Although my company specializes in high-end jobs with custom
trim, it's not often that clients request arched passageways.
When they do, I use a simple approach that allows me to
efficiently tailor the details to any size opening.
The trim detail I typically use is not a true ellipse; instead,
it's a modified form that starts as a tight radius (almost
always 12 inches) and blends into a much greater radius at the
top, then back into the same tight radius on the other side
(1). Guiding the casing stock through my Woodmaster profile
molder (800/821-6651, www.woodmastertools.com) is much easier to
do with two radiuses than with a true ellipse.
For doorways wider than 4 feet, I match the upper radius to the
width of the R.O., which results in an arch with plenty of
spring height. For narrower openings, like the 44-inch R.O.
shown here, I use a tighter upper radius to give the arch more
height. For this opening, I used 34 inches.
Each of the side jambs ends up 1 1/2 inches thick, so to leave
room for shims, I made the inside arch width 40 inches.
I lay out the arch on a scrap of sheathing, first marking its
40-inch width, then swinging a 12-inch-radius arc through each
of the two end points (2). I use an extension arm screwed to
the scrap of sheathing, and trammel points to swing the
34-inch-diameter upper arc (3).
Bending the Trim
After laying out the arch, I fasten 6-inch-tall metal
L-brackets to the plywood, spacing them about 6 inches apart
and holding them 1/8 inch off the layout line (4). This allows
room for the 1/8-inch-thick flat metal that I use to provide a
continuous clamping surface between the brackets. The last part
of the form is a second piece of flat metal, which goes around
the outside of the strips to spread out the clamping pressure
and keep the wood fibers from breaking during the bending
Because I almost always begin and end my "elliptical" arches
with a 12-inch radius, I had a couple of clamping fixtures
fabricated to make it easier to start and finish my forms. But
simply using L-brackets the whole way works, too.
The glue-up shown here measures 4 1/2 inches tall by 1 3/16
inch thick, and is made of five layers of wood. It's wide
enough that I can get a 2 1/4-inch-wide arch-top "stile" and a
length of panel molding from one setup.
To get a tight glue-up with uniform thickness, I resaw
1/4-inch-thick strips of poplar on my band saw, then lightly
plane both faces of the strips at least once. Most of the time,
I'm resawing 5/4 stock, and can get three strips from one
Glue-up. I use a glue roller to
spread yellow glue on both faces of each strip, which helps
ensure even coverage (5). After the strips are stacked, a few
wraps of 5-inch-wide shrink-wrap plastic around the ends keeps
things under control (6).
Depending on the temperature and humidity, I have between 10
and 15 minutes to finish clamping before the glue begins to set
up, so I use an impact driver to tighten the clamps (7). I
start in the middle and work toward each end, where I leave
several inches of extra material.
While the glue-up is still in the form, I mark the point where
the curved trim crosses the long layout line that indicates the
base of the arch. Later on, I will transfer this mark to the
back of the glue-up with a chisel so it doesn't get lost during
sanding or routing. When I cut the trim, the mark will tell me
exactly where the joint needs to be.
I try to wait at least three hours before unclamping, to let
the glue cure. I know from experience that it's better not to
Milling. I make indexing marks on the
back of the glue-up before ripping, which helps me keep track
of the pieces and mill them in such a way that finished parts
are installed next to each other, just the way they were glued
up. Then I use a handheld planer and a benchtop planer to clean
up one straight edge before ripping the curved stock to width
I cut the panel molding profile with a router, making a couple
of light passes before the final cut. All the excess sawdust
has to be removed before I make the final pass or it'll get in
the way of the bearing and make bumps in the molding.
To help keep the router steady — since it's sitting on
just a 1 3/16-inch edge — I add a block of wood to its
base (9). This "training wheel" looks a little funny but saves
a lot of sanding time.
Assembling the Arch
I cut four 2 1/4-inch-by-8-inch poplar rails for the paneled
arch and make a pair of pocket-screw holes at each end (10).
Two of the rails act as temporary spacers at the ends of the
curved stiles. The other two divide the top panel into three
sections, and are typically positioned 2 or 3 inches above the
point where the 12-inch radius blends into the top radius. When
I first install the rails, I turn them so the pocket screws are
on the finished side, since they will be unscrewed and removed
later on before permanent installation.
To keep the bottom of the stiles at the correct width as the
frame goes together, I also install a couple of 40-inch-long
spacers at the base.
For the curved panel itself, I rip two 11 7/8-inch-wide strips
of untempered 1/8-inch-thick hardboard. These lengths of
hardboard get glue-laminated together, and are cut long enough
to run past the end of the curved stiles by about 2 inches.
Later, when I attach the two side panels, staples through this
extra material will help fasten the sections together.
Because hardboard is so flexible, ripping it is a two-man job,
and so is stapling it in place. It's especially important to
get started straight, or you'll end up with a real mess at the
other end. Since regular trim staples would shoot right through
the hardboard, I use a Grex upholstery stapler, a lot of
staples, and a healthy bead of glue on the stiles (11).
Building the Side Jambs
Whenever I'm making paneled jambs, I like to match the layout
to the doors in the house. On a house like this one, with
10-foot-tall ceilings, there's plenty of room for an arched
top, so I make the side jambs 7 feet tall to match the doors.
The finished width of the paneled jambs is 12 1/2 inches (two
2x6 walls with a 1/2-inch space between and a layer of 1/2-inch
drywall on each face).
I pocket-screw the frames together (12), which gives me great
joints that require very little sanding (plus I don't have to
wait for glue to dry). If there's room, I add 3/4-inch-thick
plywood to the back of the frame (13), which stiffens the
assembly and provides good nailing.
To finish the side jambs, I install panel molding, fastening it
in place with a .23-gauge headless pinner (14). When I fit
panel molding, I use a block plane to put a slight underbevel
on each miter to make sure the face of the joint comes
together. The short pieces go in first; then I cut the long
pieces tight so I can bow them a little in the middle and pop
them in place.
At this point I fasten the arched top to the side jambs,
spreading glue on the hardboard that extends past the base of
the arch (15) and using pocket screws to join the straight
stiles to the curved stiles (16). With the screws pulling
everything together, I can clamp and staple the extra length of
hardboard into the back of the top rail on the straight frame
(17). Normally, pocket screws should not go into end grain, but
they seem to hold fine with the help of the hardboard.
Once both sides are fastened in place, I remove the two
temporary bottom rails from the arch. I also remove the two
upper rails — after marking their positions on the curved
stiles, because they'll be reinstalled. To hold the two
straight side jambs in alignment, I fasten another 40-inch
spreader across the bottom, at floor level.
Curved Panel Molding
Fitting joints in curved trim always involves an element of
trial and error. To make the process easier, I cut the curved
sections of panel molding from the length of trim made from the
same blank as the stile (18); this helps ensure that the curves
After installing a short length of straight molding across the
bottom of the arch (19), I cut and fit the first pair of miter
joints (20). When the bottom joints fit, I cut and fit the
upper pair of miter joints (21). Temporarily removing the rails
makes this method possible.
Once both bottom panels are trimmed out, I glue and nail one of
the rails back in place (22). I can no longer use the
pocket-screw holes, but the glue and headless pins hold the
Then I install the next section of panel molding, starting with
the lower short straight section and fitting as before. If all
goes well, by the time I have to cut and fit the last joints, I
have the angles and bevels down pat. If I get off a bit on the
last cuts, I can always make a small width adjustment on the
Curved Casing Trim
Measuring a little over a full inch in thickness, the curved
casing trim is made up of two layers of MDF glued together. I
do all the hard work of cutting the profile in the curved blank
with the help of my Woodmaster molder.
Casing blank. I use the curved jamb as a guide to lay out the
inside edge of the curved blank on a sheet of MDF; then I
rough-cut to the line with a jigsaw, and finish up the cut with
a flush trim bit mounted in a router (23). To create the
1/4-inch reveal on the casing, I make one pass with a 1/4-inch
rabbeting bit and a second pass with a flush trim bit
Next I use a compass to scribe a line 4 1/16 inches back from
the finished inside edge, and cut the outside edge to within a
fat 1/16 inch of the line with a jigsaw (25). A belt sander is
ideal for smoothing the outside curve down to the line. I use
this first curved blank as a pattern for the second blank, then
I glue and clamp the two together to achieve the thickness
needed for the molding profile (26). After the glue dries, I
clean up the edge of the second blank with a flush trim
Cutting the profile. I profile the
casing with the Woodmaster (27). This machine handles curved
and straight trim essentially the same way; I just need to make
sure that the wooden guides I clamp in place are positioned to
keep the curved blank centered (28).
Before running the blank through the molder, I mark each end of
the trim to indicate exactly where the straight lines on the
layout begin. Later, I'll use these references to mark the cut
lines. Because I have a lot of time invested in the curved
casing trim, I also do a full-scale drawing on the floor to see
how everything lines up before making the final cuts
I join the straight and curved sections of trim together (30)
with Hoffmann dovetail keys and glue (866/248-0100,
www.hoffmann-usa.com), then nail the whole
assembly to one side of the jamb before installation (31).
Having the trim in place helps to center and plumb the jamb
(32). Once the shims are in position, the temporary spacers can
be removed and the second side trimmed.
After installation, nearly all the joints need to be
finish-sanded. Although a random orbit sander works on the flat
joints, many of the moldings require hand-sanding. Experience
has taught me that even when I think I'm done sanding, I should
go back and check again after the first coat of primer.
Gary Striegler is a builder in Springdale,