Deck Design Assistant

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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 Susan Hickman, and she can be reached at 462-2009 or e-mailed at susan.hickman@townofcary.org.

FOR MORE INFORMATION, please refer to Appendix M of the 2012 NCRC and the related Accumulated Changes (search "deck").


STEP 1:

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 each direction, then multiplying the length by 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, you may want to upsize your footings based on the tributary area you plan to support.


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 by a design professional.


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 16". 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.

Total Joist length Without Cantilever With Cantilever 2' max.
Maximum girder span between posts
(2) 2x8 (2) 2x10 (2) 2x8 (2) 2x10
8' 9' 10' 7' 8'
9' 8' 10' 7' 8'
10' 8' 9' 6' 8'
11' 7' 9' 6' 7'
12' 7' 8' 6' 7'
13' 7' 8' 6' 7'
14' 6' 8' 6' 7'
15' 6' 7' 5' 6'
16' 6' 7' 5' 6'

STEP 4:

Determine your joist size.

  • 2x6 SYP #2 will span up to 9'-4" on 16" centers, or 10'-3" on 12" centers.
  • 2x8 SYP #2 will span up to 11'-10" on 16" centers, or 13'-6" on 12" centers.
  • 2x10 SYP #2 will span up to 14'-0" on 16" centers, or 16'-2" on 12" centers.
  • 2x12 SYP #2 will span up to 16'-6" on 16" centers, or 19'-1" on 12" centers.
  • 2x10 joists at 16” centers can be cantilevered 24” past the girder.
  • 2x8 joists at 12” centers can be cantilevered 20” past the girder.
  • 2x8 joists at 16” centers cannot be cantilevered.

STEP 5:

*CHANGES HAVE BEEN MADE TO HOW THE DECK IS ATTACHED* - Please see the revised Appendix M for new requirements for deck attachment.

Specify your method of attachment to the house.

If the exterior finish of 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 exterior of 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.

  • 2x10 joists at 16” centers can be cantilevered 24” past the girder.
  • 2x8 joists at 12” centers can be cantilevered 20” past the girder.
  • 2x8 joists at 16” centers cannon be cantilevered.

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:

  1. 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.
  2. Embed 4x4 posts 30" into a 12" diameter by 36" deep footing (max post height is 4' with 48 sq. ft. tributary area).
  3. Embed 6x6 posts 42" into a 20" diameter by 50" deep footing (max post height is 6' with 120 sq. ft. tributary area).
  4. 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:

Guard rails.

Please see Appendix M for specific information on the deck guard rails.


STEP 9:

Draw your deck.

This is a Deck Design Example that would be sufficient for a permit application.

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