You’ve decided
to increase your living space with an outdoor deck. Now what? What other
decisions need to be made to design and build a beautiful and durable deck.
First, you need to select your decking
. Decking is the most visual part of your
decking next to the railing, which will impact your enjoyment of your outdoor
living space like no other. But after that, we jump to the structural design of
your deck. If it doesn’t stay up, what value is beauty? Leading us to the
question of deck joist spacing.
Three things determine decking joist spacing.
- The decking’s strength to span between supports.
- The direction of the decking in relationship to the joist.
- The joist span between support.
The three most common deck joist spacing are 12” (300mm), 16” (400mm) and 24” (600mm). With most deck joist spaced at 16” (400mm).
When determining the correct deck joist spacing, it’s often a good idea to start with 16” (400mm) spacing. The majority of decking is designed for 16” (400mm) joist spacing, thus working with the greatest amount of decking. Also, after walking on and building many decks. Decking supported by 16” (400mm) spacing just feels sturdier than larger spacing. So, for me, it’s a question of does the decking need increase support, but if your budget is tight, 24” (600mm) may be an option. But evaluate this spacing against the decking span strength, direction of decking and required joist span strength. It is ensuring a strong, durable deck.
Because joist spacing and decking are so closely linked, if you haven’t already selected your decking, it would be wise to at least narrow the field down to wood or composite. Then slightly further with what type of wood or composite decking.
In short, decking material thickness determines the span
Often more material equals greater strength. There is a correlation between decking thickness and joist spacing. Treated S.P.F 2×6 decking can span 24” (600mm), but reducing it to the more common 5/4” decking can only span 16” (400mm). This correlation holds true for cedar, ipe and treated decking. Ipe is a more ridged decking and can span greater lengths as equivalent cedar decking but still the thicker, the larger possible joist spacing.
Wood Decking Joist Spacing
| Decking Material | Decking Finish Thickness | Decking Span |
| Treated S.P.F. | 5/4”x5 ½” | 16” |
| Treated S.P.F | 1 ½”x 5 ½” | 24” |
| Clear & Knotty Cedar | 1” x 5 ½” | 16” |
| Clear & Knotty Cedar | 1 ½” x 5 ½” | 24” |
| Ipe | 1” x 5 ½” | 24” |
| Ipe | 1 ½” x 5 ½” | 32” |
One note about Ipe
decking joist spacing, Ipe may be rigid enough to span 32,” but such spacing is
unlikely due to deck joist codes. We will get to that later, but for now. As
you can see, all wood decking can span 16” (400mm) on centre joist spacing, but
increasing thickness allows for 24” (600mm) spacing.
With most wood decking, it is safe to design with joist at 16” (600mm) on centre unless decking does not run perpendicular to the joist, we will get to that later. If you want to increase the joist spacing, increase the decking thickness. But with composite decking joist spacing becomes a little more complicated.
Composite Decking Joist Spacing
To start, most composite decking is thinner than conventional wood decking, most being around 1’ (25mm) thick. So, we are already starting off with a thinner material. On top of that, all composite decking is engineered. What I mean is that every decking brand and line may have different levels of rigidity and joist spanning requirements.
Most composite and PVC decking is designed for joist spacing of 16” (400mm) on centre. There are a few exceptions that you need to check before building your deck. Similar to wood, the less material in the decking either through scallops or thinner decking, less then 1” (25mm), the closer the joist will need to be.
Many jurisdictions require technical data sheets to be provided for the decking to pull a permit. As not all composite decking is the same, and there are too many variations for inspectors to know all the individual requirements.
In Calgary, where I build decks, this is the CCMC listing number. Unfortunately, even though it’s a government website and the information is for free, it is only available to professionals. Which if you’re willing to adjust during the permits process, bring the decking number, and you can adjust decking or joist spacing after they confirm the spacing. Not official data, but often the manufacturer will also provide the required joist spacing.
In the United States, ASTM D7032 Label is the standard. In short, everything designed in a lab must be tested in a lab to know its strengths and properties.
Here is a chart of some popular composite decking and their required joist spacing. These are there
maximum design spacing, but many will even suggest on their own websites that
reducing the spacing will increase the firmness of the decking. Especially on
hot days, the plastic in composite decking will become very soft and flexible.
| Composite Decking | Perpendicular Joist Spacing | 45° Joist Spacing |
| Trex all lines | 16” (400mm) | 12” (300mm) |
| TimberTech Twin Finish | 20” (510mm) | 16” (400mm) |
| TimberTech Terrain, Legacy | 16” (400mm) | 12” (300mm) |
| Azek | 16” (400mm) | 12” (300mm) |
| Cali-Bamboo | 16” (400mm) | 12” (300mm) |
| Dura-Life | 16” (400mm) | 12” (300mm) |
| Envision Composite Lumber | 16” (400mm) | 12” (300mm) |
| Veranda | 12” (300mm) | 8” (200mm) |
| Dekorators Decking | 16” (400mm) | 12” (300mm) |
| Fiberon | 16” (400mm) | 12” (300mm) |
Angle decking reducing joist spacing
As the chart shows, joist spacing needs to be reduced if you change the direction of the decking. This is the reality for both composite and wood decking. I find that you have a little more play with wood decking, but it must still be considered.
A little trick of geometry. By changing the direction of the decking, you are actually increasing the distance spanned between the two joists. A 45° angle increases the decking span by 1.414. Changing 12’ (300mm) into 16” (400mm). If the maximum decking span is 16” (400mm), the deck needs to be joisted 12” (300mm) on centre. The equivalent decking span.
Decking requiring 16” (400mm) on centre joist spacing, you will need to reduce the spacing depending on the degree of the angle of the decking. The smaller the degree in relationship to the joist, the greater the joist spacing will need to be reduced.
| Angle of Decking | Space Between Joist | Nominal Joist Spacing |
| 30° | 8 7/8” (225mm) | 8” (200mm) |
| 45° | 13 ¾” (348mm) | 12” (300mm) |
| 60° | 15 ¾” (399 mm) | 14” (350mm) |
| 90° | 17 ¾” (450mm) | 16” (400mm) |
Decreasing Deck joist spacing to Increase Joist Spanning
Both the International Residential Code and Canadian Wood Council adjust joist spans based on joist spacing. Decrease the joist spacing increases the deck’s strength allowing greater spans. When using 2×8 S.P.F. joist, the CWC increases the allowable joist span from 10’2” (3.09m) at 24” (600mm) on centre to 12’0’ (3.65m) span by reducing the joist spacing to 16” (400mm). If you reduce the joist spacing in a third down to 8” (200mm), the 2×8 joist can span 15’1” (4.6m).
Allowing you to reduce the number of concrete footings required by increasing the number of deck joists. Which, when you are digging by hand is a real chore.
Many things go into making a good deck. Joists are one of those essential parts if you would like to learn more to help you to use the right joists for your deck click here.
Preserving your deck’s joist is critical for enjoying your deck for years to come. There are many ways to prevent joist from rotting if you would like to learn about 15 ways plus my 3 favourites, click here.
