Most DIYers and carpenters have some ideas of how to connect wood together with glue, nails, screws, and bolts. However, when it comes to splicing pieces together for structural purposes, we may admit to needing help. How to connect 2 4×4 posts together is a common question on many sites, so we decided to put together a helpful guide.
To connect 2 4x4s use a half-lap joint secured with construction adhesive, two steel plates, and four to six 1/2″ or 5/8” through bolts. Posts that support roofs or platforms should lap between 20” and 24”, other structures should connect for 6” to 10” along the centerline.
In this guide, we’ll identify when to splice 4x4s and explain several current methods that are Code compliant. We’ll also identify and explain the strongest way to join 4x4s. By the end of your read, you should understand how to join 4x4s end to end, and when it is acceptable to do so.
- When to Connect Two 4X4 Posts Together?
- Extend Posts
- Build Long Skids
- Post to Beam Connection
- How To Connect 2 4X4 Posts Together: Most Popular Ways
- Butt Joint With Steel Plates
- Half Lap Joint
- Dowel Joint
- Butt Joint With Hollow Pipe Insert
- Modified Half Lap Joints
- Interlocking Tapered Scarf Joint
- Post Cap/Base Brackets
- What Is the Strongest Way to Join 2 4X4 Wood Posts?
When to Connect Two 4X4 Posts Together?
There are many reasons to join 2 4x4s together. Making a post longer, cut too short, repairing a post, building a longer piece for a skid, and post-to-beam connections are the most common reasons.
Adding a privacy barrier or roof shelter to a deck is a frequent reason to make existing railing or deck posts taller. Extending fence posts to raise the height of the barrier is another reason to lengthen 4×4 posts.
Two other reasons rest on transportation and availability issues. Carrying and connecting two 8-foot lengths is easier than transporting a 16-foot length, especially if traversing woodland trails. The availability of long 4x4s at many small suppliers is often limited due to low demand, making splicing a necessity.
Wood is susceptible to rot or damage. Repairing a 4×4 is usually easier than dismantling and rebuilding a fence, deck, or other structure. Especially if the post is set in concrete or supporting other pieces. Figuring out how to hold everything up while repairing the 4×4 may be more difficult than the repair itself.
Build Long Skids
Joining 4x4s for horizontal use instead of vertical use is done in a similar way. The orientation of the cuts should maximize lateral strength while preventing the retention of penetrating moisture. The stresses applied to the skid will also affect the type and orientation of the connection. A pull force versus a downward force affects the joint differently.
Post to Beam Connection
Fastening a 4×4 post to a beam or beam to a post can be done in a variety of ways depending upon the purpose and where the connection occurs. Using specialized brackets is a common way to secure pieces together and support a butt joint. A Mortise and Tenon connection will look better but may not provide the necessary support strength due to the dimensions of the wood.
How To Connect 2 4X4 Posts Together: Most Popular Ways
Carpentry and joinery are ancient trades packed with specialized terminology and tools. A scarf joint is any connection where two pieces of wood overlap or lap flush in the same grain direction. Connections must withstand vertical and horizontal forces or stresses, so it is important to consider those when choosing a joint. Strength and resistance to compression or tension can be improved with glue, straps or plates, and bolts.
There are a number of popular ways to connect two 4x4s together to extend their length, some are easier than others. It is advisable to check local Codes for any requirements and have a Structural Engineer check it, especially if it will support or cover people. Additionally, any cuts or holes into treated wood should be painted with an end-cut solution containing Copper Naphthenate or Sulfate.
Pro Note: When using untreated or galvanized metal on pressure-treated wood it is recommended the contact surfaces be protected with bitumen tape. The tape helps prevent corrosion, and seals against moisture. Always use stainless steel or coated fasteners with pressure-treated wood. Hot-dipped galvanized will last longer than untreated steel fasteners, but stainless will outlast them all. If near saltwater, stainless steel bolts and plates are better than hot-dipped galvanized ones.
Butt Joint With Steel Plates
The simplest way to connect two 4x4s is with a butt joint. Trim the ends to be joined so they are square and smooth. Any angle, however small, can act as a shear line, and a smooth flat surface makes for a better connection.
If attaching a 4×4 to extend an existing one or replace a damaged section, the work needs to be done where the initial post is. To connect two lengths to be erected assembled; it can be done on a flat surface like a floor, deck, saw-horses, or a table.
There are different lengths, widths, and thicknesses of steel plates or straps for use when securing a butt joint. Some are untreated steel, others are galvanized, stainless steel, or coated in ceramic or plastic. Plates may be predrilled like the Simpson Strong-Tie plates with staggered holes to prevent splitting; others need to be drilled for the fasteners. Two plates should be used to brace and reinforce opposing sides and to prevent buckling.
To drill steel plate for 1/2″ threaded rod or bolts, mark where the holes need to be. Depending on the thickness, drill a pilot hole and ream the hole larger by increasing the bit diameter a couple of times. Opposing plates should have matching holes for through bolts.
There should be two bolt holes for each connected piece. Most Codes require 1-1/2” between the edge of the wood and bolt hole, which doesn’t leave much space to off-set fasteners on a 4×4.
For through bolts or rods, secure the plate in place with clamps, ensure posts align and are level, and mark the holes. Remove the plates and bore through the posts. Treat the holes with wood preservative, clamp the plates into place, put washers on the bolts and push them through the holes, add another washer, and thread on the bolts. Check the levels again as you fully tighten the bolts.
To connect the Simpson Strong-Tie (SST) or other pre-drilled plates to the 4x4s, align and level the posts with the plates and clamp everything tightly together. Some plates have a mid-line set of holes that can align with the joint and still have enough holes for screws.
Off-set opposing plates by 1/2 the distance between rows of holes, that will make it easier to fasten them. Use appropriate fasteners like SST #9 x 2-1/2” structural-connector screws to attach the plates to the posts.
Follow the manufacturer’s instructions or Structural Engineer’s recommendations for the screw pattern.
- Quick, simple, and easy to install
- Good vertical strength
- Staggered fasteners prevent splitting
- Poor lateral strength
- Difficult to align
- Not aesthetically pleasing
Half Lap Joint
A lap joint, also known as a shiplap or half-lap scarf joint, has been used to join wood into longer lengths for more than a millennium. A half lap is a joint that overlaps by half the thickness of the pieces being joined for a less visible joint. It increases the wood to wood contact for a better connection and strength. The joint looks like a chair seat with a high straight back.
To make a half-lap joint, measure to find the centerline of the two pieces and mark both sides at the end and near where the cut will end to form the seat. Measure from the end and mark the desired length of the cut and mark it on both sides and pieces. While the depth is always half, the length of the cut will depend on use and compliance with local building codes.
There are several ways to remove the wood to do the lap; however, the lengths and location do play a part. A basic hand saw, hammer, and chisel, or all three have worked for centuries. Electric or battery-powered skill saws or reciprocating saws are a modern convenience that may make the task easier. I’ve seen a good hand sawer do up a set of half-laps in the same amount of time as a reciprocating saw, and with much more accuracy.
If both pieces are moveable, lay them out on horses or a flat surface, and measure and mark them. Using the tools of choice, remove the necessary wood from both pieces to form the seat and back of the joint. If one piece is under a deck or still holding up a fence or railing, arm and headroom may determine which tool gets used.
Careful and accurate measurement and cutting are needed so structural integrity isn’t compromised. Do not overcut either the vertical or horizontal cuts, or you create a shear line.
What the pieces will support and your location will determine the length of the lap. Posts supporting roof or platform structures have different requirements than fence post or mailbox post extensions. A 10” lap with six structural screws or three 1/2″ or 5/8” through bolts will work for a privacy fence, while a 4” to 6” lap with 4 structural screws or two 1/2” through bolts will do for a mailbox post.
Roof or deck post slices commonly need to be Code compliant. Some jurisdictions require posts to have a 24” lap with two 18” steel plates to sandwich the mid-section of the connection. Four 1/2″ or 5/8” through bolts secure the joint in a slightly staggered pattern that maintains 1-1/2” between the centerline of the bolt and the edge of the wood.
Another locale requires the lap to be 22” long with five 1/2″ or 5/8” through bolts with washers. Check before you countersink bolts as some areas don’t approve it for structural 4×4 supports. The bolts need to be staggered and be 1-1/2” in from the edge, no closer than 3” to the ends of the lap, and 4” between bolts.
Wind loads and other lateral forces may need to be considered when orienting the direction of the lap. A connection with cuts perpendicular to the force is stronger than those parallel to the stress. When splicing four corner posts to support a roof structure, it may be recommended to alternate the orientation of the lap.
To further strengthen half-lap joints use a construction grade adhesive like Liquid Nails or Loctite PL Premium. They are rated for interior-exterior use, waterproof, paintable, and even cure in sub-zero temperatures. We suggest fitting the joint together and clamping it in place, and then drilling the holes.
Separate the joint, apply the compound evenly, and then reseat the two pieces using several through bolts to hold the alignment. As the bolts are tightened, the viscosity of the glue makes the pieces want to slide out of alignment, making clamping difficult.
- Easy to make
- Very strong vertical and lateral joint
- Excellent long grain to long grain connection
- Resists twisting
- Saw skill
Lengths of 3/8” to 1-1/4” dowels can be used to add lateral strength to butt joints. Carefully measure to bore and align 1 to 4 holes in the ends of each 4×4 and drill the holes between 1” and 3” deep. Cut the dowels to lengths of 2” to 6”, sand the cuts, glue, and tap into place.
Coat the protruding piece of dowel with glue, or pour some glue in the holes of the upper pieces, and push it onto the dowels. Tap into place with a hammer if necessary.
Similar butt joint reinforcement connections may be made using two to four biscuits inset into matching grooves cut into the ends of the 4x4s. Alternatively, cut a matching groove in the butt ends and glue in a wooden spline. Dowels, biscuits, and splines all reinforce the alignment and provide some lateral reinforcement.
The use of dowels for butt joints connecting light-use posts is often helpful. However, for structural purposes, attach two steel plates with structural connector screws or 1/2” through bolts to opposite sides of the joint is commonly required to add more strength too.
- Improve lateral connection
- Inexpensive joint
- Difficult to vertically align accurately
- May produce a shear fault
Butt Joint With Hollow Pipe Insert
A hollow 10” to 24” metal pipe, 1” to 1-1/2” in diameter, inserted into a hole drilled vertically into the center of the ends of the two 4×4 to be butt joined, provides additional lateral reinforcement. The pipe is secured into the vertical holes with construction adhesive. Two 1/2″ through bolts are inserted into the top and bottom post pieces in holes bored through the wood and pipe.
- Improved lateral resistance
- Less visible connection
- Strong vertical joint
- Special tools required
- Post may twist out of alignment
- The pipe weakens the lateral strength of wood
Modified Half Lap Joints
There are numerous ways to modify a half lap. Some involve elaborate cuts and may even include a wooden key pin to secure the joint. The less complex joint retains the traditional parallel and perpendicular cuts and is easier for the novice or beginner.
The interlocking half-lap connection has an additional ‘tooth’ or tenon cut into the end of the chair back of the lap. The ‘tooth’ has a receiving channel cut into the opposing base of the other post. The two post lengths slide latterly together, so accuracy is paramount. The joint is often secured with construction adhesive combined with 1/2″ or 5/8” through bolts with washers or steel plates.
- Strong lateral and vertical seam
- Highly aesthetic joint
- High level of skill
- Require more time
Interlocking Tapered Scarf Joint
A tapered scarf or scarph joint is a diagonal joint cut on a 1:8 to 1:12 slope and is as old as the half lap joint. It is commonly found in beams, skids or other horizontal members joined together with wooden pegs or bolts and plates. The scarf joint requires some modifications when used to extend a post.
A nibbed scarf joint has the pointed ends of the slope cut perpendicular to the outside face of the timber or slightly sloped counter to the scarf. It forms a flat end or seat with the long cut being sloped – it looks like a chair with a high sloped back. The flat end, coupled with the diagonal connection across the long grains improves the vertical or compression strength.
When used to extend skids, the nibbed scarf often has a stepped slope called a hook. The hook adds another opposing plain against which lateral pull forces can ‘hook’ on. The hooked and nibbed, or nibbed scarf may also have a key or notch cut across the matching sloped faces to allow a wedge to be driven in to lock the joint together.
For increased strength, apply construction adhesive to the cut faces and connect with 1/2″ or 5/8 inch through-bolts with washers or through the steel plates.
- Stronger than half-lap or butt joints
- Withstands twists and bends
- Strong vertical and lateral connection
- Ideal for skid or post extensions
- Measurements accuracy
- Cutting skill
- More difficult than the half lap
Post Cap/Base Brackets
There are different metal brackets and sleeves that can be used to reinforce a butt joint between two 4x4s. The 12 to 18-gauge galvanized, powder-coated, or stainless steel post cap/base brackets, such as those from Simpson Strong-Tie Double 2x4 Post Cap/Base, provide a strong connection. Use structural connector screws, 10d common, or 1-1/2” #9 SST screws to fasten the brackets to the 4x4s.
The length to be extended and its purpose determines if post ties or brackets should be used. Supporting a mailbox or light fence is more acceptable than using them to lengthen a post to support a roof or deck structure. The use of a bituminous membrane or paste between the wood and metal will help prevent moisture damage and corrosion of the metal.
- A quick and simple connection
- Some vertical strength
- Easy to install
- Weak lateral resistance
- Not aesthetically appealing
What Is the Strongest Way to Join 2 4X4 Wood Posts?
Joining 4x4s together to extend their length can be as easy as a butt joint or as complex and elaborate as a keyed, double tenoned scarf joint. However, some cuts are better done where the timbers can be rolled easily and joints tested for alignment and fit. When working on posts under load or limited space, simpler connections are better.
Our choice for the strongest and easiest way to join two 4×4 posts is a half-lap joint that is glued, plated, and bolted. Cutting a half lap into an existing fixed post is easier than all other cuts, except the butt joint. The two required cuts per post are simple and doable even with limited space.
A glued, plated and bolted nibbed tapered scarf joint is stronger than a half lap but not as easy to cut in limited space. Although it too has two cuts per 4×4, accurate sawing is more difficult. The skill level and aesthetics of the finished joint influence the choice of connection. A poorly sawn joint won’t make for a strong connection.
The best way to join two 4x4s to extend a post is with a half-lap. The simple cuts make it ideal for a beginner or pro to make, and it’s easier to do in limited space. Adding glue, plates, and bolts increases the lateral and twist resistance to form a very strong connection.
Hopefully, you have a better understanding of how to connect two 4x4s, and which joint will provide the strongest connection. If you found the guide helpful, please share it with others. As always, your comments and suggestions are appreciated.
Eugene has been a DIY enthusiast for most of his life and loves being creative while inspiring creativity in others. He is passionately interested in home improvement, renovation and woodworking.
To connect 2 4x4s use a half-lap joint secured with construction adhesive, two steel plates, and four to six 1/2″ or 5/8” through bolts. Posts that support roofs or platforms should lap between 20” and 24”, other structures should connect for 6” to 10” along the centerline.How do you join two fence posts together? ›
If the post is vertical you can use a half lap seam or joint . This is where you cut two pieces identical to each other in a stair step type cut, invert one , and join the two cuts then nail , screw , or bolt the two pieces together at the seam .How do you attach a 4x4 perpendicular? ›
Miter cut the horizontal 4x4s at 45 degrees each, so that when they join together they become perpendicular. Then attach those together on top of the vertical post with 1/4″ x 6″ Tiberlok screw fasteners, as well as attach each to the vertical post, in the same manner.How do I join timber post? ›
Cut matching notches that are half as deep as the thickness of the timber and the same length as each other into the ends of your 2 pieces of timber using a table saw or a circular saw. Apply wood glue to the notches, fit the 2 pieces of timber together like a puzzle, and clamp them tightly until the glue dries.How do you secure a 4x4 post in the ground? ›
- Dig post hole so diameter of the hole is 3 times the width of the post (i.e., the hole for a 4” wood post should be about 12 inches wide). ...
- Add about 6 inches of QUIKRETE All-Purpose Gravel into the bottom of the hole. ...
- Set the post into the hole and attach 2x4 braces to adjacent sides of the post.
|Depth||50 Lb Bags|
The standard spacing for fence posts is 8 to 25 feet apart, depending on the type of fence. A standard wood fence would require posts 8 feet apart, while a high-tensile wire could span 25 feet.How do I anchor a 4x4 post without concrete? ›
Gravel: Filling the hole with gravel will help to support the fence post. This method is stronger than using dirt, but not as strong as using cement. Sand: Filling the hole with sand will help to support the fence post. Like gravel, this method is stronger than using dirt, but not as strong as using cement.How do I connect to my frame? ›
- Ask the owner of the frame to send you a code (Note: Friend codes expire after 12 hours. ...
- Open the Frameo app on your phone/tablet and press the “add friend”-icon in the top.
- Enter the friend code from the frame and press “Okay”.
- You are now permanently connected to the frame!
A butt joint is the easiest of all simple wood joints but also is the weakest. The cut end of one board butts-up against the edge of another piece at a right angle. The key to every type of wood joint is having smooth, square cuts on the boards, and the butt joint is no exception.
Using PVA glue for wood
PVA wood adhesive is specially formulated to penetrate wood fibers, resulting in glue bonds that can be even stronger than the wood itself. PVA glues create durable joints and keeps wood looking great.
There are a number of reasons why a wooden fence post may start to rot, including:
Fill the first three inches up with gravel so the end of the post doesn't come into contact with the dirt. Gravel allows water to drain quickly away from the post and into the soil. Be sure to place the post in the center of the hole. Finally, fill the entire hole up with cement to the top.Is it better to use gravel or cement for fence posts? ›
Concrete is widely considered the most secure material for setting fence posts, and the only thing that will truly keep your fence posts firmly in place. However, there are some who swear by gravel in specific circumstances. Gravel is less expensive and allows drainage that helps to prevent the rotting of buried wood.How are timber frames joined? ›
Joinery is what ties timbers together, in traditional timber framing. The ends of timbers are carved out so that they fit together like puzzle pieces. A hole about an inch in diameter is drilled right through the joint, and a wooden peg is pounded in to hold the joint together.Do you put fencing on the inside or outside of the post? ›
Fence posts should be on the outside of the fence
Fence posts that have been placed outside of the fence material will take nearly all of the animal's force. That's because they are better suited to withstand the weight of an animal than the staples used to hold the fencing in place.
You will have to figure how many boards you need to purchase to 'rip' your custom pickets. It is recommended that you use a minimum of 2 horizontal rails between posts for all fences less than 5' high, 3 rails for fences 5' to 7' high, and 4 rails for 8' high fences.How are fence rails attached when they are installed between posts? ›
The most common – and easiest – methods are to either nail or screw the rails directly to the post using rust-free fasteners or to use galvanized fence brackets. A fence bracket is an open U-shape that holds the two-by-four rail between the posts.Should you notch a 4x4 post? ›
We have seen many decks being built with guardrails that use notched 4x4's as rail posts. Regardless of the depth of the notch this is unadvisable because it significantly weakens the strength properties of the posts. Checks or cracks may propagate along the grain lines at the notch as the lumber dries and shrinks.Why do you put gravel in the bottom of a post hole? ›
If a fence post fails without any sign of a pest infestation, it's likely that the failure was caused by moisture that rotted the wood over time. To help slow such deterioration, add pea gravel or crushed stone to the bottom of the posthole.
Setting Fence Posts in Concrete
Concrete is the most secure material for setting fence posts, especially if you have sandy soil. Gravel may be okay with dense, clay-heavy soil, but in looser soil, concrete is the only thing that will truly keep your fence posts stuck in place.
The diameter of your post hole should be three times the diameter of your post. So, if you're planning on using a four-inch round or 4x4-inch square post, your post hole will need to be 12 inches in diameter. For a six-foot-high fence post, we would need a hole that's 36 inches deep and 12 inches in diameter.How long will untreated 4x4 post last? ›
If properly treated and installed, pine fence posts can last for 20–35 years; untreated pine posts might only last 3–7 years.What is the best cement for fence posts? ›
The best concrete mix for fence posts I've found is Quikrete Fast Setting Concrete Mix in a 50 lb bag. It's 4000 psi, easy to work with, sets up fast, and doesn't need to be mixed. Just pour it in the hole and soak with water.How high can a 4x4 post support? ›
A 4×4 horizontally spanning 8-feet can typically carry 500lbs safely in the center, and 1,000lbs spread evenly across the span before sagging excessively. Properly supported at 16” centers, and it can support up to 4,000lbs.What is the center of a 4x4 post? ›
The inside dimensions of the standard 4x4 post top measure 3-5/8", and the inside dimensions of the standard 6x6 measure 5-5/8".Should every fence post be set in concrete? ›
It doesn't matter if it is a do-it-yourself (DIY) project or a professional installation, wood fence or vinyl fence, your fence posts should be set in concrete. Without a properly set concrete footer, your fence posts will begin to sag until they eventually fail.How do you connect 2 4x4 posts? ›
To connect 2 4x4s use a half-lap joint secured with construction adhesive, two steel plates, and four to six 1/2″ or 5/8” through bolts. Posts that support roofs or platforms should lap between 20” and 24”, other structures should connect for 6” to 10” along the centerline.Is foam better than concrete for fence posts? ›
Concrete is more durable and can support load-bearing posts. Expanding foam works best for lightweight vinyl or mesh fences. Concrete takes longer to set and cure. Expanding foam is more expensive than concrete.Can you set posts without mixing concrete? ›
Fast-setting concrete is ideal for setting posts because there's no mixing—you simply pour the dry concrete from the bag right into the hole, then add water.
- Wet Rot Vs. Dry Rot. ...
- Choose Rot Resistant Wood. ...
- Dry the Fence Posts Out. ...
- Consider Adding Posts to Concrete. ...
- Apply Preservative to the Wooden Post. ...
- Consider Staining Your Wood. ...
- Start Digging. ...
- Fill Up the Hole.
There are three different ways to extend your wooden fence post. The two easy ways are adding metal fence post extender fastener to both posts on the front and back or adding a wood face board on the front and back of both posts.How do you stabilize a 4x4 post? ›
Stabilize Your Fence Post
You'll first want to dig a few inches of soil out from around the post's perimeter. From there, you can fill the hole with gravel and top the gravel with a rapid-set cement mix. Finally, pound the fence post back into place using a mallet.
DO Employ a Base Gravel Layer. If a fence post fails without any sign of a pest infestation, it's likely that the failure was caused by moisture that rotted the wood over time. To help slow such deterioration, add pea gravel or crushed stone to the bottom of the posthole.What do you coat the bottom of a fence post with? ›
Use Wood Preservative
Once your wood is dry, be prepared to brush the bottom half or third of the fence post with multiple coats of copper naphthenate, a wood preservative-free from chromium and arsenic.
Fill the first three inches up with gravel so the end of the post doesn't come into contact with the dirt. Gravel allows water to drain quickly away from the post and into the soil. Be sure to place the post in the center of the hole. Finally, fill the entire hole up with cement to the top.Should I seal the bottom of my fence post? ›
If you can dig down around the post when you are doing your normal fence seal and stain maintenance making sure you seal out moisture all the way to the concrete, your fence post will last longer than they would without the extra layer of moisture protection.Does expanding foam for fence posts work? ›
However, expanding foam is much easier to use, and since it can expand, you can virtually secure it to any type of elements found in the ground for the hole of the post. Both expanding foam and concrete will work great for installing your fence posts, but the expanding foam is definitely more user-friendly!What is a post extender? ›
Raises the height of an existing post.