A half lap joint may not be the prettiest joint in woodworking, but that doesn’t make it any less useful. These simple overlapping joints are extraordinarily strong in their ability to resist shearing forces, making them useful for all manner of interior and exterior applications.
The joint is essentially two pieces of material cut to half of their thickness, and then laying them on one another so they over ‘lap’ each other. The joint is then fastened with wood glue, screws, or lag bolts.
They can be made in several different ways – using a minimal number of tools – with each method having its respective advantages and disadvantages.
Let’s take a look at these various methods of how to make a half lap joint step by step.
Tools & Materials
- Circular saw
- 1” wood chisel
- Speed square
- Rough grit sandpaper
- Reciprocating saw or hand saw (optional)
Making a Half Lap Joint – Guide
Method 1: Cut Dadoes with Circular Saw
This method requires you to make numerous “fillet” cuts in the material, and then clean up the joint with a hammer and sharp chisel. It’s probably the easiest method of the three, so it’s a good choice if you’re not that experienced using a circular saw.
Step 1: Mark and Measure Half Laps
The first step is marking and measuring the half laps on both workpieces. To ensure you get an accurate measurement on both surfaces, you can clamp both pieces together and mark your cut line on both pieces at the same time.
As the old adage goes make sure to measure twice and cut once. Line up both workpieces and use a sharp pencil or marker along with a speed square to get a straight, clean line.
Step 2: Set Cutting Depth on Circular Saw
The key to proper fitting half-lap joints is removing exactly half the material from each piece. This will ensure both pieces will sit flush when attached.
Setting the right depth on your circular saw will allow you to accomplish this with ease. The posts I’m cutting in this article are exactly 4” x 4”, so I set the saw cutting depth to 2”.
Unplug the saw and loosen the depth-adjusting lever or knob and pivot the bottom plate until the measurement reads exactly half the depth of your workpiece. Double-check this by using a tape measure to read the length from the shoe to the middle of the blade.
Step 3: Make Initial Cut
The most important cuts when using this method are the outermost ones. These cuts create the outer edge of the joints – so they fit together perfectly.
Use a speed square as a guide to ensure you get a straight, accurate cut. Line up your saw blade with the line you drew in the previous step.
Then, hold the square tightly in place with one hand, and run the saw along its edge to get a perfectly perpendicular cut.
Step 4: Make a Series of Cuts to Hollow out Material
After you’ve made your initial perimeter cuts on each piece, you’ll need to make a series of cuts to form the dado. These cuts don’t need to be particularly accurate or straight but shoot for about 1/8 to 1/4-inch between cuts.
As you’ve already set the cutting depth on your saw, each cut will be at the same depth. As you cut, make sure your saw’s shoe is flat against the workpiece.
Step 5: Clean out Material with Hammer and Chisel
Use your hammer to knock out the material you hollowed out previously. This is actually really satisfying! It takes just the lightest touch to knock out the wood, so be gentle.
When you’ve knocked out as much material as possible with the hammer, it’s time to clean up the rest with a sharp chisel. Use the chisel to scrape away any bits of wood that are still attached, while being careful not to gouge deeper than necessary.
Then, clean up any remaining surface with a wood rasp or a piece of rough grit sandpaper.
Method 2: Power Saw Method
This method is a quick and effective method of making half-lap joints in a minimal amount of time. It’s a little rougher than the previous method and requires you to be comfortable with using power saws.
Step 1: Measure and Mark Half Lap Joints
The first step is to measure and mark the cuts for the half lap joint. Use a speed square and sharp pencil to mark the cut lines on the top and sides of the workpiece.
Made sure to double-check the cut lines for accuracy before you make any cuts.
Step 2: Set the Cutting Depth and make Initial Cuts
Set the cutting depth on your circular saw to exactly half the depth of the workpiece. Then make the first perpendicular cut across the piece. Use a speed square in one hand to guide your cut for more accuracy.
Step 3: Make Side Cuts
Next, you’ll need to make the two side cuts in the material to form the bottom of the half-lap joint. Depending on the thickness of the material, you may need to cut from both sides of the piece to get all the way through.
In this case, the post is 4” x 4”, so I adjusted the cutting depth to the maximum. Then cut along the cut line until the saw blade almost meets the first cut. Flip the piece over and make the same cut on the other side.
Step 4: Finish Cuts with Reciprocating Saw
Due to the blade shape of circular saws, the side cuts made previously won’t pass all the way through the material. This means you’ll be left with some excess material that needs to be cut through to complete the joint.
Use a reciprocating saw with a wood blade to finish off the cuts. Take care not to cut too far into the wood from either angle.
Method 3: Hybrid Method
This method is a slight variation from the previous method. Instead of using a reciprocating saw to finish off the cuts, use a hand saw to make the final cuts with the grain.
Using a hand saw instead of a reciprocating saw will result in a smoother final product with fewer chances for screw-ups. Reciprocating saws are a little like untamed beasts – they have a mind of their own and can cut more than you might think.