Floor Joist Blocking and Bridging (Bracing): Complete Guide

The other day I had the opportunity to experience several small kids running and playing in my friend’s living room. Besides the noise and general chaos, one thing that stood out was the surprising amount of wobbling the floor was experiencing. My friend noticed too and was wondering what the best fix might be. I mentioned floor joist blocking or bridging.

Floor joist blocking is taking solid lumber, typically the same size as your floor joists, and fastening pieces perpendicular between every pair of joists in a straight or staggered line. Cross bridging is the same idea, except that plywood or smaller pieces of lumber (2×2) are used to make an “X” between each joist in place of a solid piece of lumber.

Whichever style of blocking or bridging you choose, both will serve to solidify your floor and reduce wobble and bounce. There are many different methods to go about installing bridging and blocking, and each has specific pros and cons that may or may not be right for your floor.

In this article, we’ll take a look at what bridging and blocking are and how to install either in your own home. We’ll also go over the pros and cons of each type of joist bracing, as well as some do’s and don’ts and pro tips when it comes time to install.

Floor Joist Blocking

What is Floor Joist Blocking?

Floor joist blocking refers to solid, lateral supports installed between floor joists to evenly distribute loads placed atop floor joists. Blocking uses lumber the same size as the floor joists and is fastened either in a staggered or straight line mid-span or every 8’ depending on the length of the joist.

Blocking is effective in reducing floor wobble. When joists are not blocked or bridged, each joist is much more susceptible to movement both up and down and side to side. Installing solid wood blocking with appropriately sized nails will distribute loads across all of the joists, minimizing the movement of the joists directly beneath the load.

There are several different methods in terms of installing blocking. Most rely on using the same sized lumber as the joists themselves and fitting them between the joists in a staggered line in the middle of the joist span, fastened with nails.

Finally, one of the main problems with solid blocking is that it often causes humps in the floor above, especially if installed after the house is built. Why? The blocking lumber will dry out at a different rate than the joists.

Often the joists dry faster than the blocking simply because they’ve been there longer. When that happens, you get a slight hump from the block that can be pretty annoying.

Pros

  • Reduces floor “bounce”
  • Easy to install
  • A cheap solution for solving floor movement

Cons

  • Plumbing and electrical can make installation difficult
  • May cause hump in the floor above
  • Measurements must be precise or blocks will not fit properly

Joist Blocking Methods

There are a couple of different methods for blocking your joists with solid lumber: alternating and doing it in a straight line. Ideally, you can block in a straight line, as this provides slightly more stability as the load transfer from the floor above is directed most efficiently through blocking in a straight line.

Straight Line

Blocking floor joists

When blocking in a straight line, you’ll cut your pieces to length and begin on one end. You’ll quickly find that swinging a hammer between joists is difficult, particularly if they are spaced at 12” or 16”. Thus, a palm nailer is extremely handy. They are air-powered, however, so you’ll need an air compressor and a hose, too.

You should use nails when blocking or bridging as they can withstand shear forces much better than screws. When nailing blocking in a straight line, you’ll be able to face nail one end of the blocking through the joist. However, the other end of the block will need to be toe-nailed – two nails driven in at 45-degree angles on either side.

It should be noted that while blocking in a straight line is ideal, alternating your blocking will not result in much less – if any – reduction in load deflection. However, since you are toe-nailing blocking in a straight line for every other side of each piece of blocking, this does result in a more solid connection than face nailing both sides.

Alternating

Wood blocking

Most people choose to alternate their joist blocking because it is much easier and faster to nail than putting it in a straight line. If you are blocking your joists mid-span, then you will draw a center line across every joist. Then the blocking will be fastened in an alternating pattern on either side of that line.

This allows you to face nail each piece of blocking and avoid toe-nailing. While toe-nailing results in a stronger joint, you risk cracking or tearing out the ends of your blocking. It also takes more time as you’ll use more nails than you would alternate your blocking.

Joist Blocking Spacing

According to the IRC, joist blocking is only necessary if your joists have a depth greater than 12”. Therefore, for most houses, you are not required to have blocking or bridging if you have traditional lumber joists that are 2” in thickness and up to 12” in width as long as both ends are fastened properly.

It should be noted that if you do happen to have joists greater than 12” in width, then you should have blocking or bridging no less than every 8’. For most spans, that means either two rows or blocking/bridging or one in mid-span.

If you happen to have engineered joists or I-beams, then the code stipulates that you should follow the manufacturer’s instructions regarding bridging and blocking and not the building code.

What is Floor Joist Bridging?

Floor Joist Bridging

Floor joist bridging uses wood or metal strips or strapping to connect joists and improve load deflection. Bridging spans the joist bay by connecting the top of the width of one joist to the bottom of the width of the adjacent joist. Many types of bridging incorporate two per joist bay, creating an “X”.

Bridging in older homes often uses a “herringbone” or “X” pattern using wood strips. There is no uniform dimension to wood bridging, as some builders prefer 2×2 while others like 1×4 or even 1×2. Regardless of the size, bridging is an extremely effective method to shore up wobbly floors.

Metal bridging is also quite popular, and many current products offer “nailless” joist bridging straps. Each end has a spiked nailing plate that is hammered into the top and bottom of the joists. Traditional metal bridging is nailed on either end, usually before installing the subfloor above.

You’ll often hear the term “bridging” used interchangeably with “cross-bracing”. They aren’t the same. Bridging refers to the general act of fastening of strips between joists to deflect loads. Cross-bracing is a type of bridging, specifically referring to using bridging strips in an “x” pattern between joists.

Pros

  • Installs easily around plumbing and electrical
  • Both metal and wood strips are cheap
  • Won’t cause humps in the floor above

Cons

  • Hard to install after the subfloor above is in place
  • Nailing smaller pieces of bridging can result in splitting
  • Every piece needs to be cut at an angle

Types of Bridging

One of the big drawbacks to using wood bridging between joists is that you have to cut each piece at an angle. While finding the angle is easier than you’d think, it requires a few more steps than you’d use for metal bridging. For that reason, many new builds use metal.

Solid Wood Joist Bridging

Solid wood joist bridging is the original type of joist bridging used in home building. Typically strips of 1×2 or 1×3 were used to cross-brace every joist. While it’s hard to find data to back up whether 1×3 is better than 1×2, you can make the logical conclusion that it probably is better.

If you want to install solid wood bridging, then 1×4 is an even better option as you reduce the risk of splitting using a wider piece and it is still quite cheap.

The downside, as mentioned above, is that you have to angle cut every piece. Every piece should, however, have the same angle except for possibly the first and last joist bay, so all you have to do is set your miter saw at the appropriate angle and start making some cuts.

Steel Joist Bridging

Bridging Floor Joists

Steel joist bridging comes in several types. The first and most common type is the metal straps that are meant to be cross-braced and nailed at the top and the bottom. These straps have a bendable top and bottom that allows them to be fitted snugly into the top and bottom of each joist. They also have nail holes allowing them to be fastened easily.

Other types of metal joist bridging are the nailless type, which features a nailing plate-type fastening system on either end. Much like the version that is nailed, both ends can be bent for a perfect fit. Then the ends are hammered into the joists, as either end of the strap acts as a nailing plate.

A drawback to the nailless bridging is that the nailing plates are not as strong of a connection compared to a nail. So if joists dry out, twist, or cup, then this type of bridging can begin to pull out or slacken to the point where it is not providing any load deflection.

Wood Composite

Wood composite bridging strips act in the exact same manner as solid lumber bridging strips. However, the main drawback for these strips is that you’ll need to rip them the proper width yourself.

If you have a table saw, then this is easy. You can also accomplish this task with a circular saw, which is also quick but not accurate. However, pinpoint accuracy is not necessary for this application.

The use of plywood or OSB strips is acceptable. The thicker the strip, the better. While ½” strips could work, ⅝” or ¾” strips are better. Yes, a sheet of ¾” plywood is quite expensive. However, you’ll likely only need one or two at most.

These types of strips provide an extremely strong connection between joists. They can be bridged once or cross-bridged, and cutting them at an angle is not different than cutting solid wood at an angle.

Floor Joist Bridging Spacing

The IRC does not mandate floor joist bridging unless your floor joists are greater than 12” wide. If so, then bridging should be placed no less than 8’ on center from the next row of bridging.

The caveat to this rule is that engineered joists of all kinds, including I-beams of any material, must be installed per the manufacturer’s directions and is exempt from the bridging code.

Is Blocking (or Bridging) Required for Floor Joists?

Blocking is not required for floor joists unless joists are greater than 12” in depth (according to IRC).

Both ends should be blocked or affixed to rim joists/band joists. If so, then blocking is not required unless the ratio of the joist width to thickness is greater than 6:1. Therefore, no blocking or bridging is necessary for 12×2 joists if fastened properly on each end.

How to Install Blocking Between Joists

Installing blocking between joists requires precision measurement, as one block that is slightly larger or smaller will throw off the rest of your blocking measurements. However, as long as you measure correctly, then the process is very straightforward.

Determine Where To Block

If your joists are 16’ or less, then blocking mid-span makes sense as code dictates that less than 8’ apart is not necessary. If your joists are greater than 16’ long, then consider two rows of blocking at even intervals.

Therefore, if you have a 20’ joist, put your blocking at 6’ and 14’. You’ll have an 8’ gap between the blocking and it will be evenly spaced between each end. Of course, you could get away with one mid-span block at 10’, but many framers will tell you that every 8’ is better.

Measure Joist Size

Measure your joists. Many of you can probably just like and identify a 2×8 from a 2×10, but measure anyway. If you have a really old house, then you’ll need to measure. Always remember that a 2×10 isn’t actually 2”x10” – it’s 1 ½” by 9 ¼”. Now you know the type of lumber you’ll need to buy.

Measure Between Joists

Now you’ll need to measure between the joists. To do this, measure from the center of one joist to the center of the next. It will be close to either 12, 16, 20, or 24. If it is 16” or very close, then your joists were placed at 16” on center.

Now you can do the math in your head. The space between your joists won’t be 16”. Subtract the actual width of the lumber from 16” – so, 16” – 1.5”, which is 14 ½”. All of your blocking will be 14.5” long.

Remove Obstructions If Possible

This step is the most frustrating and time-consuming. If you are installing blocking into an older house, then there will be all manner of wires, plumbing, and HVAC between the joists. If this is the case, consider using bridging instead.

If there are only a few obstructions, see if you can temporarily remove them or move them out of the way, such as a wire or a pipe. Remember that after the blocking is installed, you’ll need to reinstall whatever you removed, meaning you’ll need to cut holes in your new blocking to reroute any electrical or plumbing.

Cut The Wood Blocking

The most important part is cutting the wood blocking accurately. Ensure you are cutting pieces that are precisely 14.5” if you have 16” on center joists, for instance. Failure to get a precise gut will result in twisted joists and wasted wood as you’ll have to recut your other blocks.

Fasten Blocking Between Joists

Use 10d nails in a staggered pattern and face nail the blocking through the joists. One nail at the top and bottom will suffice. Alternating allows you to quickly nail each block without toe-nailing.

You still will have to toe-nail the last block on each end, as you won’t be able to get behind the final joist on either end. Consider using a palm nailer to nail your blocking as swinging a hammer in a joist bay is difficult if they are 12” or 16” on center.

Pro Tip: When working around HVAC ducting, it is not realistic to cut a huge hole in your blocking to run the duct through the new block. Doing so would negate any benefit from blocking that joist as the hole would compromise its strength.

Instead, take a 2×4 and run it flat beneath the duct, joining the two joists. Cut it to the same length as your other blocks. Affix with nails just as you would for the rest of the blocking.

How to Install Bridging Between Joists

Installing Bridging is similar to blocking but with a few key differences. First, you’ll have to angle cut each piece. All the angles should be the same, and a slight variation either too acute or obtuse still allows for the strips to be used.

Secondly, nailing in wood bridging strips with a subfloor immediately above makes it very difficult to nail the top of the strip to the joist. You’ll have to use thicker – 2x lumber – strips or metal for your bridging. Thinner lumber, such as a 1×4, cannot accept a toe-nail and cannot be used with a subfloor in place.

Determine Where To Install Bridging

Just like blocking, bridging adheres to the same building code guidelines. So if your joists are 12” or less in width, no bridging is necessary. Otherwise, you’ll space your bridging no less than 8’ apart.

If you have longer joists, then including two rows of bridging evenly spaced will significantly improve the floor’s rigidity above. If your joists are 18’ long, then consider putting bridging at 5’ and 13’. That provides an 8’ gap in the middle while keeping them an even 5’ from either end of the joist.

Determine What Type of Bridging to Install

The next step is to choose your bridging. If you are installing new, then 1×4 strapping works well and is easy to cut and install without splitting. Or, if your home is older and there is an existing floor above, consider using metal bridging with nail plates, allowing you to bridge without nails.

Cut Bridging to Size

If you are using lumber or engineered wood, then you’ll need to cut your bridging to fit between the joists. There are many different ways to do this, but we’ll go through one example that doesn’t require a speed or builder’s square.

First, mark a line on the underside of a joist where you will place your row or bridging – your center mark. Then measure the width of your joist. Measure half that length, and make a mark on the joist bottom that distance above the center mark. Then do the same below your center mark. These are your edge marks.

So if your joist is 9 ¼” wide, half that length is 4 ⅝”. So you’ll measure 4 ⅝” above and below the center line. Now use a chalk line or something straight to make the same set of lines on the adjacent joist.

You are ready to measure. Simply hold a strip flat between the two joists, ensuring the top edge and bottom edge of the strip line up with the top and bottom edge marks.

Each end will form an angle against the joist. Reach up above the strip and make a mark on the strip where it meets the joist. This is your angle cut. Do the same on the other end.

When cutting, a radial arm saw is ideal. If you don’t have one, then a regular miter saw can work if you can turn it to achieve an angle. You can also buy strips, but they do not come with pre-cut angles and may not come in the size you want.

Fasten Bridging Between Joists

If using metal bridging, you’ll simply bend a portion of the bridging downward with pliers to fit the gap. Palm nailers work very well when nailing in bridging. Or you can use the nailless metal bridging.

Use 6d or 8d nails for bridging. If installing bridging on a new floor, only nail the tops of each bridge. Once the subfloor is on, then go ahead and fasten the bottoms of each bridge. Nailing both before subfloor installation could put the joists out of line.

Some contractors opt for staples instead of nails, using 1 ½” staples to install bridging. This lessens the likelihood of splitting the wood and still provides an equally effective joint.

Joist Blocking vs. Bridging – Which is Better?

Both types of joist reinforcement work well. There has been no conclusive study proving one type is better than the other. While blocking might seem like the better choice due to the solidity of the block itself, bridging, when installed properly, is just as strong.

For new installations, solid blocking makes more sense. You’ll have lots of scrap leftover from your joist installations, and it just makes sense to put it to use as blocking. There also won’t be any obstructions from electrical, ducting, or plumbing.

Bridging makes more sense for older homes. The use of metal bridging without nails or solid 2×2 or 2×3 lumber is best. This way, you’ll be able to avoid obstructions within the joist bays and not run the risk of creating a “hump” in your floor from a block that dries out at a different rate than the joist.

Whichever method you choose, make sure you are doing it right. Use appropriate fasteners, measure carefully, and consider renting or borrowing a tool or tools to make your life much easier, such as a palm nailer or arm saw.

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