How to Flatten a Live Edge Slab (and When to Let Us Do It)

Most live edge slabs are not flat when you buy them. Wide boards cup as they dry, long ones can twist, and the faces still carry saw marks from the mill. Before a slab can become a table it needs two flat, parallel faces. Here is how to do that in a home shop, and when it is worth bringing the slab to us instead.

Let the slab settle first

Wood keeps exchanging moisture with the air even after kiln drying. Edmonton's indoor air is very dry in winter, so bring the slab into the room or shop where it will be built and leave it for a week or two, stickered so air reaches both faces. Flattening a slab that is still moving means flattening it twice.

Find out how far out it is

Lay a long straightedge across the slab in several directions and look for light underneath. Two winding sticks, one at each end, show twist. Mark the high spots with chalk and measure the thinnest point. That tells you your finished thickness before you start. A cupped 2 inch (8/4) slab often finishes around 1 3/4 inches, which is why thicker slabs are a safer buy for tabletops.

The router sled method

Most slabs are too wide for a jointer or planer, so the usual home method is a router sled: a carriage that slides along two rails and holds a router above the slab. Because the rails are flat and parallel, the router cuts a flat plane no matter how the slab sits.

1. Build the base. Use a flat, rigid platform a little longer and wider than the slab, with a straight rail fixed along each long side. The rails must be parallel and in the same plane; check with your straightedge.
2. Shim the slab. Set the slab between the rails, rock it to find the high corners, and wedge shims under the low points until it sits solid. Hot glue or screw blocks to hold it. If it rocks, the cut will not be flat.
3. Set up the router. A large-diameter surfacing or spoilboard bit in a router with plenty of power works best. Set the depth so the bit just touches the highest point.
4. Take light passes. Remove about 1/16 inch or less per pass, overlapping each pass by about half the bit width. Work across the grain in steady strokes, then lower the bit and repeat until the whole face is cut.
5. Flip and repeat. Turn the slab over onto its new flat face, which no longer needs shims, and flatten the second side until the faces are parallel.
6. Sand out the ridges. A random orbit sander starting at a coarse grit removes the small steps between passes. Work up through the grits until scratches from the previous grit are gone.

Expect dust and a lot of it. Wear hearing and eye protection and a dust mask, and connect dust collection if you have it.

Deal with checks and voids

Flattening often opens up cracks you did not see. Stabilise open checks with butterfly keys so they do not grow, and fill voids, knots and bark inclusions with epoxy. Do the filling before final sanding, then flatten or sand the cured epoxy flush with the wood. Do not oil the slab before you pour, because epoxy does not bond well to an oiled surface.

When to let us do it

A router sled is a good project if you plan to make several slab pieces. For a single table it often costs more time than it saves. Consider bringing the slab to us when:

• The slab is wider or longer than any sled you can build or move.
• It is badly twisted and you would lose too much thickness by guessing.
• It is heavy enough that flipping it safely needs more than one person.
• You want to spend your shop time on the joinery, base and finish.

Our live edge slab flattening service is listed at a $220 minimum for slabs under 9 square feet and $23 per square foot for larger slabs. For example, a slab 20 inches wide and 8 feet long is about 13.3 square feet, or roughly $307 at the listed rate. Confirm your quote with the yard. We also offer sanding at $28 per square foot with a $200 minimum, and a finishing service if you want the piece ready to use.

How to book flattening

Call (780) 229-7681 with the slab length and widest width. If you bought the slab from us, give us the tag number on the listing. If the slab came from somewhere else, ask first whether we can take it. When it is done, we let you know and load it into your vehicle at the yard.

Next steps

Still choosing a slab? Read How to Choose a Live Edge Slab for a Dining Table or browse live edge slabs in Edmonton. Once it is flat, see Choosing a Finish for Hardwood Furniture. Kingma Lumber is at 4607 91 Ave NW, Edmonton, open seven days a week.