27 Uses and Tips for Deck Blocking


There are many ways to build a strong, durable deck. One of the most cost-effective means to improve a decks’ strength is through joist blocking.

Joist blocking is required for many reasons in a deck.  Joist blocking helps to lessen joist deflection, twisting while strengthening post rails and point loads.

After installing thousands of blocks, my appreciation for deck blocks only increases. Deck blocks both directly and indirectly make decks stronger. From creative designs to a firmer deck, joist blocks make the difference. Here are a few ways that joist blocking improves a deck and tips in installing.

  1. Joistblocks resist the tendency of a joist to roll. Increasing the joist strength by keeping the joist top and bottoms aligned. Directing the force on the deck directly through the joist and beam.
  2. Joist blocks reduce deflection in the joist. Like electricity travelling the most direct path, weight on a deck does the same. If the joist is allowed to bow deck, weight is transferred to the horizontal from vertical pressure. Joist are weakest horizontally being only 1 1/2” (38mm) only a fraction of the vertical dimension of the joist. Joist blocks prevent deflection ensuring the weight is born vertically not bowing the joist.
  3. Joist blocking reduces deck movement. You know that feeling when you step unto the deck and the entire deck trembles with your foot. Blocking reduces this, giving you a firm feeling as you walk across the deck.
  4. Joist blocking prevents joists from twisting. Treated wood, especially incised, is prone to twisting as it dries. A corkscrewed joist is considerably weaker than a true joist. Joist blocking keeps joists true by preventing twisting.
  5. Joist blocking should be the same dimension as the joist. The tension of the joist is in the top and bottom portion of the joist. The centre area is neutral, requiring limited support. Blocking of equal height will cross transfer the load between the top and bottom chords.
  6. Joist blocking helps to straighten joist for decking. Cutting blocking a 1/16” less then the official joist spacing will push or pull the joist to the correct spacing, removing random joist spacing. Matching the centre joists’ spacing with the fastened space on the rim and ledger.
  7. Joist blocking at midspan or third points increases the strength of the joist. Joining the blocking together with blocking the joist work together as a team. A team is always stronger than its individual parts. A deck is no different, with excess load bearing down a lone joist, blocks transfer that weight over multiply joist, for together they can carry the load.
  8. Midspan blocking does not have to be a straight line. Joist offset on a straight line still provides direct support to each other. Offsetting the blocks allow for easier installation with straight nailing but still transfers the load to each other.
  9. Centring midspan blocking below a decking board will shelter them from the rain and hide them from view.
  10. Installing midspan blocking before installing the decking will place the fasteners in a straight line, following the joist. Improving the appearance of the decking, both with face screwing and hidden fasteners.
  11. Joist Blocks transfer point loads to the joist, reinforcing the decking. Decking is only designed for 10 lbs (4.5kg) psf dead load, with an additional 40 lbs (18kg) psf of live load.
  12. Joist blocks under stair stinger transfer the load to the joist from the stringers.
  13. Joist blocking support Pergola post. The additional weight of a pergola requires additional support. Placing blocks under post locations transfers the weight to the joist.
  14. Joist blocking doubles as anchors. Awning or pergolas can have uplift with the wind. Blocking provides solid material to bolt into securing the structure through the wind.
  15. Joist blocking allows creativity in decking design. From picture frame to herringbone to insert, they are all made possible by blocking.
  16. Joist blocks provide support for mitred decking. It is impossible to support mitre decking without a blocking running at an angle to catch the mitres.
  17. Joist blocking allows better water drainage off the decking. Especially true with picture framing. Tripling up the joist to support the picture frame creates a solid basin trapping water, increasing rot. By spacing blocking 16” (400mmm), rain can freely run off the deck while the picture frame if fully supported. The same is true with inserts by spacing the joist the width of the insert plus 3” (75mm) butts are removed from the decking while allowing water to drain freely.
  18. Should joist blocking be screwed or nailed? Screws are superior in tensile strength. Nails are superior in shear strength. Nails are the ideal for framing providing both speed in installation and shear strength required in point load support.  Ringed or spiral nails will hold better and reduce squeaks. Once exception is sucking blocks or joist tight, screws are far superior in pulling material together. With a slightly twisted joist, a well-placed screw will straighten out the joist pulling it tight to the block.
  19. Glue end grains of blocking will do little to increase strength or reduce squeaks. Glue has little holding power on the end grains of wood. Think of it like glue straws together, end to end. It will hold very little. Straws and wood side by side, magical but end grain is a waste of time and glue. Additional screws or ringed nails will do more to hold the blocking to the joist.
  20. End treatment of blocking will reduce fungi growth from starting in untreated exposed wood. Preventing rot from spreading from the blocking to joist.
  21. Joist blocking reinforces railing posts. Post merely lagged to the rim joist will bend as they twist the rim. Blocking reinforces the rim by tying it together with the joist. With the additional material holding the post in place.
  22. Joist blocking around a post reinforces not only the post but also the decking butting into the post.
  23. Install blocking flat under top-mounted railing post locations. Decking is not strong enough to anchor guard rails too.
  24. Joist blocking straightens end joist. With either midspan blocking or picture, using framing blocking to pull or push out end joist, correcting a bow and straightening it out. Giving you a true edge to work off.
  25. Joist blocking should be installed flush or slightly lower than the joist. Even a 1/16” bulge will cause an ugly bump in composite decking. Being slightly low will not affect the decking because the decking will span the distance between the joists.
  26. Joist blocking can be cut from rejected joist material. A 16’ (4.8m) twisted 2×10 will make a terrible joist, but when cut into 14 7/16,” blocking the twist will be removed. Also, offcuts of material can be integrated in as joist blocking at no additional cost.
  27. Palm nailer are great for nailing blocking between tight joists. Fitting in spaces where spiker or impact driver cannot fit.

Deck blocking is about Location, Location, Location

Placing the blocking in the right location will strengthen the deck, support the blocking and provide a smooth construction experience. A sequence of installation helps to accomplish correct blocking.

  1. Install all blocking around posts or flat under railing post locations.
  2. Install straight picture framing blocking and insert blocking
  3. Install angle blocking to support decking mitre joints. These are best done on edge and short spans allow you to use 2×4 material. Cutting mitres on the angle blocks will improve the appearance and fill in the space. Gaps in material can create problems when screwing down the decking. Sistering two blocks together will provide material to attach hidden fasteners. If installing top-mounted railing post, it is often better to install a large block on flat. Supporting both the post and the mitre cuts.
  4. Snap a chalk line for mid or third span locations. Measure the spacing at the rim minus 1/16”. Cut and install all the blocks except for the last joist cavities by the picture framing joist. Watch while installing that joists are not bowing from the blocking. With the final joist cavity near the edge, run a string line along the end joist and measure the corrected distance to align the joist to the string line. Use these end blocks to correct any bows.
  5. Point load blocking should go in next. Measure out where ever there will be an extra load on the deck. Anything that will increase load permanently on the decking needs blocking.
    • Stair stringer from higher tiers
    • Pergola post
    • Planter boxes
    • Umbrella stands
  6. Double-check all locations before installing decking. It is so much easier to install of few extra blocks then have to remove deck or crawl under to add decking after the fact.

Ryan Nickel

A Red Seal carpenter, passionate about building decks to be enjoyed.

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