Cross lap joints–or “overlapping” joints–are used to create beautiful, continuous lines in wooden structures and furniture. This article will show you how to cut clean cross lap joints, especially in thicker lumber where typical woodworking tools won’t quite do the trick.
(The cross laps in this example were used to build a custom cedar arbor for a wedding ceremony. I documented the full build in a short ebook. Check it out in the “ebooks” tab if you want to tackle a fun, simple woodworking project!)
16 oz. Hammer
(Tools and materials are linked below strictly for informational purposes–no marketing revenue is generated through these links.)
To cut cross lap joints in 4×4 cedar posts like the ones in this article, it’s easiest to use a 7-1/4″ circular saw. The circular saw blade has enough clearance to make deep cross laps in the thicker wood. (For years now I’ve trusted Makita for their outstanding circular saws.)
Woodworkers will often use electric routers to make similar cross laps in thinner lumber. I’ll describe that process very briefly at the end of the article as well. Keep in mind, routers often won’t have the reach for lumber this thick. But, when applicable, they clear material with smooth precision. (I like my Bosch routers.)
Step 1: Measure And Mark For Cross Laps
The first step in cutting cross lap joints is to very accurately lay out their location. The beams in this wedding arbor need to overlap 6″ away from the ends of the framing members. So, to begin with, use a square to mark straight lines 6″ away from the beam ends.
You can then use a scrap piece of the 4×4 material to mark the other side of the joint. Using the actual material for reference, rather than relying on measurements, will often result in creating tight joints more effortlessly.
Just set a cut-off chunk of the 4×4 material on the piece you intend to mark. Sighting down on the chunk from above, line up one side with the pencil mark you just made. You can then use the opposite side as a guide to create a new pencil mark.
You can check these lines again afterwards to make sure that they are accurate. You want to just be able to barely see them while sighting down the edge of the cut-off piece from directly above. Use a very sharp pencil for more accurate marks.
Step 2: Set Saw Depth
In order to function properly, cross laps need to be exactly half the depth of the material you’re working with. This way, when you couple them, they will nestle together on a parallel line.
The 4×4 cedar posts in this article are actually 3-1/2″x3-1/2″ thick. So, the half-depth of the posts is 1-3/4″.
To set your saw depth, just unplug the saw, turn it on its side, and swivel the sole plate to its full depth (after unlocking it). Butt your tape measure to the underside of the sole plate, draw it out, and measure to the furthest reach of the blade teeth at the center of the tool.
Now, swivel the sole plate downwards until it is 1-3/4″ away from the tip of the center blade tooth (like in the picture above). Set the depth by locking the sole plate again at this position.
Step 3: Cut Cross Lap Perimeters
The most important cuts in this process are the outermost ones–the “perimeters.” These cuts will create the boundaries of the cross lap, so you want them to be tight and accurate.
To ensure squareness, use a speed square as a straight edge to guide your saw.
Set the saw on your lumber and line up the outer edge of the blade with the inner edge of the cut. Now, butt your speed square up to the sole plate at this position.
As you begin to cut, the speed square will hold the saw perfectly perpendicular to the material. Just be sure to clamp the speed square tightly with your free hand, and push the saw slowly and smoothly.
Ideally, you want your cut to take half the thickness of your marked pencil line.
Repeat the process for the opposite perimeter. You can either sight along the inner edge of your saw blade this time, or move around to the far side of the piece and perform the cut the same way you did the first time.
Step 4: Cut Fillets
To deal with the excess wood between the perimeter cuts, you’ll want to make a numerous number of similar cuts through the material. These cuts will produce standing “fillets”–thin strips of wood that will be far easier to remove than a large block.
You don’t even necessarily need to use the speed square as a guide to do this. Just run the saw through the waste wood every 1/8″ or so. The linear profile of the blade will keep it running straight mostly on its own.
Just be sure to follow saw safety rules while doing this. Keep both hands on the tool, and stand slightly to the side of the cut, not behind it. Let the blade guard fall back down into place between each pass.
And be sure not to mess up your perimeter cuts! A wandering blade can nick the edge pretty easily.
Step 5: Break Fillets
This part is always really satisfying.
The standing fillets are very weak, especially since they are produced by cutting across the wood grain. You can break one out with your fingertips. But, it’s faster (and more fun) to break them all at once.
To do this, just use a 16 oz. hammer to tap all the fillets in one direction from the side. Strike as close to the perimeter cut as possible. The fillets will force one another down in a domino fashion, resulting in a loud crack.
Just scoop the waste wood out with your hand! (You can snap out any remaining pieces with a flathead screwdriver.)
Step 6: Clean Cross Lap
The floor of the cut will still be very ragged, with some fillet bases standing higher than others. This will almost always happen in areas where the wood grain was slightly harder and more dense, or perhaps where a knot was located.
To begin cleaning out the floor of the cut, use a hammer and chisel to knock out any larger sections. You can cut towards the base of the fillet from the side using short, controlled taps with the chisel bevel facing down.
When the highest areas are knocked out, begin taking the floor to a smooth, even depth by turning the chisel bevel up and passing it back and forth through the cut field in a razing motion.
Keep the chisel flat on its back while doing this to prevent gouging. If your chisel is sharp, it will push right through the raised wood fibers, leaving an increasingly smooth surface at the bottom of the cross lap.
(FYI: the vertical walls of the cut are known as “cheeks” in woodworking.)
Step 7: Check For Fit
When the floor of the cut is smooth, use your block to test the cross lap for fit. If the joint still feels too tight, you can widen the cross lap just a bit by making one more pass on the cheek with a circular saw.
Just be sure to use your square as a guide again–and only take half a blade-width at a time! It’s very easy to overdo it and make the cross lap too wide, which is an irreversible mistake.
Step 8: Repeat Steps For Overlapping Section
If the fit on the first piece is good, you can now repeat the process for the overlapping piece.
Just be sure to keep in mind which face of the new beam the cross lap is located on. In this project, the decorative lifts at the ends of the beams were meant to point downwards. So, the cross lap was on the topside of the bottom piece, and the underside of the top piece.
If done correctly, a cross lap will create the illusion of two pieces that just pass magically through one another. This continuous sightline has a classic look, and it will really add an element of style to your structure!
Also, as I mentioned before, this entire process can be carried out on thinner lumber by simply using a router.
The process is fairly similar: use a square to mark your perimeters, set the router bit to half-depth, and then use the square to guide the sole plate of the router as it passes through the material.
If the cut is somewhat deep, or the wood especially hard, you may need to make multiple passes at increasing depths, which will probably require a router table instead of a handheld router. Or, you can also make these cuts on a table saw with stacked dado blades.
For some good tips on that process, check out this article on half-lap joints from Wood magazine.
Also, here’s a good video on carrying out the process on thin lumber with a circular saw…
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