All About Deck Spans & Loads

DECKING SPANS EXPLAINED now

The following is important information for the deck framing.

To ensured deck is safe and secure, it needs to be designed to withstand loads.  Check your local building departments codes and requirements.  If your deck frame is weak and not up to standard it could cause a collapse when under loads such as snow, or a group of people for a barbecue.  So plan correctly.  So, what is the best way to determine your structural load for your deck frame?  It is arrived at by combining the type of lumber your frame is built with , the size, and span or distance between the joists, deck boards, or beams.

Joist:  It is the member in which the deck boards are supported by and is attached to.  When attaching joist to the face of a beam or ledger it's advisable to use a metal joist hanger.

According to the National Forest Products Laboratories 2x6. 2x8, 2x10 and 2x12  are the most used sizes for joists and considered the standards of the industry in the US.  The size of your joist on your deck project will be determined by your spacing on center,  from center of the top of one joist to the other.  Joists are usually 2x6 in dimension. See the Joist span chart.

Let’s discuss deck loads:  Usually the requirement is for the deck to be able to support 40 lbs of live load and 10 lbs of dead load per square foot.  Dead load is the weight of the wood material itself.  Again confer with your local building codes.

Pay attention to how high your deck is going to be, and if it will be higher than 8 feet, it may require a heavier load.  It would be good to consult your local building department, since this may require special engineering and reinforcement. Some building contractors might use  4x8 at 24” on center. decks-n-floors.com will be referring  to only these sizes (2x6, 2x8, 2 x10 or 2x12) in our information provided on deck construction. Another advantage for using these 4 is that they are the most widely used, and in supply at your local building supplier.

Before Joist Installation:

Read the board carefully looking for joist bard defects. First look for the board's crown or bend.  This crown should be installed upwards toward the top of your deck. The crown will  eventually will settle as you complete your deck and will stiffen in time.

Also look for knots and if you will be using this board, make sure the knot goes upwards where the deck boards will sit on it.

Another defect would be a large knot at one side of the joist. If you intend to use a joist with a knot like this be sure that it is at the top side after installation. The topside of a joist is always under compression and the bottom is always under tension. So a knot on the bottom of a joist  will not hold under tension thus failing.

DECK SPANS EXPLAINED

Once you have chosen your decking product, consult the manufacturer’s recommendation as to proper spacing between the joists.  This is important because if the spacing between the joists is not enough, this could cause the decking board to flex too much making the installation regrettable.  The safest and best span for all decking products is 1 foot or 12 inch on center, that is between the center of two joists it is exactly 12 inches, again from center of the top of one joist to the other. 

Examples of standard joist spans are 12 inch on center, 16 inch on center and 24 inch or 2 foot on center.  Again depending on the recommendations for that decking board you will buy. 

What can also affect your joist span is if a spa is to be installed along with your decking.  Usually a spa is supported on its own structure, and the deck would be build around it.  To be safe, consult your local building code.  Also consult your product instructions  and recommendations concerning a spa such as how to reinforce your deck to handle such weight if it becomes attached to your deck or on top of the deck.