Joining Beams Over Posts: What Are the Options?

Smaller decks that fall within the available length of standard dimensional lumber allow for single-piece beams, but what do you do when your deck goes beyond what’s available at the hardware store? Larger decks with beam spans of greater than 20 feet require multiple beams to be joined together over a post.

Since these types of joints present a potential structural weakness in a deck, it’s crucial that they be executed properly.

Joining beams over posts can be done by either cutting a notch out of the post and splicing the beams together over the notch or by butting the ends of each beam together on top of the post and joining them using galvanized metal post caps.

In this article, we’ll explore both methods, how to properly execute each, and help you determine which is best for your deck and skill level. We’ll also review how to connect beams that meet at corner posts.

What Are Beams?

Beams are structural pieces of lumber supported by posts and footings that provide support for joists that run perpendicularly above them. A beam can consist of one large piece of lumber typically 4 inches or thicker or two to three pieces of 2x dimensional lumber held together by bolts. Beams either run on top of posts, held in place by special brackets, or fit into a notch cut out of the posts.

Why Do You Need to Join Beams Over Posts?

Decks often have greater dimensions than the maximum length of dimensional lumber available to create a beam. The longest beam one can buy is typically 20 feet. However, some lumber stores have beams that are a maximum of 16 feet long.

If you have a deck that exceeds these dimensions, the only way to provide support is to extend the length of the beams by joining the ends. In addition to the overall length, some beams also must be joined at corners.

Why Is This Type of Joint Important Structurally?

A joint between two beams presents a structural weakness in the framework of the deck. In short, a continuous piece of lumber is much stronger than two pieces of lumber joined together.

If not adequately supported, this weak point in the beam can bow under the weight of the joints and decking, creating major structural issues and an unsafe deck. For this reason, it’s crucial that the joints are only made over posts to provide the added support they need.

This means careful planning is required to ensure the proper beam lengths are purchased, so the joint ends up over the posts.

This typically does not mean purchasing the longest dimensional lumber available. For example, A deck with a line of three posts 12 feet apart will require two beams to be joined over the middle post, which is 12 feet from each outer post center to the center. In this case, the deck requires each beam to be 12 feet long to make the joint over the post.

Proper planning will ensure you purchase the proper length lumber for the beams and minimize waste.

What Are the Building Code Requirements for Joining Beams?

The International Residential Code (IRC) specifically states that splices over multi-span beams should be located over posts (R507.6). This means that each piece of the beam must extend at least one inch onto the post to be “in code.”

Ideally, the joint will take place in the middle of the post, ensuring it has plenty of coverage on both sides to prevent the joint from shifting off of the support post. Always make sure to check with local building codes to see if there are additional requirements.

What Are the Options for Joining Beams Over Posts?

When it comes to joining deck beams over posts, you have two choices. These options range from butt joints, which are perhaps the easiest option, to splicing, which is more time consuming but can result in a more aesthetically pleasing look.

In this section, we’ll review what each type of joint is as well as the pros and cons of each type. Keep in mind that whatever method you choose should be consistent with the entire deck. A post that’s using a splicing method should use a splice on all the post and beam connections, not just on those where beams are joined to achieve a uniform look.

This is also a decision to make before construction begins, as the type of joining you choose will dictate the length and depth of the support posts.

Splicing

Splice involves cutting a notch out of the post to provide support for the two ends of the beams. The notch is cut wide enough to support the entire width of the beam and deep enough so that the top of the beam is flush to the top of the post.

Once beam ends are in place, multiple bolts are installed through the beam and the remaining width of the post. This method is effective because it allows for the weight of the beam to rest entirely on the post while allowing for the beam to be securely attached to the post via the bolts.

Even though you are cutting a section of the post out because the beams are applying downward pressure on the post, it still maintains all of its structural integrity.

To execute this method, you’ll need a circular saw, a handsaw, a square, and a chisel. Begin by marking the notch so the two ends of the beam come together to fit perfectly in the notch. Otherwise, the joint will not be structurally sound.

Do this by measuring the exact width and depth of the beam. While this may sound obvious, making the required cuts before putting the beam in the ground is much easier than attempting this after the fact.

After marking the end of the post, use a circular saw to make the initial cuts. Because of the round shape of the saw’s blade, you won’t be able to complete the cuts with a circular saw alone.

Do not cut beyond the dimensions of the notch to complete the cut, as this will significantly weaken the post.

Instead, use a reciprocating saw or a handsaw to finish the cut. When the cut is complete, wedge a chisel into the cut and use a hammer to knock the notch out.

You’ll then need to use the hammer and chisel to clean up the cut so the beam can sit flush against the post in the notch. When installing the beam, ensure that each of the two ends rests on at least one inch of the post for support.

Use two galvanized bolts on the ends of each beam to secure the beam to the post. This method creates a strong joint as each beam is well secured to the post by the bolts with the weight entirely supported by the post they’re resting on.

This method is also more attractive than butt joints, as it does not involve large metal plates, which can be unsightly if the frame of the deck is visible.

That said, this method is more time consuming and involves more skill as it involves cutting notches and a fair amount of chiseling to create a notch that will fit flush with the beam ends.

Butt Joints With Post Cap

A butt joint involves resting the two beam ends squarely on top of the post. The two ends of each beam are held to the post by a post cap specially designed to connect the two beams while also holding both beams secure to the support post.

As with other joint types, it’s essential that each of the two ends of the beams have at least one inch of the post under them to ensure the entire joint is adequately supported.

When creating a butt joint, begin by installing the correct cap on top of the post. These caps typically have preset nail holes for galvanized nails or screws, making them easy to install. Once the two ends of each beam are in the cap, finish the installation by installing fasteners in the preset holes.

There are a few things to be aware of when using this method. Many caps are designed for either solid beams or multi-ply beams, so make sure to purchase a cap that is compatible with your deck design to ensure a safe and secure connection.

Butt joints are an attractive option because they don’t require additional cutting and they maintain the entire structure of the post, eliminating any loss of structural integrity. That said, metal connectors can be unsightly, making them an unattractive option for taller decks where the deck’s frame is visible.

It’s crucial to purchase the right size tie for the size of the posts on your deck.

Attaching Beam to 4×4 Post

Connectors for 4×4 posts, such as this model from Simpson Strong Ties adjustable post cap, feature two pieces. The lower part wraps around the post on both sides. Each piece includes four connection points that attach to the post, ensuring the joined beam does not shift off of the post.

The bottom of each piece stretches 2-1/2 inches down the post, creating plenty of surface area for a secure connection. The top of each piece serves as the connector for the joint. At 6 inches long and 3 inches wide, the metal piece securely holds the butted ends of the two rafters together from both sides, ensuring a strong joint.

Attaching Beam to 6×6 Post

This Simpson Strong Ties connector for 6x6 posts functions a little differently. It also comes in two pieces; however, instead of wrapping around the girth of the post, the bracket sits on top of the post with tabs that engage the post on either side. The two 2-1/4-inch tabs each have two preset drilled holes for galvanized fasteners.

Once in place, the two brackets cradle each end of the joining beams with sides that are 2.5 inches high and run the entire width of the 6×6 post. Four predrilled holes on each side securely hold the beams in place over the post.

Corner Joints

In some deck designs, it’s necessary to join two beams at a corner post.

Joining a beam over a corner can be a bit more complicated but is still very doable. It involves making multiple types of joints with the two layers of the beam to ensure the connection doesn’t come apart over time.

With this type of connection, it makes sense to install each ply of the beams separately, then attach the separate layers after the joint is made.

Begin by laying the two interior beams. Create a butt joint over the post between the two beams, using galvanized fasteners to make the connection.

Make sure the heads of the fasteners are countersunk, so the fastener heads are not jutting out from the boards. This will allow the outer pieces of the beams to sit flush to the inner layer at the joint.

Once the two beams are connected at the corner, connect the outer layers of the beams. When making this connection, use a mitered joint by making 45-degree cuts at the ends with a miter saw. Join the ends with galvanized fasteners so that both pieces have at least one inch of support from the post.

Make sure the opposite ends of each layer of the beam match up when cutting and creating the joints over the post.

By using different types of joints for each layer, the seams of the joint will be staggered, creating a stronger overall joint between the two beams. Once the joints are in place, add fasteners down the length of the beams to secure each of the beam’s two layers together.

In some cases, you may need to create a corner joint that is not at a right angle. In this case, use the same method of creating the joining for the inner layer of the two-ply beam first.

Instead of using a 90-degree butt joint, make miter cuts to both boards to account for the angle of the deck. Once this connection is made, join the outer layers of the beam using a miter cut. This will stagger the seams, creating a strong connection.

Again, make sure to secure each ply of each beam together after making the corner joint.

After the joint is created, you still need to fasten the beams to the post. Use a metal tie, such as this model from Simpson Strong Ties. This tie attaches to the outer part of the beam and the outer part of the post using galvanized fasteners. This prevents the joint from working its way off of the post over time.

How Not to Join Beams Over a Post

There’s a right way to join a beam over a post and there is a wrong way. This section will review some of the most common errors made when joining beams.

Perhaps the most common error is creating a joint that does not use the posts for support. As we mentioned earlier, joints present a structural weakness in the beam.

Neither a butt joint nor a spliced joint has the structural integrity to support the live load and a dead load of a deck without the added support of a post. An unsupported joint will eventually sag and create dangerous structural problems in the deck.

With this in mind, make the end of each beam is supported by a post and adheres to the 1-inch rule.

Some homeowners make the mistake of joining beams to a post by attaching them to the side using bolts instead of resting the beams on top of the post. This is often done by sandwiching the two layers of the beam around the post.

This method places the entire weight of the beam, joists, and decking on the fasteners instead of the post.

Unlike posts, metal bolts do not have the sheer strength to support this weight. This type of construction can cause the bolts to shear off under the load of the deck and live weight, causing parts of the decks to collapse, potentially resulting in serious injury.

Beams must rest on top of the posts to adequately support the framing of the deck.

There are also common errors made when cutting the notch for a splice joint. While notching a post to create a splice connection, one approved method for joining beams only works if done properly.

Creating too big of a notch can cause structural problems, while creating too shallow of a notch will not allow the post to support the beam adequately. With this in mind, cut a notch that is the proper depth, so the outer edge of the beam fits flush to the post.

Also, make sure to use a post size that allows you to cut a notch while still preserving enough wood to hold the beam in place on the notch securely. About half the width of the beam should be enough to accomplish this.

Conclusion

When building larger decks that demand spans longer than 20 feet or in cases where finding longer dimensional lumber is a challenge, the only way to achieve the length needed to support the deck’s joints is to join two beams over a post.

By using a splice joint or a butt joint to connect the ends of two beams, you can create a joint that is structurally sound, ensuring it does not create a weak point in the frame of your deck. As with all construction, be sure to check local building codes to ensure the type of beam joining method you used is in code.