Selecting the Right Joist for Your Deck

Decking is beautiful, but it’s the joist that makes a deck strong. The joists providing the frame for the entire deck. Hidden under the decking, but joists are the unseen hero that is making all the difference. Its when joists fail that you have a deck disaster. It’s the strength of joists that determine how safe you feel on your deck by how much it moves as you walk. It’s the joist that will be supporting you and your entire extended family for the summer reunion. O joist, I love you for you make deck real and backyards fun.

What is required of your deck joist?

Joist are essential for a deck; without joists, there is no deck. But before we explain deck joist requirements, we need to define.

What are deck joists?

Deck joist are the horizontal framing members providing the frame and support for the decking. Deck joists can be either steel or wood but are commonly wood. The skeleton of the deck, unseen but providing the structure and strength of a deck. Defining space, all while supporting the decking, allowing you to enjoy your own outdoor space.

I love joisting decks. Installing the joist and rim, the deck takes shape, going from some holes with concrete to an outdoor living space. A beam resting by itself, hinting but not defining the deck. Joist change all that, the platforms take shape, space becomes defined.

But joists not only define the deck but must be strong enough to support whatever rests on it. With deck joist, it’s the span of the joist that determines the joist’s required depth and spacing. The span is the distance between joist support. Either the distance between two beams or from ledger to beam, regardless if the beam is dropped or flush.

Required deck joist strength for the span

Rule of thumb, non-incised Spruce/Pine/Fir joist can span its nominal depth times one and a half at 16’ on centre. A 2×8 joist can span 8×1.5=12’. A 2×6 joist can span roughly 9’ at 16” on centre. Sorry metric has no such easy conversion.

If you increase the number of joists, with 12” on centre spacing, you can increase the span by 1’ compared to 16” on centre. The equation goes like this, for a 2×10 joist 12” on centre, (10×1.5) +1=16’. Now, this is only a quick number for field math. For if we go to the actual Canadian Wood Council deck joist chart, we see that a 2×10 can span 16’10” at 12” on centre and extra 10”. But with a 2×6 joist, it works out exactly to 10’. Before installing your beams, always check CWD or IRC joist charts.

Rough Joist Span Chart

Increasing joist space to 24” on centre will reduce the allowable span. A spacing I would never recommend, and decking will often not allow. With joist at 24” on centre, you will feel like a giant walking, the decking shaking and sinking with every step. But if you want to save on joist, 24” on centre is the least amount of joist allowed. Roughly the quick math for joist at 24” on centre is nominal depth plus two. A 2×6 can span 8’, and 2×12, 14’ at 24” on centre — not a great change with small joist in but a world of difference with larger joist.

If you want a really solid feeling deck or the decking requires it going to the other extreme 8” on centre. The span is nominal depth times two minus one. A 2×8 joist can span roughly 15’, (8×2)-1.

In short, the greater the span, the more joist or bigger joist the deck will need. Two other factors affect the joist span, the type of wood and treatment method.

Impact of Wood Species on Spans

Denser wood like Douglas fir can span longer distances. There is not a direct correlation, that fir will allow you to add “X” in joist length. Or, by using cedar, a softer wood will reduce the span by a certain number. Just be aware that not using the most commonly treated species will impact the joist spans either positivity or negatively in length.

Incision Reduces Joist’s Spans

If the wood is incised during treatment to increase resistance to rot, it will decrease its structural strength and span. It may be advantageous to use incised wood in high moisture area or ground-level decks, but the number of joists or depth will need to be increased to compensate. Not a clear relation, but the greater the joist depth the greater the impact. 2×6 span is barely impacted with incision, where a 2×12 span is shortened by over a foot.

Equally important in joist requirements to joist span is decking span. The material used for decking impacts how far apart the joist can be. As decking also has limits on spans.

Decking Span, impacting Joist requirements

The thicker and more rigid the decking, the greater distance it can span.  Traditional 2×6 decking can span joist at 24” on centre. Most 5/4” and solid composite decking is limited to 16” on centre running perpendicular to the joist.  Thinner, scalloped, or hollow composite decking will need 12” or less joist spacing. In short, the less decking material, the more joists required.  

Angle direction of decking will reduce the required on centre joist spacing. For example, Trex’s reduces the joist spacing according to the decking direction.

Joist spacing reduction is not limited to Trex decking, but all joist spacing should be reduced according to the decking angle. A simple case of math, at 45°, the decking span increases by 1.414, 12” becoming almost 17”.

If you are not into the math, draw the degree on the decking, set your framing square tongue along the line and put a mark of the on-centre joist spacing along the body of the square. Then measure between the two lines parallel with the decking and that is the decking’s actual span. If it’s greater, then its required perpendicular spacing, reduce the joist spacing.

The spring of the joist is determined by the span of the joist, the spring of the decking by the spacing of the joist. Deck joists impact everything.

With all decking, especially composite, always verify decking span requirements when deciding joist layout requirements. Composite decking span will vary by brand, model and design.

Deck Joist Layout Requirements

Adjusting Joist Layout for Picture Frame Decking

Once the decking required joist space is determined, you can figure out the deck’s joist layout to accommodate decking design. As already mentioned, if the decking runs at an angle, joist spacing will need to be adjusted accordingly. Picture framing will also change the joist layout. Picture framing decking has become popular in hiding decking end cuts and enhancing the appearance of the deck. Especially with composite decking where hiding end cuts are essential, but with different colours and texture available, creating beautiful accent borders and inserts.

All decking should have joist with-in 1 ½” (38mm) from the decking ends. Depending on the size of the picture frame, the first joist will need to be adjusted, starting the layout 1 ½” (38mm) past the picture frame under the ends of the main decking. The adjustment also being impacted by how much the picture frame decking overhangs the edge of the deck. I like a 1 ½” (38mm) overhang of the edge joist, making the finished fascia overhang 1” (25mm). 

With a double picture frame of 5.5” (140mm) decking, the first joist should be 11” (280mm) an ahead from the edge of the deck. Allowing for a 1 ½” (38mm) decking overhang of the joist both the border and field decking. The first joist should be set in whatever the distance is of the picture frame.

Edge Joist Requirements

Some considerations will need to be given to the edge joist, with the ledger ending on the edge of the joist preventing use of standard joist face hangers. There are two standard ways to attach the edge joist to the ledger

A concealed flange or inverted flange hanger can be used. Being fastened to the ledger inside the hanger before installing the joist. The joist will need to be cut shorter to accommodate the hanger along with the hanger installed before the joist.

Another option for edge joist is, cut the ledger 1 ½’(38mm) short, run the joist up beside the end of the ledger. Nailing the joist to the end of the ledger with five framing nails. Both the end nailing and inverted hangers will sufficiently hold an edge joist.

Adjusting the Substructure to accommodate finish Deck Size

The size of the deck substructure and joist may need to be adjusted for the decking final size to accommodate full decking pieces. Picture framed decks with 1 ½ (38mm) overhang will need the frame shrunk by 3” (75mm) less than the decking length. Most wood decking longest size is 16’ (4.8m) or composite 20’ (6.1m). The frame should be reduced to 15’9 and 19’ 9” respectively to allow a single picture frame decking board.

The deck skirting or fascia material also needs to be accounted for. Adjusting the substructure to create a consistent decking overhang and nosing depth.

Deck Joist Requirements to Survive

Joist are only as good as they are strong. If joists are rotten, they will become useless in supporting the deck. Joists need to be treated to prevent fungi rot. The most common treatment is ACQ (alkaline copper quaternary), which inhibits both fungi and insects from feeding off the joist.

There are different levels of treatment. All deck joist should be at least 0.25-PCF of treatment preventing fungi from eating the joist when allowed to dry after a rain. If the joist will be in contact with the ground or higher levels of moisture, installing joist treated for ground contact at 0.40-PCF is required.

Another way to increase joist life is with Membrane Flashing on the Joist. Installed on the top of the joist before installing decking. Creating a shield of protection against water and seal the screw holes, preventing water from being soaking into the joist. Especially worthwhile with composite or PVC decking helping the joist match the life expectancy of the decking.

If you would like to learn how to prevent joist from rotting, here is a larger article on preventing rot.

Deck Joist Selection Requirements

Deck joists must be treated but also structurally sound. Wood with large knots, cracks or wanes should be rejected. Knots and crack compromise the structural strength of the joist. Joist with wanes creates problems when installing the decking. The missing joist edge making it hard to fasten the decking with concealed fasteners and creating a dip in the decking.

Always use appearance grade lumber for joist with four square edges. Better to use defective joists as blocking where large knots and cracks can be cut off, and missing edges don’t matter. A good practice is while you are carrying the lumber to the deck area to sort into piles, good to be used as joist and poor to be used as blocking. Then during construction, the lumber us already sorted.

Blocking can be used to straighten bowed joist, but joist with large crowns will need to be removed or planned to level with the other joist. Run a string line over the joist before installing composite decking to find crowned joist. Composites’ softer nature will be impacted more by differences in joist heights.

Joist Hangers and Fasteners Requirements

The copper in wood treatment is very corrosive with metal. Therefore, all hangers, hardware and fasteners need to be either galvanized coated, organic polymer coated or stainless steel to prevent corrosion. Check the hangers that they are coated for outdoor use with treated lumber (G-185). If not coated, they will react and fail long before the wood has rotten. Causing deck failure even if the wood is still good.

Correct fasteners need to be used for all hangers and hardware — galvanized hanger nails with their thick dimeter. Screws can also be used if they are structural screws. Standard decking screws are too brittle breaking off under the pressure of a joist hanger. Structural screws are designed for sheer pressure, with the strength to hold the hanger for years to come.

Joist should be nailed with ringed nails. Ringed nails hold 40% or more compared to smooth shaft nails. It’s always a struggle to pull them out, with the ring gripping to the wood.

Rings increase the strength of nails, but there are also ways to increase the strength of deck joists, which leads to our next question.

How do I strengthen my deck joists?

Increasing Strength with Blocking

Blocking joist is one of the best ways to increase the joist strength. Teams are always stronger than individuals. Blocking transforms joist into a strong team. With each joist connected to and support joist through the blocking. The load being transfer to the many instead of the one.

Blocking is so important that the National Design Specifications (NDS) requires blocking on all 2×10 joists exceeding 8’ in length. A row of centre row of blocks down the middle of a deck will triple the joist strength. With 16’ foot joist, blocking at third spans will provide a solid feel to the deck. Blocking also prevents the joist from twisting or moving through seasonal changes.

Another location that can use the extra strength is the edge of the deck, where people naturally gravitate. Notice how everyone likes to lean on the railing while enjoying the view? Picture framing blocking is great for this as the blocks required for the picture frame strength the part of the deck requiring the most strength.

With blocking providing strength, it’s a good idea to install extra blocking where additional weight will be like under stair stringers or planters.

A good idea with blocking is to install them under a decking board. Protecting them from the rain while hiding them from view. Blocking under decking gaps will trap water, increasing the risk of rot.

A good idea is when installing blocking is to cut them a hair short. The key is a hair, blocking that is loose provides no strength but to large will bow the joist with each additional block. Often a 1/16” (1mm) less then the theoretical space works perfectly. For 16” (400mm) on centre joist, cut the blocks 14 7/16” (367mm), just a little under. Also, a good idea to run a string line every few joists to correct any deficiencies. Correcting the joist with a smaller or larger block as required. Just like you do with the decking.

For smaller joist spacing, you may need to use a palm nailer to drive the nails. As the cavity is too small to swing a hammer or fit a spiker. Where a palmer will fit nicely.

Increasing Strength through beam location

Another easy way to reduce joist deflection and joist strength is by shortening the joist span. This can be done without increasing the deck’s cost simple by incorporating cantilevers or joists overhangs. Often used to extend the length of the deck but can also be used to reduce the joist span.

Shorter spans=Less Deflection

Code allows for joist to extend past beams. Allowing you to shorten the joist span without increasing cost. Transferring a portion of the span from one side of the beam to the other.

You can reduce a 2×8 joist span from 12’ (3.65m) to 10’8” (3.25m). Reducing the span by over 10% without any extra material cost or work. Similar to blocking, reducing span by a little, reduces deflection significantly.

Joist requirements are determined by span, decking and environment. Good selection, correct hardware and blocking will all help to make your deck an oasis for years to come. May your deck carry you into your golden years with peace and pleasure. Live long my friend, enjoy the sun and breeze.