Jack Arch (Flat Lintel) Calculator
Calculate skewback rake angles (α = 60° to 75°), odd voussoir block counts with central keystone, upward camber rise (1/8" per foot), and horizontal thrust for flat masonry openings.
Interactive Skewback Angle & Thrust Simulator
Adjust the skewback angle slider to see how rake angles between 60° and 75° alter the abutment cut depth and wedging compression.
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.
| Pt # | Dist from Center (X) | Height (Y) | Fraction | Drop |
|---|
Live Plywood Scribing Simulator
🔨 Carpentry Technique: Tap finish brads at coordinate marks. Bend a flexible batten against the pins to scribe a smooth curve.
Jack Arch Structural Engineering Equations
Standard formulas for flat masonry arch design per the Brick Industry Association.
All voussoir joint lines radiate outward from the convergence pivot point (0, −y_pivot).
Ensure substantial brick jamb abutments exist on either side to resist horizontal thrust $H$.
Step-by-Step Worked Numeric Example
Suppose you are laying a flat masonry jack arch over a fireplace lintel with a clear span of W = 36.0 inches, a camber rise of H = 0.50 inch, and an arch depth of D = 8.0 inches.
Standard Brick Arch & Voussoir Opening Sizes
Opening spans, radius curves, odd brick voussoir counts (including keystone), and extrados mortar joint tapers.
| Opening Purpose | Span (W) | Crown Rise | Radius (R) | Arc Length | Voussoir Count | Extrados Joint | Recommended Substrate |
|---|---|---|---|---|---|---|---|
| Fireplace Opening (3-Foot) | 36.0" | 9.0" | R = 22.50" | L = 41.74" | 11 Bricks | 0.45" Extrados | Firebrick + Refractory Mortar |
| Fireplace Opening (4-Foot) | 48.0" | 12.0" | R = 30.00" | L = 55.85" | 15 Bricks | 0.48" Extrados | Standard Modular Clay Brick |
| Garden Gate Masonry Arch | 42.0" | 14.0" | R = 22.75" | L = 53.64" | 15 Bricks | 0.52" Extrados | Severe Weathering (SW) Brick |
| Wine Cellar Barrel Portal | 60.0" | 30.0" | R = 30.00" | L = 94.25" | 25 Bricks | 0.49" Extrados | Reclaimed Solid Clay Common |
How to Lay Out & Build a Brick Arch: 5-Stage Masonry Guide
Step-by-step master mason guide for constructing rigid timber centering, cutting radial skewbacks, spacing voussoirs, and striking falsework.
Erect & Wedge Timber Centering Formwork
Set the curved timber centering securely between supporting piers. Support centering legs on dual opposing hardwood wedges or sand jacks. Verify the timber crown matches finished springline elevation plus rise minus brick depth.
Layout & Chisel Skewback Abutment Beds
Cut skewbacks directly on supporting piers angled along the true radial sightline toward the curve pivot. Voussoirs must transfer compressive thrust perpendicular to skewback faces, never onto a flat horizontal shelf.
Dry-Fit Voussoirs & Verify Odd Unit Count
Dry-fit all voussoirs along the timber lagging using wood spacer shims. Confirm an ODD total count so a single symmetrical keystone locks the apex. Verify intrados joint width ($t_{int} approx 3/8"$) and extrados taper ($t_{ext} < 1/2"$).
Bed Voussoirs Symmetrically from Springline to Crown
Lay voussoirs in pairs from left and right springlines upward toward the apex. Never lay one side completely first, which distorts centering falsework under uneven thrust. Check every radial joint angle with your pivoted radial story line.
Drive Keystone & Execute Controlled Decentering
Butter both sides of the central keystone with mortar and firmly tap it into the apex gap until flush with adjacent voussoirs. Allow mortar to cure (minimum 48-72 hours). Gently ease opposing falsework wedges symmetrically from center outward to transfer full compressive dead load into abutment piers.
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.
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):
Key Rule: Halving the rise height doubles the outward lateral force pushing against your king studs or masonry abutment piers!
General residential partition guidelines. For load-bearing walls, second-story floor loads, or heavy masonry, consult a structural engineer and local building codes (IRC R602).
Semicircular (Roman) Arch
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.
Gothic (Pointed) Arch
The pointed apex divides the curve into steeper arcs, reducing horizontal outward vector forces compared to flatter round arches.
Segmental (Shallow) Arch
As the rise decreases relative to the span, the thrust vector angles sharply outward, exerting immense lateral force on the supporting side walls.
Jack (Flat) Lintel Arch
Flat arches rely solely on wedge friction and severe horizontal thrust to hold the voussoirs aloft. Requires massive abutments or structural tie rods.
Common Masonry Voussoir & Centering Mistakes
Prevent apex joint weakness, mortar crushing, and centering collapse during brick arch construction.
Using an Even Number of Voussoirs
Laying an even number of brick wedges in a masonry arch ring, resulting in a weak vertical mortar joint directly at the crown apex.
Always specify an ODD total count of voussoirs so that a solid, symmetrical keystone locks the apex in pure compression.
Using Uniform Thickness Mortar Joints on Curved Rings
Attempting to maintain equal mortar thickness from intrados to extrados without tapering, creating uneven bearing pressure and crushed brick edges.
Calculate wedge taper so intrados joints measure 1/4" to 3/8" while extrados joints widen proportionally based on ring depth, or use gauged wedge-cut bricks.
Improper Skewback Angle on Springline Abutments
Pouring or laying springline skewback skew blocks horizontally rather than perpendicular to the arch radius thrust line, risking horizontal slip.
Cut skewbacks precisely along the radial center line so voussoirs transfer load in 90-degree square compression against the abutment.
Field Master Pro-Tips for Clean Execution
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Frequently Asked Questions
Common carpentry, masonry, and geometric questions regarding arch calculations and construction layout.
Q1 What is a jack arch (flat masonry arch)?
A jack arch (also called a flat arch or camber arch) is a horizontal masonry lintel constructed from wedge-shaped (tapered) bricks or stone voussoirs that radiate from a single center point below the opening. The compressive wedging action and angled skewback abutments allow it to span horizontal openings without steel or concrete lintels.
Q2 What is the recommended skewback angle for a jack arch?
According to the Brick Industry Association (BIA Tech Note 31), the skewback angle (measured from the horizontal springline) should be between 60° and 75° (ideally 65° to 70°). A common rule of thumb is a skewback cut width of 1/2" to 1" per foot of span, or approximately 4 inches for a 3-foot opening.
Q3 Why should a jack arch have an upward camber?
Because horizontal masonry arches undergo slight settling under dead loads, the soffit is built with an intentional upward camber (crown rise) of 1/8" per foot of span. This prevents the optical illusion of sagging and ensures the arch remains in tight compression over time.
Q4 Why must a jack arch always have an odd number of voussoirs?
An odd number of voussoirs (e.g. 7, 9, 11, 13) guarantees a single, symmetrical central keystone at the exact apex of the span, locking the two opposing halves in equilibrium.