The rise of a Jack arch is kept at about

🏛️ What is a Jack Arch?

A Jack Arch, also known as a flat arch, is a type of arch used over openings like doors and windows. Its most distinct feature is its flat or nearly flat bottom edge (soffit). Despite its appearance, it is not a lintel or a beam; it functions as a true arch, supporting loads through compression.

• It is constructed from wedge-shaped bricks or stones called voussoirs.
• The central voussoir is the keystone.
• The angle of the end voussoirs (skewbacks) is typically set at 60 degrees to the horizontal.

📐 Why Does a "Flat" Arch Have a "Rise"?

This is the key to the question. While a Jack arch looks flat, it must have a slight upward curve to function. This slight curve is called the rise.

Definitions:

• Span: The clear horizontal distance of the opening that the arch covers.
• Rise: The vertical distance from the springing line (the level where the arch begins) to the highest point of its inner curve (intrados).

The Rule of Thumb

For a Jack arch to be structurally sound, a standard rule of thumb is applied for its rise. The rise is kept very small to maintain the flat appearance.

The standard proportion is: Rise = 1/8th to 1/12th of the Span

For example, if a window opening has a span of 120 cm (1200 mm):

• A rise of 1/8 would be: 120 cm / 8 = 15 cm
• A rise of 1/12 would be: 120 cm / 12 = 10 cm

So, the rise would be kept between 10 cm and 15 cm. This is a very subtle curve, often barely noticeable, but it is structurally vital.

🤔 Why Other Ratios Create Different Arches

The other options represent different, more visibly curved types of arches:

• 1/2 of the span: This ratio (Rise = 0.5 × Span) creates a perfect Semi-Circular Arch.
• 1/3 to 1/4 of the span: These ratios would create a Segmental Arch, which has a very noticeable curve.

Therefore, only the small ratio of 1/8 to 1/12 is appropriate for an arch that is intended to be flat in appearance.