Temporary Timber Framework & Decentering

Arch Formwork & Centering Calculator

Calculate exact timber rib radii after lagging thickness deductions (R_rib = R_finished − t_lagging), lagging slat quantities, folding strike wedges, and wet masonry dead loads.

Lagging Deduction & Decentering Masonry Support
Intrados Span
in
Intrados Rise
in
in (3/4")
in
in (Strike Drop)
Standard Centering Presets: Formwork
Timber Centering Rib & Lagging
TIMBER RIB RADIUS R_rib
29.25 in
Deducted 3/4" Lagging
LAGGING SLAT COUNT Slats
37 slats
@ 1.5" Wide on perimeter
TIMBER RIB RISE H_rib
11.25 in
11 1/4" Peak Rise
EST. MASONRY LOAD Dead Load
320 lbs
Fresh wet masonry
FINISHED INTRADOS RADIUS 30.00 in (Finished)
TIMBER RIB SOFFIT ARC 54.45 in (Rib arc length)
STRIKE WEDGE CLEARANCE 1.50" Drop for Fast Striking
Masonry Strike Mechanism Simulator

Interactive Decentering Wedge Simulator

Adjust the decentering wedge position slider to simulate driving the folding hardwood wedges to smoothly lower the timber frame away from the cured masonry soffit.

100% Fully Wedged (Supporting Wet Masonry)
Formwork Frame Drop 0.00 inches Clearance below intrados
Load Transfer Status 100% on Timber Center Pre-strike support
Safe Form Removal Locked in Place Slide frame horizontally out
Field Layout Generator

Offset-Coordinate "Mark-Out" Table & Interactive Plotter

For large arches where the swing radius exceeds your compass or room length. Simply draw your baseline, measure out horizontal intervals from center (X), and mark vertical height (Y) to plot perfect curves directly on wood or wall.

Coordinate Stepping Points (Centerline Outward)
Based on current Span: 48" • Rise: 12" • Radius: 30.00"
Step Interval:
Pt # Dist from Center (X) Height (Y) Fraction Drop
💡 Tip: Symmetrical — use identical Y heights for both left and right sides.

Live Plywood Scribing Simulator

Point 1 of 13
Plotted Offset (X): 0.00" (Center Apex)
Height Y: 12.000" (12") Drop: 0.000"
3/4" Plywood Blank Baseline X-Axis (Springline) Y: 12"

🔨 Carpentry Technique: Tap finish brads at coordinate marks. Bend a flexible batten against the pins to scribe a smooth curve.

Carpentry & Masonry Equations

Formwork Geometry & Lagging Deduction Rules

Subtract the exact thickness of the flexible wood slats to ensure the finished masonry brick course matches the architectural drawing.

1. Timber Rib Deduction Math
Finished Radius: R_fin = (W² + 4H²) / (8H)
Timber Rib Radius: R_rib = R_fin − t_lagging
Timber Rib Rise: H_rib = H_fin − t_lagging
Lagging Slat Count: N = ceil(Arc_Length / w_slat)

Layout the trammel arm pencil at radius R_rib when drawing the cut line on the plywood rib blanks.

2. Decentering & Dead Load Rules
Wet Masonry Dead Load: W_dead = Volume × 120 lbs/ft³
Pair Wedge Taper: 1:6 to 1:8 Hardwood Slope
Minimum Strike Drop: 1.0" to 2.0" (25-50mm)
Striking Sequence: Loosen all wedges symmetrically

Never strike formwork from one side only; symmetrical decentering prevents lateral arch twisting.

Step-by-Step Worked Numeric Example

Suppose you are building timber centering for a brick fireplace opening with a finished span of W = 48.0 inches, finished rise of H = 12.0 inches, and 3/4" (0.75") plywood lagging slats.

Step 1: Finished Radius R_fin = (48² + 4×12²) / 96 R_fin = 30.00 inches
Step 2: Timber Rib Radius 30.00 − 0.75 = 29.25" 29 1/4" Rib Cut Radius
Step 3: Lagging Slats 55.85" / 1.5" = 37.2 38 slats @ 1-1/2" wide
Step 4: Folding Wedges 1.5" Drop Allowance ✓ Clean formwork release
Standard Architectural Cheat Sheet

Standard Arch Opening Sizes: Quick Reference Table

Pre-calculated dimensions, radius curves, arc lengths, and material recommendations for standard residential and commercial architectural openings.

← Swipe table horizontally →
Opening Purpose Span (W) Rise (H) [Seg] Radius (R) Arc Length (L) Center Drop Min Casing Trim Recommended Substrate
Interior Single Door 30.0" (2' 6") 6.0" 21.75" 33.25" 15.75" 3' 6" 1/4" High-Flex Drywall or Flex Poly
Bedroom / Bath Door 32.0" (2' 8") 8.0" 20.00" 37.10" 12.00" 3' 10" 1/4" High-Flex Drywall / Polyurethane
Main Entry / Passage 36.0" (3' 0") 9.0" 22.50" 41.74" 13.50" 4' 3" 1/4" High-Flex / Kerfed MDF Jamb
Hallway / Corridor 42.0" (3' 6") 10.5" 26.25" 48.70" 15.75" 4' 10" Double 1/4" High-Flex Gypsum
Double Doorway / Pass-Through 48.0" (4' 0") 12.0" 30.00" 55.65" 18.00" 5' 6" Double 1/4" High-Flex or 3/8" Gypsum
French Door Opening 60.0" (5' 0") 15.0" 37.50" 69.56" 22.50" 6' 8" 1/4" Bendable Plywood / Wiggle Board
Living Room Cased Archway 72.0" (6' 0") 18.0" 45.00" 83.47" 27.00" 8' 0" Standard 1/2" Drywall (Wetted) or Plywood
Grand Room Divider / Great Hall 96.0" (8' 0") 24.0" 60.00" 111.30" 36.00" 10' 6" Standard 1/2" Drywall (Dry Bend OK)
ℹ Note: All segmental dimensions calculated at standard 1:4 rise-to-span ratio ($H = W/4$). For custom ratios, use the interactive calculator above.
IRC R305 Headroom Compliant
Jobsite Carpentry & Masonry Workflow

How to Lay Out & Build an Arch: Complete Field Guide

Follow this proven 5-stage jobsite workflow to translate calculated geometry into accurate physical templates, framing, and finish trim.

01

Establish the Springline Datum & Rough Opening

Snap a perfectly level chalk line at the springline elevation across both jambs. Measure the total rough opening width ($W$) and verify both king studs are plumb. Mark the exact center point on the horizontal springline.

✓ Set the elevation baseline and check opening squareness.
02

Plot the Curve onto 3/4" Plywood Template

For circular arcs with center points within reach, swing a trammel bar from the pivot point. For large-radius or elliptical arches, mark our calculated Offset Coordinates ($X, Y$) at 2"–3" intervals, tap finish brads at each point, and bend a flexible batten across the pins to scribe the smooth line.

✓ Transfer calculated geometry to physical material.
03

Cut & Duplicate Arch Rib Gussets

Cut the primary arch rib out of 3/4" CDX plywood or OSB using a jigsaw. Sand the curved edge smooth to eliminate high spots. Use this master rib with a flush-trim bearing router bit to produce identical duplicate ribs for the front and back wall faces.

✓ Create rigid curved framing ribs for the soffit.
04

Install 2x4 Infill Blocking & Mount Rib Assembly

Fasten 2x4 blocking (cut to wall depth minus two plywood thicknesses, typically 2.0" for 2x4 walls or 5.0" for 2x6 walls) between the dual plywood ribs at 6" to 8" on center. Anchor the completed arch assembly securely into the rough opening header and king studs.

✓ Build the structural framing sandwich.
05

Apply Finish Substrate (Drywall, Masonry, or Flex Trim)

For drywall, fasten two layers of 1/4" High-Flex drywall to the curved soffit (dampening the backing paper with a spray bottle for tight radii). Apply flexible archway corner bead and finish mud. For trim, install pre-curved polyurethane moulding matching the calculated intrados arc length.

✓ Install the curved soffit liner and finish casing.

Essential Jobsite Tools Checklist

Required for 1/16" Precision
▸ Trammel Bar / Beam Compass

For swinging long-radius arcs onto plywood sheets with pinpoint accuracy

▸ Flexible Batten / Fairing Strip

Thin strip of PVC, vinyl, or straight-grain pine to bend across offset coordinates

▸ Framing Square & Level

To ensure a true horizontal springline baseline and plumb king studs

▸ Plumb Laser or Laser Level

For projecting centerlines and verifying vertical alignment across wide spans

▸ Jigsaw / Flush-Trim Router

To cut the master curved plywood rib template and duplicate identical gussets

▸ Bar Clamps & Finish Brads

To secure flexible drywall or trim casing tightly along the curved soffit rib

Material Engineering Matrix

Material Bending Limits & Substrate Specifications

Understand the physical minimum bend radii and structural limitations of gypsum boards, bendable plywood, kerfed jambs, and flexible trim.

Drywall & Plaster ● Recommended for Tight Arches

1/4" High-Flex Gypsum Board

Min Dry Radius 24" (610 mm)
Min Wet Radius 12" (305 mm)

Specially engineered core with high tensile facing paper. Apply in two laminated 1/4" layers for 1/2" total thickness.

Fastener: 1-1/4" Type W/S Screws at 6" O.C.
Drywall & Plaster ● Standard Residential Spans

3/8" Standard Drywall

Min Dry Radius 60" (1,525 mm)
Min Wet Radius 36" (915 mm)

Moderate flexibility. Suitable for wide residential pass-throughs and hallway arches with radius over 5 feet.

Fastener: 1-1/4" Coarse Thread Screws
Drywall & Plaster ● Gentle Arches Only

1/2" Standard Drywall

Min Dry Radius 120" (3,050 mm)
Min Wet Radius 72" (1,830 mm)

Rigid board. Must not be bent dry under 10 ft radius. For wet bending, spray both sides with water and allow to soak for 15 minutes.

Fastener: 1-5/8" Drywall Screws
Wood & Millwork ● Best for Wood Arch Jambs

1/4" Bendable Plywood (Wiggle Wood)

Min Dry Radius 8" to 12" (200–305 mm)
Min Wet Radius N/A (Dry Only)

Multi-ply rotary cut veneer with cross-grain plies omitted. Available in Column Bend (8-ft axis) and Barrel Bend (4-ft axis).

Fastener: 18-Gauge Brads + Construction Adhesive
Wood & Millwork ● Custom Cased Openings

Kerfed 3/4" MDF or Clear Pine

Min Dry Radius 14" (355 mm)
Min Wet Radius N/A (Dry Only)

Saw kerfs cut to 5/8" depth (leaving 1/8" face veneer) spaced 1/2" to 3/4" apart. Fill kerfs with PVA glue during mounting for rigidity.

Fastener: Finish Screws into 2x4 Rib Blocking
Finish Trim & Casing ● Standard Finish Casing

Flexible Polyurethane Moulding

Min Dry Radius 12" to 18" (305–455 mm)
Min Wet Radius N/A (Dry Only)

Seamless imitation of wood grain that flexes effortlessly along the intrados. Order based on calculated Arc Length + 10%.

Fastener: 15-Gauge Finish Nails + Polyurethane Adhesive
Structural Mechanics & Physics

Structural Thrust Dynamics & Lateral Force Engineering

Understand how gravitational loads resolve into outward lateral forces, and how to size king studs, headers, and abutments to prevent framing deflection.

Thrust Equation

The Fundamental Law of Arch Thrust

The horizontal outward thrust (H_thrust) generated at each springline abutment is inversely proportional to the rise height (H):

H_thrust = (Total Load × Span) / (8 × Rise)

Key Rule: Halving the rise height doubles the outward lateral force pushing against your king studs or masonry abutment piers!

Framing Reinforcement Rules
✓ Span ≤ 48": Double 2x4 king studs each side
✓ Span 48"–72": Triple king studs + 2x8 structural header
✓ Span > 72": Engineered LVL header with structural tie straps
✓ Masonry Arches: Skewback width must be ≥ 1.5× wall thickness
Low Lateral Thrust

Semicircular (Roman) Arch

Lateral Thrust Ratio 20% – 30%

Because the curve meets the springline at a true vertical 90° tangent, the majority of dead load is directed vertically down into the supporting jambs.

Vector: Primarily Vertical (Downwards)
Moderate Lateral Thrust

Gothic (Pointed) Arch

Lateral Thrust Ratio 35% – 50%

The pointed apex divides the curve into steeper arcs, reducing horizontal outward vector forces compared to flatter round arches.

Vector: Steep Diagonal Vector
High Lateral Thrust

Segmental (Shallow) Arch

Lateral Thrust Ratio 75% – 90%

As the rise decreases relative to the span, the thrust vector angles sharply outward, exerting immense lateral force on the supporting side walls.

Vector: Strong Outward Diagonal Kick
Severe Lateral Thrust

Jack (Flat) Lintel Arch

Lateral Thrust Ratio 150% – 200%

Flat arches rely solely on wedge friction and severe horizontal thrust to hold the voussoirs aloft. Requires massive abutments or structural tie rods.

Vector: Extreme Horizontal Force
Trade Gotchas & Quality Control

Common Arch Construction Mistakes & How to Avoid Them

Prevent costly jobsite errors, structural failures, drywall cracking, and misaligned casing trim by reviewing these critical trade gotchas.

⚠️ DRYWALL & FRAMING

Exceeding Drywall Minimum Bending Radius

Common Mistake:

Attempting to force standard 1/2" drywall around tight curves (under 10 ft radius) dry, causing catastrophic gypsum core fracture and unsightly faceted creases.

Pro Trade Solution:

Use two layers of 1/4" High-Flex gypsum (bends dry to 24" radius, wet to 12" radius). For ultra-tight arches, use kerfed 3/4" MDF or 1/4" bendable plywood (wiggle board).

⚠️ STRUCTURAL LOAD

Underestimating Lateral Horizontal Thrust

Common Mistake:

Treating shallow segmental or flat jack arches like standard post-and-beam lintels without reinforcing the side supporting piers or king studs.

Pro Trade Solution:

Install double or triple king studs tightly fastened with structural framing screws. For masonry arches, ensure skewbacks are angled toward the radius center with stout abutments.

⚠️ MASONRY & BRICK

Using an Even Number of Voussoirs

Common Mistake:

Laying an even number of brick wedges in a masonry arch ring, resulting in a weak vertical mortar joint directly at the crown apex.

Pro Trade Solution:

Always specify an ODD total count of voussoirs so that a solid, symmetrical keystone locks the apex in pure compression.

⚠️ ELLIPTICAL LAYOUT

Using Stretchy Twine for Two-Pin Layout

Common Mistake:

Using standard cotton twine or jute string to draw elliptical curves. Elastic stretch distorts the constant sum (2a), producing an egg-shaped or irregular arch.

Pro Trade Solution:

Use non-stretch braided nylon mason line, steel aircraft cable, or high-tensile braided fishing line with zero elasticity for the string loop.

⚠️ TRIM & FINISHING

Ordering Casing by Span Width Instead of Arc Length

Common Mistake:

Ordering flexible polyurethane or wood moulding based on the horizontal rough opening width ($W$), leaving the installer short by 15% to 57%.

Pro Trade Solution:

Always order moulding based on the true calculated Intrados Arc Length (L_intrados) plus a minimum 10% allowance for trimming springline plumb cuts.

⚠️ LEVELING & DATUM

Uneven Springline Heights Across Jambs

Common Mistake:

Measuring the arch rise from the finished floor without verifying that the left and right springline points are level, resulting in a tilted, lopsided arch crown.

Pro Trade Solution:

Shoot a laser level line across the opening to establish an absolute level springline datum before tracing or mounting curved plywood ribs.

Field Master Pro-Tips for Clean Execution

★ Pre-curve flexible drywall overnight around a cylindrical form before hanging to relax internal surface tension.
★ When making plywood rib templates, clamp both ribs together and sand simultaneously for identical profiles.
★ Always leave centering formwork and shoring wedges in place until masonry mortar achieves full 7-day compressive cure strength.
★ For flexible casing moulding, acclimatize the material to room temperature (70°F+) for 24 hours prior to bending.
Complete Tool Index

Arch Calculators & Trade Solvers Directory

Quickly filter across our specialized trade tools, pure geometry solvers, and architectural reference guides.

Arch Shape Calculators

Tier 1 Shapes (9)

Trade & Jobsite Tools

Tier 2 Trades (10)

Geometric Solvers & Math

Tier 3 Math (5)

Authority References

Tier 4 Guides (4)
Expert Knowledge Base

Frequently Asked Questions

Common carpentry, masonry, and geometric questions regarding arch calculations and construction layout.

Q1 What is arch centering formwork?

Centering (or an arch center) is the temporary timber framework built to support masonry voussoir bricks or stone blocks during construction until the central keystone is driven into place and the mortar cures, making the arch self-supporting.

Q2 How do you calculate the radius of the timber centering rib?

The radius of the timber rib must be deducted by the thickness of the flexible lagging slats attached to its curved top edge. Formula: Rib Radius = Finished Arch Intrados Radius − Lagging Slat Thickness. For a 30.0" finished radius arch with 3/4" (0.75") plywood lagging: Rib Radius = 30.00" − 0.75" = 29.25 inches.

Q3 What are decentering wedges (folding wedges) in arch formwork?

Decentering wedges (or folding wedges) are pairs of opposing hardwood timber wedges placed beneath the temporary vertical support posts. When the mortar cures, the wedges are gently driven inward/outward with a mallet to lower the centering frame smoothly by 1 to 2 inches, transferring compressive loads onto the arch ring without shocking the fresh masonry.

Q4 When is it safe to strike (remove) arch centering formwork?

For standard Portland-lime structural mortar (Type S or M), centering formwork should remain in place for a minimum of 48 to 72 hours in warm weather, and 7 to 14 days for massive stone vault bridges before striking the decentering wedges.