Building a Coffered Ceiling, continued

Finish Work

We start with the beam bottoms, which are made from 3/4-inch stock ripped 1/16 inch wider than the plywood box. This ensures a tight joint where the sides overlap (Figure 3). Although we try to avoid splices, large rooms can make joints along the beam's bottom unavoidable. We locate them out of sight from the main entry and use biscuits and yellow glue to create tight joints.

Figure 3.A Bosch miter finder helps achieve a perfect joint at beam intersections (top), while biscuits help prevent the joints from moving over time (center). The carpenter uses a scrap of the side material to help center the bottom piece (bottom).

The beams shown here are cherry, and we prefinished them in the shop with several coats of tung oil. We try to minimize nail holes on site, nailing about every 20 inches with 16-gauge finish nails. We fill the holes with soft, rub-in-type putty.

Once the bottoms are covered, we install the side pieces. Routing a bead or chamfer along the bottom edge creates an interesting shadow line and makes the crown molding appear larger. We miter and glue the corner joints and fasten with trim nails (Figure 4).

Figure 4.To avoid wasting stock, the side pieces are only as wide as they need to be (left); the crown molding covers the rest (right).

Installing the Crown

Putting up the crown takes more time than any other step, but a couple of tricks keep things moving. Back in the shop, we rough-cut most of the crown pieces to make handling easier. Sometimes we even pre-cope one end, so the field carpenters can quickly cut the piece to length and nail it up. With two carpenters on the job, one can install the pieces while the other cuts to length and copes when necessary.

Because the carpenter doing the nailing works from stilts, we'll sometimes stock the material on temporary cleats to make it accessible (Figure 5).

Figure 5.Storing the beam stock on high cleats keeps it in easy reach of a carpenter working from stilts.

We install the longest perimeter crown pieces first. Then we move to the individual coffers, starting with one of the shorter sides, where we install a piece with two square cuts. The next piece, on one long side, is coped to the first piece on one end and cut square at the other, as is the third piece, on the opposite long side.

The final piece, coped at both ends, winds up on a short side, which minimizes waste if we make mistakes while coping.

For accurate measuring, we use an improvised tool -- a telescopic leg from a camera tripod. By extending it inside the coffer, we can make accurate measurements on both long and short points (Figure 6).

Figure 6.A telescoping leg from a camera tripod serves as an accurate inside-dimension measuring tool. The leg is locked into place, then handed down to the carpenter making the cuts.