A deck is an extension of the living space and should be a solid platform for your outdoor activities. A wobbly or bouncy deck is a cause for concern and doesn’t instill confidence in the skills of the builder. An easy deterrent or fix is the use of deck joist blocking.
Blocking, according to the building code, is required at the open end between joists to prevent rotation or at 8-foot maximums if the joists are greater than 2×12 in dimensions. Most builders, though, recognize that blocking every 4 to 6 feet unifies the frame and improves structural strength.
In this guide, we’ll discuss what joist blocking is and why it is necessary, plus requirements, methods, and spacing. Additionally, we’ll explain several ways to install blocking for different purposes. The guide will provide you with all the information you need for blocking deck joists.
- What is Joist Blocking in Deck Construction?
- Do Deck Joists Need Blocking?
- Joist Blocking Requirements
- Deck Joist Blocking Methods
- Can You Use Joist Hangers for Blocking?
- Deck Blocking Spacing
- Should You Nail or Screw Deck Blocking?
- How to Install Blocking Between Joists
- Deck Blocking for Picture Frame
- Blocking for Rail Posts
What is Joist Blocking in Deck Construction?
Deck joist blocking is pieces of wood cut from joist material and installed perpendicular to the joist to connect or bridge the gap between joists. The blocks are commonly uniform in length, being 14-1/2” for joists spaced at 16” centers, 10-1/2” for 12” centers, and 22-1/2” for 24” centers.
The 2018 International Building Code (IRC) Section 502.7 requires deck joists to be blocked with 2-by material the same depth as the joists at the ends to prevent rotation. A band or rim joist, or a header are acceptable alternatives to end blocking. Section 502.7.1 of the IRC identifies that joists greater than 2×12 require blocking at intervals of not more than 8 feet.
There are different ways to connect joists to minimize lateral movement.
- Strapping is the use of a 1×3 or greater board across the underside of joists every 4 to 6 feet from outside joist to outside joist. The strap is attached to the bottom of each joist. The straps hold the spacing and provide limited resistance to joist rotation or flexing. They are easy to install during or after construction. However, they are horizontal and tend to hold dirt, moisture, and nests and rots out easily.
- Blocking is short lengths of solid blocks cut from joist material and attached between joists. Not as quick or easy to install as strapping, they provide significantly more structural support. Often installed during construction in an alternating or straight line between joists they disappear under a single deck. The slender vertical profile also sheds moisture better and lasts longer.
- Cross bracing deck joists is done using 2×2 material mitered at each end and fastened to form an X between joists. Cross bracing joists better distributes the top load from one joist to the bottom of adjacent joists under deflection than strapping or blocking, although blocking is almost as good. Cutting and installing cross joist braces is time-consuming, requires additional material, and the ends frequently split during fastening, so need to be replaced. Predrilling helps but takes more time.
Most deck builders use scrap or unusable joist boards to cut into blocks for use at mid-span or 8’ spacing between beams. Blocking is also frequently used to create a frame or racetrack to support deck board designs such as picture frames, herringbone, zipper, parquet, and diamond patterns. Additionally, blocking provides joist braces to support posts for railings and other structures, including stairs.
Pro Note: When using pressure-treated wood, remember that all cuts and drill holes exposing untreated wood should be painted with preservatives to reduce rot. It should also be noted that preservative chemicals corrode unprotected metal, so use fasteners rated for treated wood.
Do Deck Joists Need Blocking?
Blocking between joists is required under the IRC at the open ends of joists to prevent rotation or every 8 feet if the joist dimensions are greater than 2×12. Joists are commonly fastened at the bottom to mid support beams and to deck boards on the top surface, which some consider equivalent to blocking to prevent rotation.
Many carpenters install blocking at the mid-span or 4 to 6-foot intervals. There are, however, many reasons joist bridging is needed for decks.
- Blocking floor joists ensure the spacing between joists remains uniform and helps straighten curves or twists.
- Joists that overlap at a beam should be blocked to prevent twisting, ensure spacing, and transfer compression and flexing stresses between multiple joists.
- Joist bridging prevents joists from twisting or rotating when under loads or due to moisture-induced expansion or contraction.
- Blocking midway between support beams strengthens the structure and decreases deflection and bounce by distributing the forces between connected joists.
- Bridging between joists reduces lateral movement caused by walking or running across the parallel joists.
- Solid blocking with wood the same width and thickness as the joists effectively cross transfers loads between the top and bottom portion or cords of the joists as loads are applied.
- Joist blocking spacing at half or third points strengthens the deck frame by improving the connections and distributing forces to more support members.
- Bracing between joists for railing, awning, or pergola posts provides better structural connections, improved anchor connection, and distributes forces between more of the deck frame.
- Additional blocking is frequently added between rim or band joists and the joist parallel to it to increase stability and reduce flexing. The extra blocks also can be spaced to improve railing post attachment.
- Blocking provides support and strengthens stair-to-deck connections.
- Deck bridging provides support for picture frames and other deck board designs such as diamonds, herringbone, or parquet while allowing moisture to pass through.
- Blocking makes it easier to construct irregular, semi-circular, or circular decks. The blocks provide support for decking, prevent joists from twisting, and help maintain expanding or unique spacing.
Joist Blocking Requirements
Blocking between joists strengthens floor structures, reduces bounce, prevents twisting, and helps maintain spacing between joists. The 2018 IRC requires blocking with 2-by material of the same depth as the joists at the open ends of joists to prevent rotation.
A use of rim or band joist can be substituted for blocking. The IRC also requires blocking or the use of 1×3 strapping every 8 feet or less between joists of greater than 2×12 dimensions.
The American Wood Council’s 2015 edition of the National Design Specifications (NDS) requires 2×10 and 2×12 joists to be blocked at intervals of 8 feet or less. The NDS also requires blocking between end joists or the use of rim or band boards to prevent rotation.
Many builders will block joists every 4 to 6 feet or at midpoints between supports regardless of joist dimensions. A check of local code requirements is recommended to ensure compliance.
Deck Joist Blocking Methods
Installing a joist brace between parallel boards helps stiffen the framework and transfer loads between members. Builders frequently have a preference when it comes to installing blocks based on experience, costs of material and labor, maximum effect, and code compliance. Here are the more common ways to block joists.
Full Depth Solid Blocking
Using blocks cut from the same material as the joists ensure uniform dimensions and full support from the top to bottom of the joists. It is also a good cost-saving use of joist scraps and unusable twisted or bowed joist timbers.
Diagonal Cross Bracing
Diagonal bracing requires the use of 2×2 material mitered at each end. Two pieces are used to form an X between all joists to transfer top and bottom deflection and load between members. Cross bracing deck joists requires additional material and time to cut and install.
Additionally, the ends often split when fastening into place which requires the use of a new piece. Predrilling solves the splitting problem but increases the cost and installation time.
Alternating blocking is an easy way to install solid blocking between joists. Measure where the blocks are needed at both ends of the deck and snap a chalk line. Alternating block placement on either side of the line from joist to joist makes it easier to direct fasten than in from above or below.
Many deck framers will measure so that the staggered line of blocks falls under a deck board to protect them from moisture and make them invisible from above.
The layout for straight-line blocking also uses the chalk line, except the blocks are all on the same side of the line. The layout allows direct fastening of one end of the blocks while the other end must be toe nailed into place. The straight-line of joist bridging easily hides beneath a deck board from above and looks better if visible from below.
Can You Use Joist Hangers for Blocking?
The use of joist hangers is acceptable for attaching blocking between joists and is even mentioned as an option in my local code; however, their use is uncommon when installing blocking. The added material and labor costs make their use questionable.
Deck Blocking Spacing
The benefits of mid-beam joist span deck blocking are felt in the structural stability of most decks. Many local codes identify their use and have set requirements, and Inspectors often look for them. Some experts question the use of mid-span deck blocking since the joist ends are secure, the top edges fastened to decking, and the bottom to beams which all work to prevent rotation.
2-by blocks the same depth as the joists are required by the IRC at open joist ends to prevent rotation or the use of rim or band joists. Additional blocking every 8’ or less is also mandated for joists larger than 2×12. The NDS requires 2-by blocking every 8’ or less for 2×10 and 2×12 joists.
Many builders add deck blocks between all joist dimensions at mid-beam spans or spacing every 8 feet, whichever is less, to prevent bounce and sway. The deck bridging may not be structurally required by codes but do improve the firmness or feel of the deck. They are also easier to install during construction than retrofitting them afterward.
Should You Nail or Screw Deck Blocking?
To fasten deck blocking 10d nails are commonly used. Nails have greater shear strength and screws greater pulling and holding ability. Some builders use specially designed and manufactured structural screws instead of nails to pull twisted joists against the blocks and to prevent squeaks.
The thicker structural screws are more expensive and take longer to drive. Standard screws or deck screws are not recommended for structural use as they can snap due to stress forces.
Nail guns are often used by builders, especially where swing space is difficult, as it can be between joists. In really tight spots a palm hammer is helpful. To prevent nail movement or squeaks, use 2-1/2” spiral or ringed nails.
Whichever is used, make sure it is rated for the type of wood it is holding together. Chemicals used to treat wood against insects and rot will corrode unprotected metal and cause it to fail.
How to Install Blocking Between Joists
Installing blocking is easier to do prior to the laying down of the decking. The height of the deck off the ground may influence whether the joist braces are installed from above or below.
Additionally, the space between the joists can affect the install. Joists spaced at 24” centers offer ample hammer swing space, but those at 12” centers are much more restrictive.
Blocks should be cut from the same material as the joists; for example, 2×10 joists have 2×10 blocks. To minimize waste and cost, cut blocks from cut-offs or planks unfit for use as joist due to twists or other defects. Remember to treat the cut ends of pressure-treated wood with a preservative to prevent rot of fungal growth.
Protect Blocking With Flashing
Moisture trapped, or sitting on wood will lead to rot and structural damage. To protect the wood and your investment use bituminous adhesive tape, tar paper strips, or flashing on horizontal framing surfaces. The more protection provided, the longer the deck structure will last.
Mark the Layout of Joist Blocking
Measure the distance between the ledger and beam or beam and beam and divide it by two to determine the halfway point between joist supports. Mark the mid-span on both ends of the deck structure and snap a chalk line across all joists in between. If the distance is greater than 8 feet, divide the span by three and snap two equally spaced chalk lines.
Deck Blocking Installation
1. Measure the distance between two joists at the ledger or beam. Joists at 12”oc should be spaced 10-1/2”, 16”oc 14-1/2” apart, and 24”oc a 22-1/2” separation. Measure and cut the blocks from the same material as the joists.
Drawing cut lines using a square helps for straight square cuts. The blocks should fit snugly between the joists to maintain spacing and help form a cohesive web of support. Ensure all cuts are straight and square.
2. To nail the joist braces into place use 10d or equivalent fasteners. The nails should be 1-1/2” from the top and bottom edges of the joists and be spaced 2” or more apart, so a 2×8 block should have 2 nails per end.
3a. Align the block on the left or right side of the string line mark and flush with the top and bottom of the joist for alternate block placement. The nails are driven through the joists into the ends of the blocks. The blocks alternate on the left or right of the string marking.
3b. For straight-line alignment the blocks should be all on one side of the chalk line or centered on it. The first block will be nailed through parallel joists into each block end. Subsequent blocks will be nailed through a joist at one end and toe nailed at the other end.
4. Check periodically to ensure the spacing between joists is accurate. Twists, crooks, and rough ends can throw off alignment.
5. Before blocking the last outside joist, run a string line or laser along it. Measure and adjust the gap if necessary, and use ladder blocking to correct the spacing or bowing.
6. Once the mid or third span bridging is installed, add blocking for picture frames or inserts and post supports. It is also easier to install blocking for stairs and railing or pergola posts prior to laying down the decking. If there will be heavy objects like a hot tub or built-in planters, extra blocking will help spread the additional load to other supports.
7. If using tracks or joist caps either lower the blocks below the upper edge of the joist by 1/2″ or trim the blocks to be flush with the bottom of the joist. The tracks or protective joist caps are an alternative to bituminous tape, tar paper strips, or flashing.
Blocks should be flush with the top and bottom of the joists at mid-beam span or 8 feet, whichever is less.
Alternate blocking floor joists at mid-beam span creates a zipper looking bridge across the joists maintaining spacing and unifying the framework. It provides a more solid structure by reducing the sway and bounce by sharing the forces across the joist bridging.
Using additional joist braces every foot between the parallel end joists forms a ladder-like web that ties the edge more firmly and improves the strength and stability of the edge. It also helps to share any additional stress forces – such as the semicircular feature.
Deck Blocking for Picture Frame
Deck boards commonly run perpendicular or diagonally to the joists. Stylized designs often have deck boards running parallel to joists. To ensure the ends of decking are supported, it may be necessary to construct a support frame using blocking.
Additional pieces of joist material or smaller dimension lumber may be cut with mitered ends to fit between joists and square-cut blocks to form a framework to support inserts or frames.
Deck perimeter picture framing requires blocks every 12 or 16 inches between outside joists to support decking laid parallel to the joists. It may necessitate lowering the blocking below the joist edge by 1-1/2” and laying a 2×4 flat on the blocking. The flat board provides support and a fastening surface for the picture frame and the ends of deck boards.
Installing supports vertically so the narrow edge is up improves drainage, and minimizes rot potential. Some framers prefer to lay the supports horizontally for broader exposure. While that will provide more nailing surface, it will trap moisture between the deck board and support, accelerating rot.
Picture framing and inserts need additional bridging for support and fastening. The use of flashing, tarpaper, or bituminous tape helps protect surfaces from rot.
Blocking for Rail Posts
Railing posts fasten to the inside or outside of the rim or end joists. Posts mounted inside are frequently boxed with blocking cut from joist dimension material and fastened to parallel joists. The blocks form a sleeve for the post and provide additional fastening surfaces for bolts.
They also improve post stability and transfer forces against the top of the post or railings away from the rim or band joists. Some extra pieces may be required to support deck board ends at posts too depending on board runs.
Blocking creates a sleeve for the railing post and additional fastening material. It transfers forces between parallel joists and away from rim joists, improving stability.
Boreholes and end cuts in pressure-treated lumber should be painted with preservatives to prevent moisture damage. Protect horizontal surfaces with a moisture barrier for the same reason. The sleeve works against inward and outward forces applied to the post, for a safer and more secure railing.
Deck joist blocking and bridging at mid-beam span or intervals less than 8 feet decreases bounce and sway. It unifies the joist framework for a stronger, more cohesive deck. Although only required at the open joist ends or for larger dimension joists, many builders block all joists every 4’ to 6’ or mid-span regardless of joist size.
I hope you found the guide informative and helpful and have a better understanding of deck bridging. If you found the guide useful, please share it with others. As always, your comments and suggestions are appreciated.