Jack Arches Calculating and templating a masonry opening

Images and figures courtesy Mortar Net Solutions

by Steven Fechino

Jack arches are a structural element in masonry construction that have been used in the built environment for centuries. Jack arches, also known as flat or straight arches, are structural elements that provide support at openings in masonry.

At some point, mason contractors may need to incorporate a jack arch into their work. The following are step-by-step instructions for constructing a custom jack arch using a template.

To calculate the cuts for a custom jack arch, a cut template needs to be created first. Three integral pieces of information are needed to begin the process. For the purposes of this article, the arch’s technical terms will be used, and the field terminology will be included in parentheses. First, one must know the width of the rough opening. Second, one must know how many courses vertically the arch will extend. The jack arch will have full-width units at the extrados (top of the arch) and cut to a thinner width at the intrados (bottom of the arch or lintel location). Third, one must know what angle they will want the arch’s left and right ends to look like; this is called the skewback—the voussoir (arch brick) at the extrados. Working with the degree of skewback can simplify the building and cutting of materials for the arch.

This layout math for a jack arch can be applied to whatever different sizes of brick that are being installed.

Light commercial façade use

The following is an example of a jack arch on a small building, for instance, a bank or a light commercial façade.

This project requires modular brick, 9.5 mm (0.375 in.) joints, and four courses of vertical arch height. Full brick is needed at the extrados and, with a 1.2-m (4 ft) opening, it will have full-brick widths at the top of the arch and workable cuts at the bottom of the arch (see Figure 1, page 45).

It is easier to build an arch with an odd number of bricks; however, in this case, an odd number will not fit properly. This means the centre of the arch will be the head joint of the mid span of the arch. There will not be a key brick (or centre brick) in this case. To determine the arch material, one should calculate the extrados based on the masonry rough opening, the width of each of the skewbacks added to the intrados, the dimensions of modular brick, plus one mortar joint for each brick. After all of this is computed, an additional mortar joint should be added to balance the arch, which is typical of all arches.

• Mortar joint: 9.5 mm

• Modular brick (height): 57 mm (2.25 in.)

• Modular brick: 9.5 + 57 mm = 66.5 mm (2.625 in.)

This example uses 66.5 mm (2.625 in.) as the width of the voussoir plus a mortar joint (arch brick plus mortar joint).

The jack arch will be sitting on a lintel that will not alter the calculations but will simplify the thought process because of the façade bond. The jack arch will extend to the fourth vertical course of wall façade brick. The extrados is where the jack arch voussoirs (arch brick) extend to; for this example, look at four courses. Four courses of the façade bond (running bond or English bond, typically) of modular brick plus each mortar joint will be 66.5 mm x 4 courses = 266 mm (2.625 in./course x 4 courses = 10.5 in.) above the lintel (the lintel also represents the intrados of the arch or the bottom of the arch).

The skewback is anywhere from 60 to 70 degrees. The higher the skewback’s degree, the farther the strike point is from the arch’s bottom. The strike point is used to pivot the arch’s width as well as to mark every brick used for cutting. The strike point is drawn from the left and right skewback following the skewback angle to a point in the centre of the masonry opening; the closer to the intrados, the strike point is located to the bottom of the arch. By calculating the number of bricks at the top of the arch, the skewback angle will be defined. This is just one way of doing it; there are several different methods, however.

Calculating the degree of skewback

To determine the degree of skewback, one must calculate full brick widths at the top of the extrados. In this case, to calculate an angle that will allow a 1.2 m (4 ft) intrados, plus the additional length for extrados, x millimetres/inches on the left and the right widths.

Knowing the rough opening is 1.2 m, and the skewback needs to be less than 152 mm (6 in.) wide on each side is based on experience. Jack arches seldom extend further than a brick; this is where the 60- to 70-degree skewback comes into play (see Figure 2).

In this example, by drawing the 63-degree skewback on cardboard, it was useful to see how far left and right the arch top would extend once the skewback angle was determined. The 63-degree skewback will add 133 mm (5.25 in.) to each side of the intrados, making the extrados 266 mm wider than the intrados.

1.2 m (rough opening) + 0.27 m = 1.46 m (4.8 ft) 1.46 m (opening at extrados) ÷ 67 mm (2.6 in.) (brick and mortar joint) = 22.3 bricks, which is perfect because it means 22 full-width bricks with head joints are included. There will be 7.3 mm (0.29 in.) for the extra mortar joint required for balancing that is 3.3 mm
(0.125 in.) off which can be bonded out.

To determine the brick’s dimension at the bottom course of the arch, the following calculation is required:

1.2 m (rough opening) ÷ 22 brick opening (determined at the extrados or the top of the arch)

= 55 mm (2.16 in.) brick and mortar joint.

After performing these calculations, the typical solution is to adjust and tighten the head joints or recalculate until a dimension for 22 bricks and 23 head joints will fit into the 1.2 m rough-opening dimension.

Twenty-three head joints x 9.5 mm per head joint = 219 mm (8.63 in.), which is between 16 and 19 mm (0.625 and 0.75 in.). Therefore, in this case, one should use 219 mm for head joints. This leaves the following calculation for a 1.2 m rough opening 219 mm = 1000 mm (8.63 in. = 39.37 in.)

To calculate a more exact width for the cuts at the intrados one should use the following formula:

1000 mm ÷ 22 bricks = 45.4 mm per brick (39.37 in. ÷ 22 bricks = 1.8 in. per brick

A template can be beneficial

After one has calculated the necessary measurements, a template can be made so the brick can be cut. Using 6.4 mm (0.25 in.) plywood is helpful as it is more durable than cardboard or poster board. It can also be easily stored if multiple arches are required for the project. When using the template, one should use a fine-tipped marker as it will lead to sharp lines and keep the markings more accurate.

When using the template, a few items are needed such as: a 1.2 x 2.4 m (4 x 8 ft) sheet of plywood, a set of clamps, a sturdy work surface, a straight edge, markers, jig saw, drill, and a 16d nail (89-mm [3.5-in.] long) are all useful in marking the template since there is a 1.2 m rough opening. Once the template has been completed, one can then turn the plywood and mark out the opening.

Simplifying the process

• Draw a 38 mm (1.5 in.) parallel or horizontal with the 2.4 m (8 ft) side of the plywood; this will be the extrados (top of the arch).

• Draw a line 305 mm (12 in.) parallel of horizontal with the 2.4 m side of the plywood; this will be the intrados (bottom of the arch).

• Draw a perpendicular or vertical line 1.2 m (4 ft) from the left or right side of the plywood, this will be the centre of the rough opening.

• From the left of the template, draw a perpendicular or vertical line at 0.6 m and 1.8 m (2 ft and 6 ft); this will represent the jambs or reveals of the rough opening.

• Next, draw a 133 mm (5.25 in.) on each side of the jamb at the intrados; this will layout the extrados total width
(see Figure 3).

Draw a line extending from the top of the extrados to the intrados (follow the skewback angle) and down to the centre of the masonry opening. The intersection of this point will be the strike point. This point needs to be the exact centre of the rough opening and the key voussoir’s centre (key brick). From the strike point, all template lines will be generated from this point on (see Figure 3A).

• Layout the façade coursing onto the template along with mortar joint; it is easier for one to draw the entire arch. However, only one half can be completed, and duplicate cuts can be drawn (see Figure 4).

• From the extrados’ top, draw 22 evenly spaced ticks; this will be one head joint and one full-width modular brick. The measurement to work with is 67 mm (2.625 in.) with an additional head joint on the opposite side of the layout for balance. All the ticks on the extrados will connect to the strike point. Once finished, the voussoir (arch brick) layout can be started (see Figure 5).

• Once the voussoirs are laid out, look at horizontal bed joints for an esthetically pleasing project.

• Label all template pieces, then take pictures of them.

• Double-check calculations and the template markings before starting the next step.

• Using a jig saw, cut out the jack arch to ensure it is in the proper orientation. After making sure it is correct, cut out the individual templates.

• With all the pieces cut out, the arch can be laid up (see Figure 6).

Jack arches are an important element when building in masonry construction. By having the knowledge on how to properly calculate measurements and create a template for a jack arch, masons will have the confidence to incorporate this unique, cost-effective element into a future project.

Steven Fechino is the engineering and construction manager for Mortar Net Solutions. He provides engineering support services and product training. Fechino has a Bachelor of Science degree in civil engineering technology and two associate degrees in civil engineering and drafting and design, specializing in building construction. He can be contacted via email at sfechino@mortarnet.com.

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