This page is intended to assist Cary citizens with
the design requirements of average decks. All situations will not be
covered by this worksheet, but we hope that most design questions can
be answered using the following specifications. Please note that all
of the following requirements are minimums and we recommend
exceeding the minimums in all cases. CAUTION: If you intend to
install a hot tub on your deck, a structural engineer will be required
for the design. Of course, if you have questions, help is only a phone
call away. Our residential plan reviewer is Harry VanWagenen, and he can be reached at 460-4977 or e-mailed at harry.vanwagenen@townofcary.org.
STEP 1:
Determine your footing size.
Footing size is based on the tributary area of deck
that loads onto each footing. Tributary area can be calculated by
measuring halfway to the next support (girder, house band, post) in
every direction, then multiplying the length times the width of that
rectangle. Find the resulting square footage in the following chart,
which gives the minimum footing size. Note that you cannot interpolate
between sizes (you must use the next larger size on the chart).
| TRIBUTARY AREA |
PRECAST 4X8X16 SOLID BLOCKS |
POURED CONCRETE |
| Up to
36 sq ft |
8"x16"x4" deep |
8"x16"x6" deep |
| Up to 40 sq ft |
12"x12"x4" deep |
12"x12"x6" deep |
| Up to 70 sq ft |
16"x16"x8" deep |
16"x16"x8" deep |
| Up to 100 sq ft |
|
16"x24"x8" deep |
| Up to 150 sq ft |
|
24"x24"x8" deep |
All footings must be at least 12" below grade
to the bottom of the footing.
If you ever plan to construct a roof over your
deck, use one size larger than required in the above chart.
STEP 2:
Determine your post size.
Post size is determined solely on height of your
deck, measured from footing to girder. 4x4 treated posts can be used
up to 8’ high. 6x6 treated posts are okay up to 20’ high. Decks
over 20’ high must be engineered.
STEP 3:
Determine your girder size and span.
Girders support the floor joists, and are connected
to each post. The girder can be attached to the posts by bolting the
girder boards to the side of the post using two 5/8" diameter
galvanized through-bolts per connection, or by notching the post at
least 1" under each girder board and bolting with one 5/8"
galvanized through-bolt per connection, or by resting the girder on
top of the post and securing with a treated wood block nailed to the
side of the post and the girder. Breaks in girders must occur at
posts, with a minimum of 1-1/2" bearing on the post. Girders may cantilever past posts up
to 15". The girder can be dropped below the joists, or the joists
can be flush with the girder and be attached with hangers or notched
for a 2x2 ledger.
Girder size is based on the tributary load of the
deck supported. The following table will provide maximum girder spans
for doubled 2x8 or 2x10 girders, given the total length of joists
loading on the girder.
| |
MAXIMUM GIRDER SPAN
BETWEEN POSTS |
|
TOTAL JOIST LENGTH
|
TWO 2X8 GIRDER
|
TWO 2X10 GIRDER
|
| 8' |
10' |
12' |
| 9' |
9' |
10' |
| 10' |
8' |
10' |
| 11' |
8' |
10' |
| 12' |
8' |
9' |
| 13' |
7' |
9' |
| 14' |
7' |
9' |
| 15' |
7' |
8' |
| 16' |
7' |
8' |
| 18' |
6' |
7' |
| 20' |
6' |
7' |
| 22' |
5' |
7' |
| 24' |
5' |
6' |
STEP 4:
Determine your joist size.
2x6 SYP #2 will span up to 9’-9" on 16"
centers, or 10’-9" on 12" centers.
2x8 SYP #2 will span up to 12’-10" on
16" centers, or 14’-2" on 12" centers.
2x10 SYP #2 will span up to 16’-1" on
16" centers, or 18’-0" on 12" centers.
2x12 SYP #2 will span up to 18’-10" on
16" centers, or over 20’ on 12" centers.
Joists may cantilever past girders up to 24".
STEP 5:
Specify your method of attachment to the house.
If the house is siding, you can remove the siding
and fibrous sheathing, install flashing, and bolt a treated band to
the house band using 5/8" galvanized through-bolts at 20" on
center (42" on center if joists span 8’ or less). In addition,
nail with three 12d galvanized nails on 6" centers (two 12d
galvanized on 8" centers if joists span 8’ or less). Attach a
2x2 treated ledger to the bottom of the band using three nails per
joist space, or use an approved hanger.
If the house is brick veneered, attach the band to
the house as above, but bolt through the brick and the house band on
16" centers (28" if joists span 8’ or less).
You can make the deck freestanding by installing a
girder within 2’ of the house, and let the joists cantilever past
the girder to the house. This unattached method requires extra
bracing.
STEP 6:
Determine bracing requirements.
Freestanding decks require bracing in both
directions (parallel to the house and perpendicular to the house), and
attached decks over four feet high require bracing parallel to the
house, using one of these methods:
- Install 4x4 diagonal bracing from girder to a point at least
one-third down each post. Secure each end with one 5/8"
galvanized through bolt.
- Embed 4x4 posts 30" into a 12" diameter by 36"
deep footing (max post height is 4’ with 48 sq. ft. tributary
area).
- Embed 6x6 posts 42" into a 20" diameter by 50"
deep footing (max post height is 6’ with 120 sq. ft. tributary
area).
- Install 2x6 diagonal bracing (as close to 45 degrees as
possible) from post to post with one 5/8" galvanized through
bolt per connection.
All bays (post to post) may not need bracing. A
good rule of thumb is to brace at least half of bays, and a minimum of
two bays. Thus, if you have three support posts, both bays should have
bracing. If you have four posts, two of the three bays should have
bracing. With five posts, two of the four bays should have bracing,
and so forth. If the embedment method of bracing is used, all posts
must conform to the embedment requirement.
STEP 7:
Specify your decking material.
Assuming that you are using treated pine decking,
the following spans apply:
Nominal 1" (dressed to ¾") decking will
span joists at 12" centers.
Nominal 1" tongue and groove decking will span
joists at 16" centers.
Nominal 5/4" decking will span joists at
24" centers.
Nominal 2" decking will span joists at
32" centers.
STEP 8:
Draw your deck.
This is a Deck Design Example that would be sufficient
for a permit application.
