deck (DOC) by ashrafp

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									                                BASIC DECK DESIGN




Designing the substructure
Start by combining your deck’s surface dimensions with the information given in Tables l, 2, 3
and 4 to work out the proper number, spans, and spacing of joists, beams, and posts for the deck.
Notice that the tables are based on the deck’s weight and the loads its surface will carry, as well
as the size and wood species of the different structurals. The deck’s height is also an important
consideration. In addition, be sure to take into account any special stresses that may affect your
deck such as high winds or heavy snowfall.

Deck loads and height
Building codes in many areas require a substructure to be strong enough to bear 40 pounds per
square foot (psf) of “live” load plus 10 pounds per square foot (psf) of “dead” weight. To carry
such loads, decks higher than 12 feet above grade (even at only one post) may require cross
bracing and special post designs.
The tables and other design information presented here are based on this 40 plus 10 psf loading
at deck heights up to 12 feet. If your deck is to be higher or to bear greater loads (heavy snow,
large planters, a fountain), get professional help with the design.
Allowable spans and spacing
Spans and spacings are very important considerations in deck design. A span is the distance
decking, joists or beams must bridge; spacing is the nearness of joist to joist, beam to beam, or
post to post (see illustration below).
The maximum safe spans and spacings for lumber of different dimensions depend on the wood
species (Table 1). Softwoods fall into three major numerical groups according to differences in
bending strength, stiffness, and ability to withstand compression. The accompanying tables give
maximum spans and spacings for common sizes of lumber in each group: Table 2, decking
spans; Table 3, joist spans and spacing; and Table 4, beam spans and spacing plus size and
placement of posts.
Start designing the substructure using Tables 1 and 2 by noting the maximum span your deck
surfacing lumber can bridge. This span is the maximum allowable spacing between joists (or
beams if joists are not used).
If you’ re working with joists, see Table 3 after checking the spacing from Table 2 to find the
maximum spans acceptable for differently sized joists of the wood species you intend to use.
These joist spans in turn limit beam spacings found in Table 4 (third column).
Both beam spacing and beam size limit spans between posts as indicated in Table 4. The
minimum size for posts is mainly governed by their height and the width of the beams they
support (post width should always at least equal beam width). As you work out spans and
spacing, remember that those given in the tables are maximum limits. You can always choose
shorter spans, closer spacings, or larger joists, beams, and posts than given in the tables. Using
the maximums, you will also find that many equally strong combinations exist.
A good method to use is to roughly sketch out a number of different combinations before
selecting a final one.

Further information & Tips
Keep in mind that, despite the inherent strength in a deck’s basic members, bracing may be
needed to give it necessary lateral stability and to help distribute loads evenly.
Adding a deck to a house is a manageable project for homeowners with basic carpentry skills.
One pitfall the amateur encounters involves attaching the deck to the house. The best-case
scenario has a freestanding deck that is from the house. This method is the most material- and
labor-intensive. The worst-case scenario has a ledger board bolted or nailed directly against the
house. This method will be the quickest and easiest and will guarantee moisture decay between
the house and ledger. Builders use several techniques to keep the ledger away from but still
attached to the house. Slim wedges, washers and special hardware are all employed to provide a
gap between the house and deck structure, insuring that water will not collect there. Also be sure
your fasteners are securely anchored to the house framing.

Piers
Piers must be a minimum of 12 inches in diameter. The bottom of the pier must be a minimum of
18 inches below finished grade (frost line) and a minimum of 12 inches into undisturbed native
material. The top of the pier should be a minimum of 6 inches above finished grade. All wood in
contact with the concrete must be pressure treated. This can be in the form of a piece of pressure
treated wood attached to the pier. Tar paper between the beam and the pier is not acceptable. If
the beams resting on the pier are less than 6 inches above finished grade they must be pressure
treated.
Railings
If your deck is going to be 2 feet or more above the ground you must have a railing around it and
next to any stairs you may have. The railing must have the elements spaced so that there is no
place a 4 inch diameter ball can pass through it. There are may different styles of railings. A few
are shown below.




Railing #l: This horizontal design is extremely easy to build because there are so few pieces to
put together. Once you lay out the vertical spacing of the horizontal 2-by-4’s, you can join them
on any rail post or use full-length boards that span several posts at a time. Horizontal lines tend
to stop the eye, making this a good choice if you want to hide an undesirable view. One caution:
horizontal boards create a ladder-like design that small children can climb.




Railing #2: A California classic, this design uses 2-by-2’s as balusters nailed to horizontal 2-by-4
rails. The baluster ends have been chamfered (trimmed at a 45-degree angle) to reduce their bulk
for a more finished appearance.
Railing #3: A variation on Railing #2, this one eliminates the lower rail, saving a bit of lumber
and labor. In this design, the rim joist acts as the lower rail.




Railing #4: Here, even the posts and upper rail has been eliminated. The balusters provide all the
strength for the rail, so they must be fastened to the rim joist with at least two screws each. On
top, a single 2-by-6 on edge yields a clean, modern look.
Cable Railing: This open design is nothing more than tightly-stretched steel cable, spaced evenly
from top to bottom. Turnbuckles and eyebolts hold the cable to the posts at each end.
Intermediate posts must be drilled to allow the cable to pass through. This railing is a great
choice when a nearly unobstructed view is important.
TABLE 1: STRENGTH GROUPINGS OF COMMON SOFTWOODS
GROUP I
Douglas-fir (Coast type)
Larch, Western
Pines, Southern
GROUP II
Baldcypress (Red, Yellow, and White Cypress)
Douglas-fir (South type)
Fir, Western True (White, Red, Yellow, Balsam, and Noble)
Hemlock, Eastern and Western
GROUP III
Cedar (Incense-cedar, Port Orford, and Western Red)
Pine (Idaho White, Lodgepole, Northern White, Ponderosa, and Sugar)
Redwood
Spruce (Engelmann and Sitka)

								
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