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Deck Joist Sizing and Spacing

Joists are the repeated structural members that are used to build a deck frame. The minimum size joist to be used in deck construction is not a stand-alone consideration, and is related to the number of footings and beams that will be installed. In general terms, joists spaced 16 inches on center can span 1.5 times in feet their depth in inches. A 2x8 up to 12 feet; 2x10 to 15 feet and 2x12 to 18 feet. The larger the deck, the larger the joists.  In typical deck construction, with a ledger on one side of the joist and beam on the other, the size of the joists is driven by the size of the deck based on the general maximum spans mentioned above.

 

 Joist Spacing (o.c.)    12" 16" 24" 
 Species  Size  Allowable Span
 Southern Pine  2x6  9'-11" 9'-0"  7'-7" 
 2x8  13'-1"  11'-10"  9'-8"
 2x10  16'-2"  14'-0" 11'-5" 
 2x12 18'-0"   16'-6" 13'-6" 

 Douglas Fir-Larch,

Hem-Fir,

Spruce-Pine-Fir

 2x6 9'-8"   8'-4" 6'-10" 
2x8  12'-8"   11'-1" 9'-1" 
2x10  15'-8"   13'-7" 11'-1" 
2x12  18'-0"   15'-9" 12'-10" 

 Redwood,

Western Cedars,

Poderosa Pine, Red Pine

 2x6 8'-10"   8'-0" 6'-10" 
2x8  11'-8"   10'-7" 8'-8" 
 2x10 14'-11"  13'-0"  10'-7" 
 2x12  17'-5"  15'-1" 12'-4" 

 

For conventional guard rail post installation, bolted to the side of the framing, larger framing will provide for more strength in the rails. For upper level decks 2x10 is recommended as the minimum size to use for strong guard post connections. 2x6 joists should only be used on ground-level decks that do not require, and will not provide for, any guards.  Similar to guards, if stairs are to be hung from the side of an upper-level deck, 2x10 as a minimum is recommended, as it will exceed the depth of the first step.

Joist hangers are used to attach the ends of joists to the face of a beam or a ledger board. If you identify a crown in the board you should always install it upwards. The crown will eventually settle after completing construction and should stiffen in the proper position after drying.

The layout term for joists is “On Center” which is the center to center measurement from one joist to the next. Most decks use 16" on center spacing for joists. Most decking is not strong enough to support longer spans than 16". Some builders reduce joist spacing to 12" on center to strengthen the deck frame or to increase maximum allowable joist spans.

Before building a composite deck always read the installation instructions from the manufacturer paying special attention to the required joist spacing.  Most composite decking materials require 16" on center spacing for straight decking and 12" on center joist spacing for 45 degree angle diagonal decking.  Also many composite decking materials require 12" or even 10" on center stair stringers spacing to support composite stair treads.  If you are reinstalling composite decking over an existing frame you may need to install new intermediate joists or stair stringers to meet the installation requirements. 


Laying out the first joist position on an angle.

Using a scrap piece of wood as a guide to locate the first joist in the situation of an angled corner.


Use a sharpie or construction pencil to mark your joists.

Marking joist locations at 16" on center spacing on the header with a marker.


Toe nailing a joist to the rim joist.

Installing a joist over a beam. Make sure the top of the joist is flush with the header.


Use a chisel to notch beams to create a level frame.

Notching a joist over a beam. This joist was 1/4" higher than all the others. You can notch out a section of the joist or add shims to a narrow joist to even out the top of the deck frame.


Always fill all the nail holes for deck harware.

Using a palm-nailer or hammer to install a Simpson Strong Tie LUS210Z joist hangers with 1-1/2" teco nails to the ledger board.


Inspect the board and trim off the rougher edge.

Measuring the 2x10 pressure treated joist length to prepare for cutting. Use a speed square to square off your joists.


Lift with your knees!

Installing 2x10 pressure treated joists at 16" on center. Be sure to install the joists crown side up.


You can use a pneumatic palm nailer.

Installing a Simpson Strong Tie H2.5Z Hurricane Tie for every joist to beam connection.

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