The Deck Beam Span Determines The Strength of Your Deck

Plan in advance by knowing the correct deck beam span to use. During your planning stage, don’t forget to consider the spacing for your beams. You should also know the size of the post you are considering to use.

See the Deck Post Size Table to make that determination because it will affect the size of beam you choose.

The Deck Beam Span Table on this page makes it easy to determine how far apart your posts and therefore the span of the beam should be.

But let’s first explain what the numbers in the chart actually mean, because it can be a bit confusing.

If you have seen the Deck Joist Span Table, you already understand the relationship between joist span and joist spacing. The closer you space joists, the longer you can span joists – up to a point.

For determining beam spans (distance a beam can span between supporting posts), consider the following concept. The more weight, or the longer the supported length of joist that a beam must carry, the shorter the span of the beam – for a particular size of beam.

First Step:

Determine the length of the joist actually supported by the beam. This distance is half the distance to the next beam in either direction. If a joist is attached from a ledger to a beam, then the distance is the midway point between the supported ends (either a ledger or beam) of the joist plus any cantilevered portion of the joist past the beam.

It makes sense if you think about it. If a joist spans between two beams or a beam and a ledger, each beam (or ledger) supports half of the joist.

The length of joist span actually supported by a beam is what determines the force that bears upon that beam and therefore the distance that a given size of beam can span before another post is required to support that beam.

Second Step:

Refer to the Deck Beam Span Table below to assist in determining the maximum span of a given beam between posts. Obviously, the larger the beam, the greater the distance it can span between posts. A Redwood 4x6 beam should span no more than 6' between supporting posts.

Beam Span Table
Beam Span between Posts or Ledgers
Wood Species Beam Size 4' 5' 6' 7' 8' 9' 10' 11' 12'
W. cedar. Redwood 4x6 6 6              
3x8 7 6          
4x8 8 7 6 6      
3x10 9 8 7 6 6 6  
4x10 10 9 8 8 7 7 6 6
3x12 11 10 9 8 7 7 6 6
4x12 12 11 10 9 9 8 7 7
Hemlock 4x6 6 6 6            
3x8 8 8 7 6          
4x8 9 8 7 7 6 6  
3x10 11 10 9 8 8 7 7 6 6
4x10 12 11 10 9 9 8 8 7 7
3x12   12 10 10 9 8 8 9 7
4x12     12 11 10 10 9 8 8
Douglas Fir, Pine 4x6 6 6 6            
3x8 9 8 7 6 6 6 6    
4x8 10 9 8 7 7 6 6 6
3x10 11 10 9 9 8 7 7 7 6
4x10 12 11 10 9 9 8 8 7 7
3x12              
4x12              

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