Furniture made from slabs of wood with natural edges still intact has a soulful, sculptural quality, thanks largely to the handiwork of Mother Nature. And creating projects with waterfall joints—a miter joint with the continuous-grain appearance of water going over a falls—only adds to the beauty.
But cutting tight-fitting, precise miters on stock with no straight reference edges can be tricky. Thankfully, this technique helps you achieve that waterfall look, using tools found in most shops. (This also works with more typical straight-edged lumber too unwieldy for a tablesaw.)
Flatten and stabilize the slab
Your natural-edge slab will likely have rough-sawn faces and bark-covered edges. In order to create perfect-fitting miters, though, both faces must be flat and parallel. You can achieve this by running the slab through a wide planer or drum sander, but if that's not an option, use a router and shop-made jig. This requires patience as you make repeated shallow passes across the width and along the length of the slab, but it's effective. When finished, sand the slab smooth with 60- or 80-grit abrasive.
Next, evaluate any splits, knots, bark inclusions, voids, or other defects in the slab. Filling or repairing them ensures long-term stability, but keeping everything as natural as possible usually looks best. Splits can open up more as the slab adjusts to a new environment's humidity, so stabilize these with bow-tie keys, as shown below. Before adding these to your slab, though, determine the location of the miter joint and mark it. You don't want to later cut the miter through a bow tie you painstakingly installed.
To highlight the bow-tie keys, use a contrasting wood species. Make them 1⁄2 " thick and visually pleasing in length and width. Graduated sizes add even more flair. Adhere the bow ties to the slab with double-faced tape, and trace around them with a marking knife. Identify each bow tie and matching mortise so you can pair them up later.
Take the plunge. Remove the bow ties and tape, and trace each knife line with a sharp pencil to increase its visibility for the next step. Using a plunge router and 1⁄4 " upcut spiral bit, rout the mortise for each bow tie in incremental depths, keeping about 1⁄16 " inside the marked outline. Rout to a depth slightly less than the thickness of your bow ties: in this case, 7⁄16 ".
Use a chisel—as wide as will fit—to pare away the remaining waste, seating the chisel in the knife-scored outline as you go and cutting perpendicular to the slab surface as you work around the perimeter. Use a small square to check your progress: If the walls are not perpendicular, the bow tie will not seat in the bottom. Check each bow tie's fit as you go, but avoid inserting it too deep to remove.
Glue each bow tie into its mortise using two-part clear epoxy, which will fill any small voids. Allow 24 hours for the epoxy to cure, and then trim each bow tie flush to the slab surface. We used a plunge router with a flat-bottom dado clean-out bit, and elevated the router base by taping on a pair of 1⁄8 "-thick runners so the router could pass over the bow tie. Sand smooth when finished.
Prep the slab's edges
The natural edges grab the spotlight (and deservedly so), so treat them like the stars of the show. Although it's tempting to leave the bark on, don't. It eventually will fall off, in pieces, so get rid of it now. If it doesn't peel off easily, chip or slice it away with rounded chisels (sharp corners will gouge the wood), putty knives, and wire brushes. You might find hidden surprises beneath the bark, such as small burrs, ripply edges, or insect tunnels; leave these intact, if practical, because they add character.
With the bark removed, treat the edges to suit your taste. If you prefer a slightly bumpy texture, leave the stringy cambium fibers and bristly points. For a smoother edge, sand away the roughness with abrasive flapwheels of sequential grit.
Miter the waterfall joint
Because you won't be able to use a tablesaw, radial-arm saw, or sliding mitersaw to cut the miters, we recommend using a portable circular saw equipped with a 60-tooth blade and a simple straightedge jig, as shown below.
Set the circ saw for a 90° full-depth cut, and trim one edge of the jig to provide zero-clearance support. Position that edge on the cutline, clamping it as close to perpendicular to the slab's edges as you can by eyeballing it. Then cut across the slab.
Make the jig from 1⁄4 " hardboard or plywood about 10" longer than your slab's width. The jig should be about 10" wide, with a 3⁄4 "-tall centered fence attached. Using this setup, and following the photos below, cut mating miters for each joint.
If your saw cannot cut through the slab's full thickness, finish the cut with a handsaw.
With the slab now in two pieces, tilt the saw blade to 45° and trim the other edge of the jig for zero-clearance support. Reposition the jig on the slab's top face with the mitered edge aligned with the tip of the just-cut end of the slab, clamp it in place, and make a miter cut. Finish with a handsaw, if needed. Repeat for the other slab section.
To smooth the hand-sawn area to match the circ-sawn area, attach a simple subbase to a trim router. Install any flat-bottom bit with the cutter extending beyond the subbase's edge. Adjust the bit depth so it just grazes the circ-saw cut face. Then, with the subbase resting on the smooth circ-sawn surface, rout back and forth to smooth and flatten the handsawn portion of the miter. A backer board protects the heel of the miter from chip-out as you rout the final pass. Repeat for the miter on the other slab section.
Call for reinforcements
Regardless of whether you're making a table, desk, or bench, it's crucial to strengthen the miter joint beyond just glue. Do this by concealing a loose tenon or two in the joint, as shown below. This also helps align the joint. For extra support, add a piece of angle iron (or similar steel bracket) to the bottom face.
Our slab was too short to make waterfall-joint legs on both ends, so we opted instead for one black-painted welded-steel leg [Sources]. Screws through slotted holes in this leg bracket allow for seasonal wood movement.
After assembling your project, finish-sand the faces to 220 grit. Be careful to not oversand along the miter joint and compromise the waterfall effect. (If your miter has a slight gap, gently rub a smooth screwdriver shank or burnishing tool over the joint to close it up.) Complete the project with a finish that highlights the natural-edge slab. We used two coats of an oil-varnish mix to bring out the figure in this redwood.
Using a plunge router and 1⁄2 " upcut spiral bit, set your router's edge guide to roughly center the bit along the miter, and then rout a stopped mortise in 1⁄4 "-deep increments. We routed 11⁄4 "-deep mortises. Repeat for the other slab segment.
Make loose tenons from 1⁄2 "-thick stock, with the grain aligned with the slabs' grain. This allows the tenon to expand and shrink with the slab even after being glued in place, preventing splitting. We used two tenons with a 1⁄4 " gap between them, rather than one long tenon, to allow more room for movement.
To hide the steel support, recess it into the slabs' bottom faces. Cut the support to length, and, with the two slabs dry-assembled, lay out the mortise locations. Use a plunge router and straightedge guide to mortise each piece. Attach the support with screws that fit loosely in the support's holes (to allow for expansion and contraction); widen the holes if needed. When satisfied with the fit of the miter, disassemble all parts, add glue, reassemble, clamp, and reattach the support with screws.
Natural-edge slab: Jewell Hardwoods, Clackamas, Oregon, 503-785-3935, jewellhardwoods.com.
Welded-steel leg: Factor Fabrication, Des Moines, Iowa, 515-635-1942, factorfabrication.com.