Large timber semi-circles look impressive, but once you break them down, they’re straightforward to build. The key is accurate cutting and a calm glue-up.
Let’s say you need a semi-circle with a bottom width of 1500mm. Here’s how to approach it.
Step 1: Work out the basics
If the width across the bottom is 1500mm, that’s your diameter.
Diameter: 1500mm
Radius: 750mm
From there, you decide how many pieces (segments) you want to use.
More segments = smoother curve
Fewer segments = quicker build
For something this size, 12 segments for a full circle (so 6 for a semi-circle) works well.
Step 2: Set your mitre angle
For a 12-segment full circle:
360° ÷ 12 = 30° per segment
Each joint is cut at half that
So set your saw to 15°
That gives you 6 pieces to form your semi-circle.
If you want a smoother finish straight off the saw, go for 16 segments (8 per half):
360° ÷ 16 = 22.5°
Mitre angle = 11.25°
It’s more cutting, but less sanding later.
Step 3: Cutting your pieces
In theory, you can calculate exact lengths using circumference maths. In practice, it’s better to:
Cut your mitres accurately
Leave your pieces slightly over length
Dry assemble
Trim to suit
Timber isn’t perfect and saws aren’t either. Dry fitting saves headaches.
Step 4: Gluing up without stress
Don’t try to glue all six joints in one go, especially if you’re on your own.
Instead:
Glue two or three segments together
Let them cure
Join the larger sections
Use:
Good quality PVA or PU glue
Band clamps or ratchet straps
Cauls to keep faces flush
If alignment is critical, biscuits or dominos help, but tight mitres and proper clamping are usually enough.
Step 5: Keep it flat
Segmented arcs love to twist.
Glue up on:
A flat bench
Or a sheet of MDF
Apply pressure downward as well as inward if needed.
Step 6: Fairing the curve
Even with accurate mitres, you’ll need to smooth the curve.
Best approach:
Belt sander to remove high spots
Long sanding block to even it out
Router with a trammel jig if you want a perfect radius
If it needs to fit into something, leave it slightly oversized and trim to final size once fully cured.
Timber choice matters
If it’s being painted, laminated MDF around a former can be quicker and more stable.
If it’s exposed timber:
Use straight grain
Avoid knots near mitres
Think about seasonal movement
Large arcs exaggerate movement, especially across the grain.
Final thoughts
This type of job isn’t difficult, it just rewards accuracy. Get your mitres spot on, take your time with glue-up, and the sanding will be minimal.
If you run a joinery shop, this sort of feature piece is a great way to show proper craftsmanship. Clean joints, smooth curve, no filler — that’s what people notice.