
on the footing. Attaching columns with 1/2-inch bolts and wedge
anchors adds stability and resists wind lift.
We dig the footings using shovels because moving digging equipment
onto the site of an existing residence causes lawn and property
damage. Where soil conditions dictate deep footings, say 6 feet,
we'll dig down to 3 feet at twice the footing width. This creates a
ledge we can stand on when digging the bottom 3 feet. We've been
known to shorten shovel handles while working in a tight spot. It's
awkward and difficult but often the only option we have.
On steep slopes, we key the footings into the ground (Figure 4) by
digging the uphill half of the footing hole several inches deeper
than the front to gain additional bite in the ground.
Figure 4. Footings for stacked decks need to
be larger to support the increased loads. On sloping sites, special
measures need to be taken to prevent the footing from sliding
downhill; two such are digging a deeper footing, and digging the
uphill side of the footing in deeper to provide a
As the elevation increases, so does the overall level of difficulty
of building decks. For one thing, scaffolding gets more extensive.
And the higher the work zone, the more time it takes simply to move
people, tools, and material.
On a deck just 12 feet up, for example, one carpenter standing on
the ground can still hand lumber to another on the framing or a
scaffold. As heights approach 20 feet, however, the crew has to
pass material from man to man on successive scaffold levels. Even
lighter, smaller objects, like tools and hardware, take more time
to move — usually with a rope and bucket.
I have found that multiple sections of pipe scaffolding with leg
adapters, stabilizers, bracing, and walkboards work well on most
sites (Figure 5). We start out with scaffolds along what will
become the two sides of the deck; the sections can be relocated as
needed during construction. Allow for extra walkboards on all
levels to assist in transferring materials to the deck level.
Scaffolding even helps with demolition of an existing deck, making
the process faster and safer.
Figure 5. Scaffolding is essential to provide
a safe and efficient workplace at a height. Be sure to rent enough
to scaffold the sides and the width of the deck, and have extra
walk boards to facilitate material handling.
On sloped sites, you need to create a stable, level base for the
low end of the scaffold; we use timber cribbing. Scaffolding always
needs to be tied to the house and together. The crew fastens
eyebolts to the house to secure the scaffold.
In most cases, the crews raise the columns and beams from the
scaffolding. On a new construction site, cranes or forklifts may be
used to handle steel columns and other materials. For existing
residences, however, heavy equipment may not be an option because
of tight lots, or risk of damage to the landscaping or
Stacking Decks on Wood Columns
For single-level decks up to 12 feet in height, I prefer to use
6x6s as columns. Beyond that elevation, my company uses steel
columns, with some exceptions.
For multilevel decks, I prefer to stack the different levels using
6x6 posts in a continuous load path that incorporates the lower
level's floor framing. This approach allows the crew to work its
way up one level at a time, with no need for scaffolding: After the
first level is framed and decked, the crew has a platform from
which to build the next level.
Where one post stacks atop a lower level, we nail four or more
floor joists together and run them directly over the lower column,
and directly under where the upper column will land (Figure 6).
This supports the upper deck structure and provides a solid-wood
load path from column to column. We fit two thicknesses of solid
bridging between the quadruple joist and the joists to either side,
over the beam. Strapping ties the upper and lower columns together
with the beam, creating a continuous tie to resist wind lift. At
the house, fabricated steel brackets on both sides of the quad
joist attach it to the ledger.
Figure 6. Multiple joists provide a continuous
load path between upper-level and lower-level wood columns.
Diagonally laid decking adds lateral stability.
The size and number of decks that are being built and stacked
determines the column layout. Tighter spacing of the columns can
distribute loads over more footings and let you stack wood columns
on wood floor framing without worrying about concentrated loads
crushing the wood.
We anchor the 6x6 columns to the footings using brackets the
engineer calls out for the circumstance (Figure 7). To attach the
main beam, the crew notches the tops of 6x6s to fully receive the
doubled 2x10 or 2x12 beam (Figure 8). That leaves a section of the
post about 2 1/2 inches thick that comes flush with the top of the
beam. We double-bolt each post-and-beam connection with 1/2-inch
Figure 7. Using approved column bases takes on
more importance on stacked decks. Be sure the ones used can handle
the increased loads.
Another option for supporting upper decks is to use independent 6x6
columns (they can be up to 20 feet in height) that go through the
lower deck's framing adjacent to the lower deck columns,
distributing the upper-deck loads directly to the footings. You can
stabilize the longer 6x6 by tying it into the first-floor framing
and bolting it to the lower-level column. Steel columns can be used
in this same fashion.
Figure 8. Notching 6x6 columns to receive a
doubled 2-by beam provides a spot to positively anchor the beam to
the column using bolts.
Using double columns from the footings to the first-level beam
gives the structure a larger bearing base; and using twice the
number of bolts where the beam bears on both columns assists with
locking in the beam against beam roll.
Many jurisdictions require steel columns for decks above a certain
height. For decks higher than about 13 feet, it's advisable to use
steel regardless, because of the added stiffness of the column and
its connections (Figure 9). This should be engineered. Our
structural engineer specs 4-inch or larger schedule 40 square
tubing on high decks.
Figure 9. For decks elevated 13 or more feet,
steel columns are needed to provide rigidity. Have the footings in
place to determine column height before ordering.
I keep the column count as low as possible on elevated single-level
decks, while allowing for the loads, because I want to handle as
few steel columns and footings as possible. Those things are heavy
— 16 pounds per foot for a 4-inch column, and 20 pounds per
foot for a 5-inch column.
You need to factor in the turn-around time for steel-column orders,
to minimize delays on the job. I like to establish the deck level
on the house and pour footings before ordering the columns.
With the footings done and scaffolding erect, it's time to build.
We usually get the beam to the top of the scaffold before raising
the columns. It's best if the beam is built up of long 2x10s or
2x12s, which can be raised to the top of the scaffold and then
assembled. That way, the columns don't get in the way of raising
the beam, and the beam spanning the two scaffolds helps to brace
them. It's possible to span greater distances using a treated
glulam or steel I-beam, but those require the use of a crane.
To raise a column from scaffolding, the scaffolding has to be set
up and secured to the house. We wrap the top of the column with a
heavy-duty nylon strap, which is hooked to a 5/8-inch rope that
runs to a block and tackle affixed to the scaffold's top. A
slipknot keeps the strap from sliding past the saddle welded to the
Place the column perpendicular to the house, its top over the
footing and its base closer to the house. Hoist slowly while two
men on the ground move the base end of the column toward the
footing. This relieves stress on the scaffolding. Raise the column
until the base can be set on the footing.
When the column is correctly placed on the footing, we tie it off
to the scaffold and drill into the pier for 4-inch-by-1/2-inch
wedge anchors and bolts through the column's base plate. Then we
bolt the column to the footing.
With both columns set, the crew drops the beam into the saddles on
the columns, and braces it to the house — first with some
2x4s running to the ledger, and then with joists (Figure 10).
Angled 2-by bracing fastened to stakes in the ground is also used
as needed. Raising shorter columns in place may simply require a
couple of men on the scaffolding with ropes or straps and a couple
of men on the ground to help.
Figure 10. Bracing the beams on steel columns
is done first with temporary 2x4s tied to the ledger, then more
permanently using joists.
Using steel columns to stack decks can be a problem because code
doesn't allow for steel columns to bear on wood. On two-level
decks, however, it is possible to use square steel columns if you
have brackets welded to receive the beam for the lower deck. The
benefit of one continuous column is that you don't have to worry
about beam roll or the crushing stress created by stacking
structure on structure.
When pricing elevated decks, you've got to allow for several items
you don't encounter on most other decks. In addition to typical
charges, I include the cost of a structural engineer and a
geotechnical engineer; the cost of scaffold rental, as well as set
up and teardown; and the extra cost of steel columns. And if you're
stacking decks, don't forget the cost of the fabricated brackets to
tie the quad joists to the ledger.
Footings can be surprisingly costly, particularly those on sloped
sites. Of course, footing requirements vary geographically, but I
allow as much as $500 per footing on some sites. I also include the
cost of raising steel columns, which usually takes 4 to 5 man-hours
Over and above these line items is an elevation charge. This is an
attempt to cover the slowdown in production that occurs when crews
work at high elevations, and to pay them a premium for doing such
work. As a general rule, I add $2 per square foot at 12 feet in
elevation, and another $2 per square foot for every 2 feet in
elevation above that.
And sometimes, I walk away from a project. For example, if very
large beams are required and there's no way to get in with a crane,
it's just not worth the risk.
Bobby Parks is the vice president and operations manager at
Archadeck of North Atlanta, Ga.